-
This section describes the LLVM Analysis Passes.
-
-
-
-
-
This is a simple N^2 alias analysis accuracy evaluator.
- Basically, for each function in the program, it simply queries to see how the
- alias analysis implementation answers alias queries between each pair of
- pointers in the function.
-
-
This is inspired and adapted from code by: Naveen Neelakantam, Francesco
- Spadini, and Wojciech Stryjewski.
-
-
-
-
-
-
A basic alias analysis pass that implements identities (two different
- globals cannot alias, etc), but does no stateful analysis.
-
-
-
-
-
-
-
-
-
-
- A pass which can be used to count how many alias queries
- are being made and how the alias analysis implementation being used responds.
-
-
-
-
-
-
-
Dependence analysis framework, which is used to detect dependences in
- memory accesses.
-
-
-
-
-
-
- This simple pass checks alias analysis users to ensure that if they
- create a new value, they do not query AA without informing it of the value.
- It acts as a shim over any other AA pass you want.
-
-
-
- Yes keeping track of every value in the program is expensive, but this is
- a debugging pass.
-
-
-
-
-
-
-
- This pass is a simple dominator construction algorithm for finding forward
- dominator frontiers.
-
-
-
-
-
-
-
- This pass is a simple dominator construction algorithm for finding forward
- dominators.
-
-
-
-
-
-
-
- This pass, only available in opt, prints the call graph into a
- .dot graph. This graph can then be processed with the "dot" tool
- to convert it to postscript or some other suitable format.
-
-
-
-
-
-
-
- This pass, only available in opt, prints the control flow graph
- into a .dot graph. This graph can then be processed with the
- "dot" tool to convert it to postscript or some other suitable format.
-
-
-
-
-
-
-
- This pass, only available in opt, prints the control flow graph
- into a .dot graph, omitting the function bodies. This graph can
- then be processed with the "dot" tool to convert it to postscript or some
- other suitable format.
-
-
-
-
-
-
-
- This pass, only available in opt, prints the dominator tree
- into a .dot graph. This graph can then be processed with the
- "dot" tool to convert it to postscript or some other suitable format.
-
-
-
-
-
-
-
- This pass, only available in opt, prints the dominator tree
- into a .dot graph, omitting the function bodies. This graph can
- then be processed with the "dot" tool to convert it to postscript or some
- other suitable format.
-
-
-
-
-
-
-
- This pass, only available in opt, prints the post dominator tree
- into a .dot graph. This graph can then be processed with the
- "dot" tool to convert it to postscript or some other suitable format.
-
-
-
-
-
-
-
- This pass, only available in opt, prints the post dominator tree
- into a .dot graph, omitting the function bodies. This graph can
- then be processed with the "dot" tool to convert it to postscript or some
- other suitable format.
-
-
-
-
-
-
-
- This simple pass provides alias and mod/ref information for global values
- that do not have their address taken, and keeps track of whether functions
- read or write memory (are "pure"). For this simple (but very common) case,
- we can provide pretty accurate and useful information.
-
-
-
-
-
-
-
- This pass collects the count of all instructions and reports them
-
-
-
-
-
-
-
- This analysis calculates and represents the interval partition of a function,
- or a preexisting interval partition.
-
-
-
- In this way, the interval partition may be used to reduce a flow graph down
- to its degenerate single node interval partition (unless it is irreducible).
-
-
-
-
-
-
-
Bookkeeping for "interesting" users of expressions computed from
- induction variables.
-
-
-
-
-
-
Interface for lazy computation of value constraint information.
-
-
-
-
-
-
LibCall Alias Analysis.
-
-
-
-
-
-
This pass statically checks for common and easily-identified constructs
- which produce undefined or likely unintended behavior in LLVM IR.
-
-
It is not a guarantee of correctness, in two ways. First, it isn't
- comprehensive. There are checks which could be done statically which are
- not yet implemented. Some of these are indicated by TODO comments, but
- those aren't comprehensive either. Second, many conditions cannot be
- checked statically. This pass does no dynamic instrumentation, so it
- can't check for all possible problems.
-
-
Another limitation is that it assumes all code will be executed. A store
- through a null pointer in a basic block which is never reached is harmless,
- but this pass will warn about it anyway.
-
-
Optimization passes may make conditions that this pass checks for more or
- less obvious. If an optimization pass appears to be introducing a warning,
- it may be that the optimization pass is merely exposing an existing
- condition in the code.
-
-
This code may be run before instcombine. In many cases, instcombine checks
- for the same kinds of things and turns instructions with undefined behavior
- into unreachable (or equivalent). Because of this, this pass makes some
- effort to look through bitcasts and so on.
-
-
-
-
-
-
-
- This analysis is used to identify natural loops and determine the loop depth
- of various nodes of the CFG. Note that the loops identified may actually be
- several natural loops that share the same header node... not just a single
- natural loop.
-
-
-
-
-
-
-
- An analysis that determines, for a given memory operation, what preceding
- memory operations it depends on. It builds on alias analysis information, and
- tries to provide a lazy, caching interface to a common kind of alias
- information query.
-
-
-
-
-
-
-
This pass decodes the debug info metadata in a module and prints in a
- (sufficiently-prepared-) human-readable form.
-
- For example, run this pass from opt along with the -analyze option, and
- it'll print to standard output.
-
-
-
-
-
-
-
- This is the default implementation of the Alias Analysis interface. It always
- returns "I don't know" for alias queries. NoAA is unlike other alias analysis
- implementations, in that it does not chain to a previous analysis. As such it
- doesn't follow many of the rules that other alias analyses must.
-
-
-
-
-
-
-
- The default "no profile" implementation of the abstract
- ProfileInfo interface.
-
-
-
-
-
-
-
- This pass is a simple post-dominator construction algorithm for finding
- post-dominator frontiers.
-
-
-
-
-
-
-
- This pass is a simple post-dominator construction algorithm for finding
- post-dominators.
-
-
-
-
-
-
-
-
-
-
-
- This pass, only available in opt, prints the call graph to
- standard error in a human-readable form.
-
-
-
-
-
-
-
- This pass, only available in opt, prints the SCCs of the call
- graph to standard error in a human-readable form.
-
-
-
-
-
-
-
- This pass, only available in opt, prints the SCCs of each
- function CFG to standard error in a human-readable form.
-
-
-
-
-
-
-
Pass that prints instructions, and associated debug info:
-
-
- - source/line/col information
- - original variable name
- - original type name
-
-
-
-
-
-
-
Dominator Info Printer.
-
-
-
-
-
-
- This pass, only available in opt, prints out call sites to
- external functions that are called with constant arguments. This can be
- useful when looking for standard library functions we should constant fold
- or handle in alias analyses.
-
-
-
-
-
-
-
- The PrintFunctionPass class is designed to be pipelined with
- other FunctionPasses, and prints out the functions of the module
- as they are processed.
-
-
-
-
-
-
-
- This pass simply prints out the entire module when it is executed.
-
-
-
-
-
-
-
- This pass is used to seek out all of the types in use by the program. Note
- that this analysis explicitly does not include types only used by the symbol
- table.
-
-
-
-
-
-
Profiling information that estimates the profiling information
- in a very crude and unimaginative way.
-
-
-
-
-
-
-
- A concrete implementation of profiling information that loads the information
- from a profile dump file.
-
-
-
-
-
-
-
Pass that checks profiling information for plausibility.
-
-
-
-
- The RegionInfo pass detects single entry single exit regions in a
- function, where a region is defined as any subgraph that is connected to the
- remaining graph at only two spots. Furthermore, an hierarchical region tree is
- built.
-
-
-
-
-
-
-
- The ScalarEvolution analysis can be used to analyze and
- catagorize scalar expressions in loops. It specializes in recognizing general
- induction variables, representing them with the abstract and opaque
- SCEV class. Given this analysis, trip counts of loops and other
- important properties can be obtained.
-
-
-
- This analysis is primarily useful for induction variable substitution and
- strength reduction.
-
-
-
-
-
-
-
Simple alias analysis implemented in terms of ScalarEvolution queries.
-
- This differs from traditional loop dependence analysis in that it tests
- for dependencies within a single iteration of a loop, rather than
- dependencies between different iterations.
-
- ScalarEvolution has a more complete understanding of pointer arithmetic
- than BasicAliasAnalysis' collection of ad-hoc analyses.
-
-
-
-
-
-
-
Provides other passes access to information on how the size and alignment
- required by the target ABI for various data types.
-
-
-
-
This section describes the LLVM Transform Passes.
-
-
-
-
-
ADCE aggressively tries to eliminate code. This pass is similar to
- DCE but it assumes that values are dead until proven
- otherwise. This is similar to SCCP, except applied to
- the liveness of values.
-
-
-
-
-
-
A custom inliner that handles only functions that are marked as
- "always inline".
-
-
-
-
-
-
- This pass promotes "by reference" arguments to be "by value" arguments. In
- practice, this means looking for internal functions that have pointer
- arguments. If it can prove, through the use of alias analysis, that an
- argument is *only* loaded, then it can pass the value into the function
- instead of the address of the value. This can cause recursive simplification
- of code and lead to the elimination of allocas (especially in C++ template
- code like the STL).
-
-
-
- This pass also handles aggregate arguments that are passed into a function,
- scalarizing them if the elements of the aggregate are only loaded. Note that
- it refuses to scalarize aggregates which would require passing in more than
- three operands to the function, because passing thousands of operands for a
- large array or structure is unprofitable!
-
-
-
- Note that this transformation could also be done for arguments that are only
- stored to (returning the value instead), but does not currently. This case
- would be best handled when and if LLVM starts supporting multiple return
- values from functions.
-
-
-
-
-
-
-
This pass combines instructions inside basic blocks to form vector
- instructions. It iterates over each basic block, attempting to pair
- compatible instructions, repeating this process until no additional
- pairs are selected for vectorization. When the outputs of some pair
- of compatible instructions are used as inputs by some other pair of
- compatible instructions, those pairs are part of a potential
- vectorization chain. Instruction pairs are only fused into vector
- instructions when they are part of a chain longer than some
- threshold length. Moreover, the pass attempts to find the best
- possible chain for each pair of compatible instructions. These
- heuristics are intended to prevent vectorization in cases where
- it would not yield a performance increase of the resulting code.
-
-
-
-
-
-
-
This pass is a very simple profile guided basic block placement algorithm.
- The idea is to put frequently executed blocks together at the start of the
- function and hopefully increase the number of fall-through conditional
- branches. If there is no profile information for a particular function, this
- pass basically orders blocks in depth-first order.
-
-
-
-
-
-
- Break all of the critical edges in the CFG by inserting a dummy basic block.
- It may be "required" by passes that cannot deal with critical edges. This
- transformation obviously invalidates the CFG, but can update forward dominator
- (set, immediate dominators, tree, and frontier) information.
-
-
-
-
-
-
- This pass munges the code in the input function to better prepare it for
- SelectionDAG-based code generation. This works around limitations in it's
- basic-block-at-a-time approach. It should eventually be removed.
-
-
-
-
-
-
- Merges duplicate global constants together into a single constant that is
- shared. This is useful because some passes (ie TraceValues) insert a lot of
- string constants into the program, regardless of whether or not an existing
- string is available.
-
-
-
-
-
-
-
This file implements constant propagation and merging. It looks for
- instructions involving only constant operands and replaces them with a
- constant value instead of an instruction. For example:
-
add i32 1, 2
-
becomes
-
i32 3
-
NOTE: this pass has a habit of making definitions be dead. It is a good
- idea to to run a DIE (Dead Instruction Elimination) pass
- sometime after running this pass.
-
-
-
-
-
-
- Dead code elimination is similar to dead instruction
- elimination, but it rechecks instructions that were used by removed
- instructions to see if they are newly dead.
-
-
-
-
-
-
-
- This pass deletes dead arguments from internal functions. Dead argument
- elimination removes arguments which are directly dead, as well as arguments
- only passed into function calls as dead arguments of other functions. This
- pass also deletes dead arguments in a similar way.
-
-
-
- This pass is often useful as a cleanup pass to run after aggressive
- interprocedural passes, which add possibly-dead arguments.
-
-
-
-
-
-
-
- This pass is used to cleanup the output of GCC. It eliminate names for types
- that are unused in the entire translation unit, using the find used types pass.
-
-
-
-
-
-
-
- Dead instruction elimination performs a single pass over the function,
- removing instructions that are obviously dead.
-
-
-
-
-
-
-
- A trivial dead store elimination that only considers basic-block local
- redundant stores.
-
-
-
-
-
-
-
A simple interprocedural pass which walks the call-graph, looking for
- functions which do not access or only read non-local memory, and marking them
- readnone/readonly. In addition, it marks function arguments (of pointer type)
- 'nocapture' if a call to the function does not create any copies of the pointer
- value that outlive the call. This more or less means that the pointer is only
- dereferenced, and not returned from the function or stored in a global.
- This pass is implemented as a bottom-up traversal of the call-graph.
-
-
-
-
-
-
-
- This transform is designed to eliminate unreachable internal globals from the
- program. It uses an aggressive algorithm, searching out globals that are
- known to be alive. After it finds all of the globals which are needed, it
- deletes whatever is left over. This allows it to delete recursive chunks of
- the program which are unreachable.
-
-
-
-
-
-
-
- This pass transforms simple global variables that never have their address
- taken. If obviously true, it marks read/write globals as constant, deletes
- variables only stored to, etc.
-
-
-
-
-
-
-
- This pass performs global value numbering to eliminate fully and partially
- redundant instructions. It also performs redundant load elimination.
-
-
-
-
-
-
-
- This transformation analyzes and transforms the induction variables (and
- computations derived from them) into simpler forms suitable for subsequent
- analysis and transformation.
-
-
-
- This transformation makes the following changes to each loop with an
- identifiable induction variable:
-
-
-
- - All loops are transformed to have a single canonical
- induction variable which starts at zero and steps by one.
- - The canonical induction variable is guaranteed to be the first PHI node
- in the loop header block.
- - Any pointer arithmetic recurrences are raised to use array
- subscripts.
-
-
-
- If the trip count of a loop is computable, this pass also makes the following
- changes:
-
-
-
- - The exit condition for the loop is canonicalized to compare the
- induction value against the exit value. This turns loops like:
-
for (i = 7; i*i < 1000; ++i)
- into
- for (i = 0; i != 25; ++i)
- - Any use outside of the loop of an expression derived from the indvar
- is changed to compute the derived value outside of the loop, eliminating
- the dependence on the exit value of the induction variable. If the only
- purpose of the loop is to compute the exit value of some derived
- expression, this transformation will make the loop dead.
-
-
-
- This transformation should be followed by strength reduction after all of the
- desired loop transformations have been performed. Additionally, on targets
- where it is profitable, the loop could be transformed to count down to zero
- (the "do loop" optimization).
-
-
-
-
-
-
-
- Bottom-up inlining of functions into callees.
-
-
-
-
-
-
-
- This pass instruments the specified program with counters for edge profiling.
- Edge profiling can give a reasonable approximation of the hot paths through a
- program, and is used for a wide variety of program transformations.
-
-
-
- Note that this implementation is very naïve. It inserts a counter for
- every edge in the program, instead of using control flow information
- to prune the number of counters inserted.
-
-
-
-
-
-
-
This pass instruments the specified program with counters for edge profiling.
- Edge profiling can give a reasonable approximation of the hot paths through a
- program, and is used for a wide variety of program transformations.
-
-
-
-
-
-
-
- Combine instructions to form fewer, simple
- instructions. This pass does not modify the CFG This pass is where algebraic
- simplification happens.
-
-
-
- This pass combines things like:
-
-
-
%Y = add i32 %X, 1
-%Z = add i32 %Y, 1
-
-
- into:
-
-
-
%Z = add i32 %X, 2
-
-
- This is a simple worklist driven algorithm.
-
-
-
- This pass guarantees that the following canonicalizations are performed on
- the program:
-
-
-
- - If a binary operator has a constant operand, it is moved to the right-
- hand side.
- - Bitwise operators with constant operands are always grouped so that
- shifts are performed first, then
ors, then
- ands, then xors.
- - Compare instructions are converted from
<,
- >, ≤, or ≥ to
- = or ≠ if possible.
- - All
cmp instructions on boolean values are replaced with
- logical operations.
- add X, X is represented as
- mul X, 2 ⇒ shl X, 1- Multiplies with a constant power-of-two argument are transformed into
- shifts.
- - … etc.
-
-
-
-
-
-
-
- This pass loops over all of the functions in the input module, looking for a
- main function. If a main function is found, all other functions and all
- global variables with initializers are marked as internal.
-
-
-
-
-
-
-
- This pass implements an extremely simple interprocedural constant
- propagation pass. It could certainly be improved in many different ways,
- like using a worklist. This pass makes arguments dead, but does not remove
- them. The existing dead argument elimination pass should be run after this
- to clean up the mess.
-
-
-
-
-
-
-
-
-
-
-
- Jump threading tries to find distinct threads of control flow running through
- a basic block. This pass looks at blocks that have multiple predecessors and
- multiple successors. If one or more of the predecessors of the block can be
- proven to always cause a jump to one of the successors, we forward the edge
- from the predecessor to the successor by duplicating the contents of this
- block.
-
-
- An example of when this can occur is code like this:
-
-
-
if () { ...
- X = 4;
-}
-if (X < 3) {
-
-
- In this case, the unconditional branch at the end of the first if can be
- revectored to the false side of the second if.
-
-
-
-
-
-
-
- This pass transforms loops by placing phi nodes at the end of the loops for
- all values that are live across the loop boundary. For example, it turns
- the left into the right code:
-
-
-
for (...) for (...)
- if (c) if (c)
- X1 = ... X1 = ...
- else else
- X2 = ... X2 = ...
- X3 = phi(X1, X2) X3 = phi(X1, X2)
-... = X3 + 4 X4 = phi(X3)
- ... = X4 + 4
-
-
- This is still valid LLVM; the extra phi nodes are purely redundant, and will
- be trivially eliminated by InstCombine. The major benefit of
- this transformation is that it makes many other loop optimizations, such as
- LoopUnswitching, simpler.
-
-
-
-
-
-
-
- This pass performs loop invariant code motion, attempting to remove as much
- code from the body of a loop as possible. It does this by either hoisting
- code into the preheader block, or by sinking code to the exit blocks if it is
- safe. This pass also promotes must-aliased memory locations in the loop to
- live in registers, thus hoisting and sinking "invariant" loads and stores.
-
-
-
- This pass uses alias analysis for two purposes:
-
-
-
- - Moving loop invariant loads and calls out of loops. If we can determine
- that a load or call inside of a loop never aliases anything stored to,
- we can hoist it or sink it like any other instruction.
- - Scalar Promotion of Memory - If there is a store instruction inside of
- the loop, we try to move the store to happen AFTER the loop instead of
- inside of the loop. This can only happen if a few conditions are true:
-
- - The pointer stored through is loop invariant.
- - There are no stores or loads in the loop which may alias
- the pointer. There are no calls in the loop which mod/ref the
- pointer.
-
- If these conditions are true, we can promote the loads and stores in the
- loop of the pointer to use a temporary alloca'd variable. We then use
- the mem2reg functionality to construct the appropriate SSA form for the
- variable.
-
-
-
-
-
-
-
- This file implements the Dead Loop Deletion Pass. This pass is responsible
- for eliminating loops with non-infinite computable trip counts that have no
- side effects or volatile instructions, and do not contribute to the
- computation of the function's return value.
-
-
-
-
-
-
-
- A pass wrapper around the ExtractLoop() scalar transformation to
- extract each top-level loop into its own new function. If the loop is the
- only loop in a given function, it is not touched. This is a pass most
- useful for debugging via bugpoint.
-
-
-
-
-
-
-
- Similar to Extract loops into new functions,
- this pass extracts one natural loop from the program into a function if it
- can. This is used by bugpoint.
-
-
-
-
-
-
-
- This pass performs a strength reduction on array references inside loops that
- have as one or more of their components the loop induction variable. This is
- accomplished by creating a new value to hold the initial value of the array
- access for the first iteration, and then creating a new GEP instruction in
- the loop to increment the value by the appropriate amount.
-
-
-
-
-
-
-
A simple loop rotation transformation.
-
-
-
-
-
-
- This pass performs several transformations to transform natural loops into a
- simpler form, which makes subsequent analyses and transformations simpler and
- more effective.
-
-
-
- Loop pre-header insertion guarantees that there is a single, non-critical
- entry edge from outside of the loop to the loop header. This simplifies a
- number of analyses and transformations, such as LICM.
-
-
-
- Loop exit-block insertion guarantees that all exit blocks from the loop
- (blocks which are outside of the loop that have predecessors inside of the
- loop) only have predecessors from inside of the loop (and are thus dominated
- by the loop header). This simplifies transformations such as store-sinking
- that are built into LICM.
-
-
-
- This pass also guarantees that loops will have exactly one backedge.
-
-
-
- Note that the simplifycfg pass will clean up blocks which are split out but
- end up being unnecessary, so usage of this pass should not pessimize
- generated code.
-
-
-
- This pass obviously modifies the CFG, but updates loop information and
- dominator information.
-
-
-
-
-
-
-
- This pass implements a simple loop unroller. It works best when loops have
- been canonicalized by the -indvars pass,
- allowing it to determine the trip counts of loops easily.
-
-
-
-
-
-
-
- This pass transforms loops that contain branches on loop-invariant conditions
- to have multiple loops. For example, it turns the left into the right code:
-
-
-
for (...) if (lic)
- A for (...)
- if (lic) A; B; C
- B else
- C for (...)
- A; C
-
-
- This can increase the size of the code exponentially (doubling it every time
- a loop is unswitched) so we only unswitch if the resultant code will be
- smaller than a threshold.
-
-
-
- This pass expects LICM to be run before it to hoist invariant conditions out
- of the loop, to make the unswitching opportunity obvious.
-
-
-
-
-
-
-
- This pass lowers atomic intrinsics to non-atomic form for use in a known
- non-preemptible environment.
-
-
-
- The pass does not verify that the environment is non-preemptible (in
- general this would require knowledge of the entire call graph of the
- program including any libraries which may not be available in bitcode form);
- it simply lowers every atomic intrinsic.
-
-
-
-
-
-
-
- This transformation is designed for use by code generators which do not yet
- support stack unwinding. This pass supports two models of exception handling
- lowering, the 'cheap' support and the 'expensive' support.
-
-
-
- 'Cheap' exception handling support gives the program the ability to execute
- any program which does not "throw an exception", by turning 'invoke'
- instructions into calls and by turning 'unwind' instructions into calls to
- abort(). If the program does dynamically use the unwind instruction, the
- program will print a message then abort.
-
-
-
- 'Expensive' exception handling support gives the full exception handling
- support to the program at the cost of making the 'invoke' instruction
- really expensive. It basically inserts setjmp/longjmp calls to emulate the
- exception handling as necessary.
-
-
-
- Because the 'expensive' support slows down programs a lot, and EH is only
- used for a subset of the programs, it must be specifically enabled by the
- -enable-correct-eh-support option.
-
-
-
- Note that after this pass runs the CFG is not entirely accurate (exceptional
- control flow edges are not correct anymore) so only very simple things should
- be done after the lowerinvoke pass has run (like generation of native code).
- This should not be used as a general purpose "my LLVM-to-LLVM pass doesn't
- support the invoke instruction yet" lowering pass.
-
-
-
-
-
-
-
- Rewrites switch instructions with a sequence of branches, which
- allows targets to get away with not implementing the switch instruction until
- it is convenient.
-
-
-
-
-
-
-
- This file promotes memory references to be register references. It promotes
- alloca instructions which only have loads and
- stores as uses. An alloca is transformed by using dominator
- frontiers to place phi nodes, then traversing the function in
- depth-first order to rewrite loads and stores as
- appropriate. This is just the standard SSA construction algorithm to construct
- "pruned" SSA form.
-
-
-
-
-
-
-
- This pass performs various transformations related to eliminating memcpy
- calls, or transforming sets of stores into memset's.
-
-
-
-
-
-
-
This pass looks for equivalent functions that are mergable and folds them.
-
- A hash is computed from the function, based on its type and number of
- basic blocks.
-
- Once all hashes are computed, we perform an expensive equality comparison
- on each function pair. This takes n^2/2 comparisons per bucket, so it's
- important that the hash function be high quality. The equality comparison
- iterates through each instruction in each basic block.
-
- When a match is found the functions are folded. If both functions are
- overridable, we move the functionality into a new internal function and
- leave two overridable thunks to it.
-
-
-
-
-
-
-
- Ensure that functions have at most one ret instruction in them.
- Additionally, it keeps track of which node is the new exit node of the CFG.
-
-
-
-
-
-
-
This pass performs partial inlining, typically by inlining an if
- statement that surrounds the body of the function.
-
-
-
-
-
-
-
- This file implements a simple interprocedural pass which walks the call-graph,
- turning invoke instructions into call instructions if and
- only if the callee cannot throw an exception. It implements this as a
- bottom-up traversal of the call-graph.
-
-
-
-
-
-
-
- This pass reassociates commutative expressions in an order that is designed
- to promote better constant propagation, GCSE, LICM, PRE, etc.
-
-
-
- For example: 4 + (x + 5) ⇒ x + (4 + 5)
-
-
-
- In the implementation of this algorithm, constants are assigned rank = 0,
- function arguments are rank = 1, and other values are assigned ranks
- corresponding to the reverse post order traversal of current function
- (starting at 2), which effectively gives values in deep loops higher rank
- than values not in loops.
-
-
-
-
-
-
-
- This file demotes all registers to memory references. It is intended to be
- the inverse of -mem2reg. By converting to
- load instructions, the only values live across basic blocks are
- alloca instructions and load instructions before
- phi nodes. It is intended that this should make CFG hacking much
- easier. To make later hacking easier, the entry block is split into two, such
- that all introduced alloca instructions (and nothing else) are in the
- entry block.
-
-
-
-
-
-
-
- The well-known scalar replacement of aggregates transformation. This
- transform breaks up alloca instructions of aggregate type (structure
- or array) into individual alloca instructions for each member if
- possible. Then, if possible, it transforms the individual alloca
- instructions into nice clean scalar SSA form.
-
-
-
- This combines a simple scalar replacement of aggregates algorithm with the mem2reg algorithm because often interact,
- especially for C++ programs. As such, iterating between scalarrepl,
- then mem2reg until we run out of things to
- promote works well.
-
-
-
-
-
-
-
- Sparse conditional constant propagation and merging, which can be summarized
- as:
-
-
-
- - Assumes values are constant unless proven otherwise
- - Assumes BasicBlocks are dead unless proven otherwise
- - Proves values to be constant, and replaces them with constants
- - Proves conditional branches to be unconditional
-
-
-
- Note that this pass has a habit of making definitions be dead. It is a good
- idea to to run a DCE pass sometime after running this pass.
-
-
-
-
-
-
-
- Applies a variety of small optimizations for calls to specific well-known
- function calls (e.g. runtime library functions). For example, a call
- exit(3) that occurs within the main() function can be
- transformed into simply return 3.
-
-
-
-
-
-
-
- Performs dead code elimination and basic block merging. Specifically:
-
-
-
- - Removes basic blocks with no predecessors.
- - Merges a basic block into its predecessor if there is only one and the
- predecessor only has one successor.
- - Eliminates PHI nodes for basic blocks with a single predecessor.
- - Eliminates a basic block that only contains an unconditional
- branch.
-
-
-
-
-
-
-
This pass moves instructions into successor blocks, when possible, so that
- they aren't executed on paths where their results aren't needed.
-
-
-
-
-
-
-
- performs code stripping. this transformation can delete:
-
-
-
- - names for virtual registers
- - symbols for internal globals and functions
- - debug information
-
-
-
- note that this transformation makes code much less readable, so it should
- only be used in situations where the strip utility would be used,
- such as reducing code size or making it harder to reverse engineer code.
-
-
-
-
-
-
-
- performs code stripping. this transformation can delete:
-
-
-
- - names for virtual registers
- - symbols for internal globals and functions
- - debug information
-
-
-
- note that this transformation makes code much less readable, so it should
- only be used in situations where the strip utility would be used,
- such as reducing code size or making it harder to reverse engineer code.
-
-
-
-
-
-
-
- This pass loops over all of the functions in the input module, looking for
- dead declarations and removes them. Dead declarations are declarations of
- functions for which no implementation is available (i.e., declarations for
- unused library functions).
-
-
-
-
-
-
-
This pass implements code stripping. Specifically, it can delete:
-
- - names for virtual registers
- - symbols for internal globals and functions
- - debug information
-
-
- Note that this transformation makes code much less readable, so it should
- only be used in situations where the 'strip' utility would be used, such as
- reducing code size or making it harder to reverse engineer code.
-
-
-
-
-
-
-
This pass implements code stripping. Specifically, it can delete:
-
- - names for virtual registers
- - symbols for internal globals and functions
- - debug information
-
-
- Note that this transformation makes code much less readable, so it should
- only be used in situations where the 'strip' utility would be used, such as
- reducing code size or making it harder to reverse engineer code.
-
-
-
-
-
-
-
- This file transforms calls of the current function (self recursion) followed
- by a return instruction with a branch to the entry of the function, creating
- a loop. This pass also implements the following extensions to the basic
- algorithm:
-
-
-
- - Trivial instructions between the call and return do not prevent the
- transformation from taking place, though currently the analysis cannot
- support moving any really useful instructions (only dead ones).
-
- This pass transforms functions that are prevented from being tail
- recursive by an associative expression to use an accumulator variable,
- thus compiling the typical naive factorial or fib implementation
- into efficient code.
-
- TRE is performed if the function returns void, if the return
- returns the result returned by the call, or if the function returns a
- run-time constant on all exits from the function. It is possible, though
- unlikely, that the return returns something else (like constant 0), and
- can still be TRE'd. It can be TRE'd if all other return
- instructions in the function return the exact same value.
-
- If it can prove that callees do not access theier caller stack frame,
- they are marked as eligible for tail call elimination (by the code
- generator).
-
-
-
-
-
-
-
This section describes the LLVM Utility Passes.
-
-
-
-
-
- Same as dead argument elimination, but deletes arguments to functions which
- are external. This is only for use by bugpoint.
-
-
-
-
-
-
- This pass is used by bugpoint to extract all blocks from the module into their
- own functions.
-
-
-
-
-
-
This is a little utility pass that gives instructions names, this is mostly
- useful when diffing the effect of an optimization because deleting an
- unnamed instruction can change all other instruction numbering, making the
- diff very noisy.
-
-
-
-
-
-
-
- Ensures that the module is in the form required by the Module Verifier pass.
-
-
-
- Running the verifier runs this pass automatically, so there should be no need
- to use it directly.
-
-
-
-
-
-
-
- Verifies an LLVM IR code. This is useful to run after an optimization which is
- undergoing testing. Note that llvm-as verifies its input before
- emitting bitcode, and also that malformed bitcode is likely to make LLVM
- crash. All language front-ends are therefore encouraged to verify their output
- before performing optimizing transformations.
-
-
-
- - Both of a binary operator's parameters are of the same type.
- - Verify that the indices of mem access instructions match other
- operands.
- - Verify that arithmetic and other things are only performed on
- first-class types. Verify that shifts and logicals only happen on
- integrals f.e.
- - All of the constants in a switch statement are of the correct type.
- - The code is in valid SSA form.
- - It is illegal to put a label into any other type (like a structure) or
- to return one.
- - Only phi nodes can be self referential: %x = add i32 %x, %x is
- invalid.
- - PHI nodes must have an entry for each predecessor, with no extras.
- - PHI nodes must be the first thing in a basic block, all grouped
- together.
- - PHI nodes must have at least one entry.
- - All basic blocks should only end with terminator insts, not contain
- them.
- - The entry node to a function must not have predecessors.
- - All Instructions must be embedded into a basic block.
- - Functions cannot take a void-typed parameter.
- - Verify that a function's argument list agrees with its declared
- type.
- - It is illegal to specify a name for a void value.
- - It is illegal to have an internal global value with no initializer.
- - It is illegal to have a ret instruction that returns a value that does
- not agree with the function return value type.
- - Function call argument types match the function prototype.
- - All other things that are tested by asserts spread about the code.
-
-
-
- Note that this does not provide full security verification (like Java), but
- instead just tries to ensure that code is well-formed.
-
-
-
-
-
-
-
- Displays the control flow graph using the GraphViz tool.
-
-
-
-
-
-
-
- Displays the control flow graph using the GraphViz tool, but omitting function
- bodies.
-
-
-
-
-
-
-
- Displays the dominator tree using the GraphViz tool.
-
-
-
-
-
-
-
- Displays the dominator tree using the GraphViz tool, but omitting function
- bodies.
-
-
-
-
-
-
-
- Displays the post dominator tree using the GraphViz tool.
-
-
-
-
-
-
-
- Displays the post dominator tree using the GraphViz tool, but omitting
- function bodies.
-
-
-
-
-
-
-
-
-
- 
- 
-
- Reid Spencer
- LLVM Compiler Infrastructure
- Last modified: $Date$
-
-
-
-
diff --git a/docs/Passes.rst b/docs/Passes.rst
new file mode 100644
index 00000000000..ed721666638
--- /dev/null
+++ b/docs/Passes.rst
@@ -0,0 +1,1264 @@
+..
+ If Passes.html is up to date, the following "one-liner" should print
+ an empty diff.
+
+ egrep -e '^
| -.* | .* |
$' \
+ -e '^
.*$' < Passes.html >html; \
+ perl >help <<'EOT' && diff -u help html; rm -f help html
+ open HTML, "
) {
+ m:^| -.* | .* |
$: or next;
+ $order{$1} = sprintf("%03d", 1 + int %order);
+ }
+ open HELP, "../Release/bin/opt -help|" or die "open: opt -help: $!\n";
+ while () {
+ m:^ -([^ ]+) +- (.*)$: or next;
+ my $o = $order{$1};
+ $o = "000" unless defined $o;
+ push @x, "$o| -$1 | $2 |
\n";
+ push @y, "$o -$1: $2\n";
+ }
+ @x = map { s/^\d\d\d//; $_ } sort @x;
+ @y = map { s/^\d\d\d//; $_ } sort @y;
+ print @x, @y;
+ EOT
+
+ This (real) one-liner can also be helpful when converting comments to HTML:
+
+ perl -e '$/ = undef; for (split(/\n/, <>)) { s:^ *///? ?::; print " \n" if !$on && $_ =~ /\S/; print "
\n" if $on && $_ =~ /^\s*$/; print " $_\n"; $on = ($_ =~ /\S/); } print " \n" if $on'
+
+====================================
+LLVM's Analysis and Transform Passes
+====================================
+
+.. contents::
+ :local:
+
+Written by `Reid Spencer `_
+ and Gordon Henriksen
+
+Introduction
+============
+
+This document serves as a high level summary of the optimization features that
+LLVM provides. Optimizations are implemented as Passes that traverse some
+portion of a program to either collect information or transform the program.
+The table below divides the passes that LLVM provides into three categories.
+Analysis passes compute information that other passes can use or for debugging
+or program visualization purposes. Transform passes can use (or invalidate)
+the analysis passes. Transform passes all mutate the program in some way.
+Utility passes provides some utility but don't otherwise fit categorization.
+For example passes to extract functions to bitcode or write a module to bitcode
+are neither analysis nor transform passes. The table of contents above
+provides a quick summary of each pass and links to the more complete pass
+description later in the document.
+
+Analysis Passes
+===============
+
+This section describes the LLVM Analysis Passes.
+
+``-aa-eval``: Exhaustive Alias Analysis Precision Evaluator
+-----------------------------------------------------------
+
+This is a simple N^2 alias analysis accuracy evaluator. Basically, for each
+function in the program, it simply queries to see how the alias analysis
+implementation answers alias queries between each pair of pointers in the
+function.
+
+This is inspired and adapted from code by: Naveen Neelakantam, Francesco
+Spadini, and Wojciech Stryjewski.
+
+``-basicaa``: Basic Alias Analysis (stateless AA impl)
+------------------------------------------------------
+
+A basic alias analysis pass that implements identities (two different globals
+cannot alias, etc), but does no stateful analysis.
+
+``-basiccg``: Basic CallGraph Construction
+------------------------------------------
+
+Yet to be written.
+
+``-count-aa``: Count Alias Analysis Query Responses
+---------------------------------------------------
+
+A pass which can be used to count how many alias queries are being made and how
+the alias analysis implementation being used responds.
+
+``-da``: Dependence Analysis
+----------------------------
+
+Dependence analysis framework, which is used to detect dependences in memory
+accesses.
+
+``-debug-aa``: AA use debugger
+------------------------------
+
+This simple pass checks alias analysis users to ensure that if they create a
+new value, they do not query AA without informing it of the value. It acts as
+a shim over any other AA pass you want.
+
+Yes keeping track of every value in the program is expensive, but this is a
+debugging pass.
+
+``-domfrontier``: Dominance Frontier Construction
+-------------------------------------------------
+
+This pass is a simple dominator construction algorithm for finding forward
+dominator frontiers.
+
+``-domtree``: Dominator Tree Construction
+-----------------------------------------
+
+This pass is a simple dominator construction algorithm for finding forward
+dominators.
+
+
+``-dot-callgraph``: Print Call Graph to "dot" file
+--------------------------------------------------
+
+This pass, only available in ``opt``, prints the call graph into a ``.dot``
+graph. This graph can then be processed with the "dot" tool to convert it to
+postscript or some other suitable format.
+
+``-dot-cfg``: Print CFG of function to "dot" file
+-------------------------------------------------
+
+This pass, only available in ``opt``, prints the control flow graph into a
+``.dot`` graph. This graph can then be processed with the :program:`dot` tool
+to convert it to postscript or some other suitable format.
+
+``-dot-cfg-only``: Print CFG of function to "dot" file (with no function bodies)
+--------------------------------------------------------------------------------
+
+This pass, only available in ``opt``, prints the control flow graph into a
+``.dot`` graph, omitting the function bodies. This graph can then be processed
+with the :program:`dot` tool to convert it to postscript or some other suitable
+format.
+
+``-dot-dom``: Print dominance tree of function to "dot" file
+------------------------------------------------------------
+
+This pass, only available in ``opt``, prints the dominator tree into a ``.dot``
+graph. This graph can then be processed with the :program:`dot` tool to
+convert it to postscript or some other suitable format.
+
+``-dot-dom-only``: Print dominance tree of function to "dot" file (with no function bodies)
+-------------------------------------------------------------------------------------------
+
+This pass, only available in ``opt``, prints the dominator tree into a ``.dot``
+graph, omitting the function bodies. This graph can then be processed with the
+:program:`dot` tool to convert it to postscript or some other suitable format.
+
+``-dot-postdom``: Print postdominance tree of function to "dot" file
+--------------------------------------------------------------------
+
+This pass, only available in ``opt``, prints the post dominator tree into a
+``.dot`` graph. This graph can then be processed with the :program:`dot` tool
+to convert it to postscript or some other suitable format.
+
+``-dot-postdom-only``: Print postdominance tree of function to "dot" file (with no function bodies)
+---------------------------------------------------------------------------------------------------
+
+This pass, only available in ``opt``, prints the post dominator tree into a
+``.dot`` graph, omitting the function bodies. This graph can then be processed
+with the :program:`dot` tool to convert it to postscript or some other suitable
+format.
+
+``-globalsmodref-aa``: Simple mod/ref analysis for globals
+----------------------------------------------------------
+
+This simple pass provides alias and mod/ref information for global values that
+do not have their address taken, and keeps track of whether functions read or
+write memory (are "pure"). For this simple (but very common) case, we can
+provide pretty accurate and useful information.
+
+``-instcount``: Counts the various types of ``Instruction``\ s
+--------------------------------------------------------------
+
+This pass collects the count of all instructions and reports them.
+
+``-intervals``: Interval Partition Construction
+-----------------------------------------------
+
+This analysis calculates and represents the interval partition of a function,
+or a preexisting interval partition.
+
+In this way, the interval partition may be used to reduce a flow graph down to
+its degenerate single node interval partition (unless it is irreducible).
+
+``-iv-users``: Induction Variable Users
+---------------------------------------
+
+Bookkeeping for "interesting" users of expressions computed from induction
+variables.
+
+``-lazy-value-info``: Lazy Value Information Analysis
+-----------------------------------------------------
+
+Interface for lazy computation of value constraint information.
+
+``-libcall-aa``: LibCall Alias Analysis
+---------------------------------------
+
+LibCall Alias Analysis.
+
+``-lint``: Statically lint-checks LLVM IR
+-----------------------------------------
+
+This pass statically checks for common and easily-identified constructs which
+produce undefined or likely unintended behavior in LLVM IR.
+
+It is not a guarantee of correctness, in two ways. First, it isn't
+comprehensive. There are checks which could be done statically which are not
+yet implemented. Some of these are indicated by TODO comments, but those
+aren't comprehensive either. Second, many conditions cannot be checked
+statically. This pass does no dynamic instrumentation, so it can't check for
+all possible problems.
+
+Another limitation is that it assumes all code will be executed. A store
+through a null pointer in a basic block which is never reached is harmless, but
+this pass will warn about it anyway.
+
+Optimization passes may make conditions that this pass checks for more or less
+obvious. If an optimization pass appears to be introducing a warning, it may
+be that the optimization pass is merely exposing an existing condition in the
+code.
+
+This code may be run before :ref:`instcombine `. In many
+cases, instcombine checks for the same kinds of things and turns instructions
+with undefined behavior into unreachable (or equivalent). Because of this,
+this pass makes some effort to look through bitcasts and so on.
+
+``-loops``: Natural Loop Information
+------------------------------------
+
+This analysis is used to identify natural loops and determine the loop depth of
+various nodes of the CFG. Note that the loops identified may actually be
+several natural loops that share the same header node... not just a single
+natural loop.
+
+``-memdep``: Memory Dependence Analysis
+---------------------------------------
+
+An analysis that determines, for a given memory operation, what preceding
+memory operations it depends on. It builds on alias analysis information, and
+tries to provide a lazy, caching interface to a common kind of alias
+information query.
+
+``-module-debuginfo``: Decodes module-level debug info
+------------------------------------------------------
+
+This pass decodes the debug info metadata in a module and prints in a
+(sufficiently-prepared-) human-readable form.
+
+For example, run this pass from ``opt`` along with the ``-analyze`` option, and
+it'll print to standard output.
+
+``-no-aa``: No Alias Analysis (always returns 'may' alias)
+----------------------------------------------------------
+
+This is the default implementation of the Alias Analysis interface. It always
+returns "I don't know" for alias queries. NoAA is unlike other alias analysis
+implementations, in that it does not chain to a previous analysis. As such it
+doesn't follow many of the rules that other alias analyses must.
+
+``-no-profile``: No Profile Information
+---------------------------------------
+
+The default "no profile" implementation of the abstract ``ProfileInfo``
+interface.
+
+``-postdomfrontier``: Post-Dominance Frontier Construction
+----------------------------------------------------------
+
+This pass is a simple post-dominator construction algorithm for finding
+post-dominator frontiers.
+
+``-postdomtree``: Post-Dominator Tree Construction
+--------------------------------------------------
+
+This pass is a simple post-dominator construction algorithm for finding
+post-dominators.
+
+``-print-alias-sets``: Alias Set Printer
+----------------------------------------
+
+Yet to be written.
+
+``-print-callgraph``: Print a call graph
+----------------------------------------
+
+This pass, only available in ``opt``, prints the call graph to standard error
+in a human-readable form.
+
+``-print-callgraph-sccs``: Print SCCs of the Call Graph
+-------------------------------------------------------
+
+This pass, only available in ``opt``, prints the SCCs of the call graph to
+standard error in a human-readable form.
+
+``-print-cfg-sccs``: Print SCCs of each function CFG
+----------------------------------------------------
+
+This pass, only available in ``opt``, printsthe SCCs of each function CFG to
+standard error in a human-readable fom.
+
+``-print-dbginfo``: Print debug info in human readable form
+-----------------------------------------------------------
+
+Pass that prints instructions, and associated debug info:
+
+#. source/line/col information
+#. original variable name
+#. original type name
+
+``-print-dom-info``: Dominator Info Printer
+-------------------------------------------
+
+Dominator Info Printer.
+
+``-print-externalfnconstants``: Print external fn callsites passed constants
+----------------------------------------------------------------------------
+
+This pass, only available in ``opt``, prints out call sites to external
+functions that are called with constant arguments. This can be useful when
+looking for standard library functions we should constant fold or handle in
+alias analyses.
+
+``-print-function``: Print function to stderr
+---------------------------------------------
+
+The ``PrintFunctionPass`` class is designed to be pipelined with other
+``FunctionPasses``, and prints out the functions of the module as they are
+processed.
+
+``-print-module``: Print module to stderr
+-----------------------------------------
+
+This pass simply prints out the entire module when it is executed.
+
+.. _passes-print-used-types:
+
+``-print-used-types``: Find Used Types
+--------------------------------------
+
+This pass is used to seek out all of the types in use by the program. Note
+that this analysis explicitly does not include types only used by the symbol
+table.
+
+``-profile-estimator``: Estimate profiling information
+------------------------------------------------------
+
+Profiling information that estimates the profiling information in a very crude
+and unimaginative way.
+
+``-profile-loader``: Load profile information from ``llvmprof.out``
+-------------------------------------------------------------------
+
+A concrete implementation of profiling information that loads the information
+from a profile dump file.
+
+``-profile-verifier``: Verify profiling information
+---------------------------------------------------
+
+Pass that checks profiling information for plausibility.
+
+``-regions``: Detect single entry single exit regions
+-----------------------------------------------------
+
+The ``RegionInfo`` pass detects single entry single exit regions in a function,
+where a region is defined as any subgraph that is connected to the remaining
+graph at only two spots. Furthermore, an hierarchical region tree is built.
+
+``-scalar-evolution``: Scalar Evolution Analysis
+------------------------------------------------
+
+The ``ScalarEvolution`` analysis can be used to analyze and catagorize scalar
+expressions in loops. It specializes in recognizing general induction
+variables, representing them with the abstract and opaque ``SCEV`` class.
+Given this analysis, trip counts of loops and other important properties can be
+obtained.
+
+This analysis is primarily useful for induction variable substitution and
+strength reduction.
+
+``-scev-aa``: ScalarEvolution-based Alias Analysis
+--------------------------------------------------
+
+Simple alias analysis implemented in terms of ``ScalarEvolution`` queries.
+
+This differs from traditional loop dependence analysis in that it tests for
+dependencies within a single iteration of a loop, rather than dependencies
+between different iterations.
+
+``ScalarEvolution`` has a more complete understanding of pointer arithmetic
+than ``BasicAliasAnalysis``' collection of ad-hoc analyses.
+
+``-targetdata``: Target Data Layout
+-----------------------------------
+
+Provides other passes access to information on how the size and alignment
+required by the target ABI for various data types.
+
+Transform Passes
+================
+
+This section describes the LLVM Transform Passes.
+
+``-adce``: Aggressive Dead Code Elimination
+-------------------------------------------
+
+ADCE aggressively tries to eliminate code. This pass is similar to :ref:`DCE
+` but it assumes that values are dead until proven otherwise. This
+is similar to :ref:`SCCP `, except applied to the liveness of
+values.
+
+``-always-inline``: Inliner for ``always_inline`` functions
+-----------------------------------------------------------
+
+A custom inliner that handles only functions that are marked as "always
+inline".
+
+``-argpromotion``: Promote 'by reference' arguments to scalars
+--------------------------------------------------------------
+
+This pass promotes "by reference" arguments to be "by value" arguments. In
+practice, this means looking for internal functions that have pointer
+arguments. If it can prove, through the use of alias analysis, that an
+argument is *only* loaded, then it can pass the value into the function instead
+of the address of the value. This can cause recursive simplification of code
+and lead to the elimination of allocas (especially in C++ template code like
+the STL).
+
+This pass also handles aggregate arguments that are passed into a function,
+scalarizing them if the elements of the aggregate are only loaded. Note that
+it refuses to scalarize aggregates which would require passing in more than
+three operands to the function, because passing thousands of operands for a
+large array or structure is unprofitable!
+
+Note that this transformation could also be done for arguments that are only
+stored to (returning the value instead), but does not currently. This case
+would be best handled when and if LLVM starts supporting multiple return values
+from functions.
+
+``-bb-vectorize``: Basic-Block Vectorization
+--------------------------------------------
+
+This pass combines instructions inside basic blocks to form vector
+instructions. It iterates over each basic block, attempting to pair compatible
+instructions, repeating this process until no additional pairs are selected for
+vectorization. When the outputs of some pair of compatible instructions are
+used as inputs by some other pair of compatible instructions, those pairs are
+part of a potential vectorization chain. Instruction pairs are only fused into
+vector instructions when they are part of a chain longer than some threshold
+length. Moreover, the pass attempts to find the best possible chain for each
+pair of compatible instructions. These heuristics are intended to prevent
+vectorization in cases where it would not yield a performance increase of the
+resulting code.
+
+``-block-placement``: Profile Guided Basic Block Placement
+----------------------------------------------------------
+
+This pass is a very simple profile guided basic block placement algorithm. The
+idea is to put frequently executed blocks together at the start of the function
+and hopefully increase the number of fall-through conditional branches. If
+there is no profile information for a particular function, this pass basically
+orders blocks in depth-first order.
+
+``-break-crit-edges``: Break critical edges in CFG
+--------------------------------------------------
+
+Break all of the critical edges in the CFG by inserting a dummy basic block.
+It may be "required" by passes that cannot deal with critical edges. This
+transformation obviously invalidates the CFG, but can update forward dominator
+(set, immediate dominators, tree, and frontier) information.
+
+``-codegenprepare``: Optimize for code generation
+-------------------------------------------------
+
+This pass munges the code in the input function to better prepare it for
+SelectionDAG-based code generation. This works around limitations in it's
+basic-block-at-a-time approach. It should eventually be removed.
+
+``-constmerge``: Merge Duplicate Global Constants
+-------------------------------------------------
+
+Merges duplicate global constants together into a single constant that is
+shared. This is useful because some passes (i.e., TraceValues) insert a lot of
+string constants into the program, regardless of whether or not an existing
+string is available.
+
+``-constprop``: Simple constant propagation
+-------------------------------------------
+
+This file implements constant propagation and merging. It looks for
+instructions involving only constant operands and replaces them with a constant
+value instead of an instruction. For example:
+
+.. code-block:: llvm
+
+ add i32 1, 2
+
+becomes
+
+.. code-block:: llvm
+
+ i32 3
+
+NOTE: this pass has a habit of making definitions be dead. It is a good idea
+to to run a :ref:`Dead Instruction Elimination ` pass sometime
+after running this pass.
+
+.. _passes-dce:
+
+``-dce``: Dead Code Elimination
+-------------------------------
+
+Dead code elimination is similar to :ref:`dead instruction elimination
+`, but it rechecks instructions that were used by removed
+instructions to see if they are newly dead.
+
+``-deadargelim``: Dead Argument Elimination
+-------------------------------------------
+
+This pass deletes dead arguments from internal functions. Dead argument
+elimination removes arguments which are directly dead, as well as arguments
+only passed into function calls as dead arguments of other functions. This
+pass also deletes dead arguments in a similar way.
+
+This pass is often useful as a cleanup pass to run after aggressive
+interprocedural passes, which add possibly-dead arguments.
+
+``-deadtypeelim``: Dead Type Elimination
+----------------------------------------
+
+This pass is used to cleanup the output of GCC. It eliminate names for types
+that are unused in the entire translation unit, using the :ref:`find used types
+` pass.
+
+.. _passes-die:
+
+``-die``: Dead Instruction Elimination
+--------------------------------------
+
+Dead instruction elimination performs a single pass over the function, removing
+instructions that are obviously dead.
+
+``-dse``: Dead Store Elimination
+--------------------------------
+
+A trivial dead store elimination that only considers basic-block local
+redundant stores.
+
+``-functionattrs``: Deduce function attributes
+----------------------------------------------
+
+A simple interprocedural pass which walks the call-graph, looking for functions
+which do not access or only read non-local memory, and marking them
+``readnone``/``readonly``. In addition, it marks function arguments (of
+pointer type) "``nocapture``" if a call to the function does not create any
+copies of the pointer value that outlive the call. This more or less means
+that the pointer is only dereferenced, and not returned from the function or
+stored in a global. This pass is implemented as a bottom-up traversal of the
+call-graph.
+
+``-globaldce``: Dead Global Elimination
+---------------------------------------
+
+This transform is designed to eliminate unreachable internal globals from the
+program. It uses an aggressive algorithm, searching out globals that are known
+to be alive. After it finds all of the globals which are needed, it deletes
+whatever is left over. This allows it to delete recursive chunks of the
+program which are unreachable.
+
+``-globalopt``: Global Variable Optimizer
+-----------------------------------------
+
+This pass transforms simple global variables that never have their address
+taken. If obviously true, it marks read/write globals as constant, deletes
+variables only stored to, etc.
+
+``-gvn``: Global Value Numbering
+--------------------------------
+
+This pass performs global value numbering to eliminate fully and partially
+redundant instructions. It also performs redundant load elimination.
+
+.. _passes-indvars:
+
+``-indvars``: Canonicalize Induction Variables
+----------------------------------------------
+
+This transformation analyzes and transforms the induction variables (and
+computations derived from them) into simpler forms suitable for subsequent
+analysis and transformation.
+
+This transformation makes the following changes to each loop with an
+identifiable induction variable:
+
+* All loops are transformed to have a *single* canonical induction variable
+ which starts at zero and steps by one.
+* The canonical induction variable is guaranteed to be the first PHI node in
+ the loop header block.
+* Any pointer arithmetic recurrences are raised to use array subscripts.
+
+If the trip count of a loop is computable, this pass also makes the following
+changes:
+
+* The exit condition for the loop is canonicalized to compare the induction
+ value against the exit value. This turns loops like:
+
+ .. code-block:: c++
+
+ for (i = 7; i*i < 1000; ++i)
+
+ into
+
+ .. code-block:: c++
+
+ for (i = 0; i != 25; ++i)
+
+* Any use outside of the loop of an expression derived from the indvar is
+ changed to compute the derived value outside of the loop, eliminating the
+ dependence on the exit value of the induction variable. If the only purpose
+ of the loop is to compute the exit value of some derived expression, this
+ transformation will make the loop dead.
+
+This transformation should be followed by strength reduction after all of the
+desired loop transformations have been performed. Additionally, on targets
+where it is profitable, the loop could be transformed to count down to zero
+(the "do loop" optimization).
+
+``-inline``: Function Integration/Inlining
+------------------------------------------
+
+Bottom-up inlining of functions into callees.
+
+``-insert-edge-profiling``: Insert instrumentation for edge profiling
+---------------------------------------------------------------------
+
+This pass instruments the specified program with counters for edge profiling.
+Edge profiling can give a reasonable approximation of the hot paths through a
+program, and is used for a wide variety of program transformations.
+
+Note that this implementation is very naïve. It inserts a counter for *every*
+edge in the program, instead of using control flow information to prune the
+number of counters inserted.
+
+``-insert-optimal-edge-profiling``: Insert optimal instrumentation for edge profiling
+-------------------------------------------------------------------------------------
+
+This pass instruments the specified program with counters for edge profiling.
+Edge profiling can give a reasonable approximation of the hot paths through a
+program, and is used for a wide variety of program transformations.
+
+.. _passes-instcombine:
+
+``-instcombine``: Combine redundant instructions
+------------------------------------------------
+
+Combine instructions to form fewer, simple instructions. This pass does not
+modify the CFG This pass is where algebraic simplification happens.
+
+This pass combines things like:
+
+.. code-block:: llvm
+
+ %Y = add i32 %X, 1
+ %Z = add i32 %Y, 1
+
+into:
+
+.. code-block:: llvm
+
+ %Z = add i32 %X, 2
+
+This is a simple worklist driven algorithm.
+
+This pass guarantees that the following canonicalizations are performed on the
+program:
+
+#. If a binary operator has a constant operand, it is moved to the right-hand
+ side.
+#. Bitwise operators with constant operands are always grouped so that shifts
+ are performed first, then ``or``\ s, then ``and``\ s, then ``xor``\ s.
+#. Compare instructions are converted from ``<``, ``>``, ``≤``, or ``≥`` to
+ ``=`` or ``≠`` if possible.
+#. All ``cmp`` instructions on boolean values are replaced with logical
+ operations.
+#. ``add X, X`` is represented as ``mul X, 2`` ⇒ ``shl X, 1``
+#. Multiplies with a constant power-of-two argument are transformed into
+ shifts.
+#. … etc.
+
+``-internalize``: Internalize Global Symbols
+--------------------------------------------
+
+This pass loops over all of the functions in the input module, looking for a
+main function. If a main function is found, all other functions and all global
+variables with initializers are marked as internal.
+
+``-ipconstprop``: Interprocedural constant propagation
+------------------------------------------------------
+
+This pass implements an *extremely* simple interprocedural constant propagation
+pass. It could certainly be improved in many different ways, like using a
+worklist. This pass makes arguments dead, but does not remove them. The
+existing dead argument elimination pass should be run after this to clean up
+the mess.
+
+``-ipsccp``: Interprocedural Sparse Conditional Constant Propagation
+--------------------------------------------------------------------
+
+An interprocedural variant of :ref:`Sparse Conditional Constant Propagation
+`.
+
+``-jump-threading``: Jump Threading
+-----------------------------------
+
+Jump threading tries to find distinct threads of control flow running through a
+basic block. This pass looks at blocks that have multiple predecessors and
+multiple successors. If one or more of the predecessors of the block can be
+proven to always cause a jump to one of the successors, we forward the edge
+from the predecessor to the successor by duplicating the contents of this
+block.
+
+An example of when this can occur is code like this:
+
+.. code-block:: c++
+
+ if () { ...
+ X = 4;
+ }
+ if (X < 3) {
+
+In this case, the unconditional branch at the end of the first if can be
+revectored to the false side of the second if.
+
+``-lcssa``: Loop-Closed SSA Form Pass
+-------------------------------------
+
+This pass transforms loops by placing phi nodes at the end of the loops for all
+values that are live across the loop boundary. For example, it turns the left
+into the right code:
+
+.. code-block:: c++
+
+ for (...) for (...)
+ if (c) if (c)
+ X1 = ... X1 = ...
+ else else
+ X2 = ... X2 = ...
+ X3 = phi(X1, X2) X3 = phi(X1, X2)
+ ... = X3 + 4 X4 = phi(X3)
+ ... = X4 + 4
+
+This is still valid LLVM; the extra phi nodes are purely redundant, and will be
+trivially eliminated by ``InstCombine``. The major benefit of this
+transformation is that it makes many other loop optimizations, such as
+``LoopUnswitch``\ ing, simpler.
+
+.. _passes-licm:
+
+``-licm``: Loop Invariant Code Motion
+-------------------------------------
+
+This pass performs loop invariant code motion, attempting to remove as much
+code from the body of a loop as possible. It does this by either hoisting code
+into the preheader block, or by sinking code to the exit blocks if it is safe.
+This pass also promotes must-aliased memory locations in the loop to live in
+registers, thus hoisting and sinking "invariant" loads and stores.
+
+This pass uses alias analysis for two purposes:
+
+#. Moving loop invariant loads and calls out of loops. If we can determine
+ that a load or call inside of a loop never aliases anything stored to, we
+ can hoist it or sink it like any other instruction.
+
+#. Scalar Promotion of Memory. If there is a store instruction inside of the
+ loop, we try to move the store to happen AFTER the loop instead of inside of
+ the loop. This can only happen if a few conditions are true:
+
+ #. The pointer stored through is loop invariant.
+ #. There are no stores or loads in the loop which *may* alias the pointer.
+ There are no calls in the loop which mod/ref the pointer.
+
+ If these conditions are true, we can promote the loads and stores in the
+ loop of the pointer to use a temporary alloca'd variable. We then use the
+ :ref:`mem2reg ` functionality to construct the appropriate
+ SSA form for the variable.
+
+``-loop-deletion``: Delete dead loops
+-------------------------------------
+
+This file implements the Dead Loop Deletion Pass. This pass is responsible for
+eliminating loops with non-infinite computable trip counts that have no side
+effects or volatile instructions, and do not contribute to the computation of
+the function's return value.
+
+.. _passes-loop-extract:
+
+``-loop-extract``: Extract loops into new functions
+---------------------------------------------------
+
+A pass wrapper around the ``ExtractLoop()`` scalar transformation to extract
+each top-level loop into its own new function. If the loop is the *only* loop
+in a given function, it is not touched. This is a pass most useful for
+debugging via bugpoint.
+
+``-loop-extract-single``: Extract at most one loop into a new function
+----------------------------------------------------------------------
+
+Similar to :ref:`Extract loops into new functions `, this
+pass extracts one natural loop from the program into a function if it can.
+This is used by :program:`bugpoint`.
+
+``-loop-reduce``: Loop Strength Reduction
+-----------------------------------------
+
+This pass performs a strength reduction on array references inside loops that
+have as one or more of their components the loop induction variable. This is
+accomplished by creating a new value to hold the initial value of the array
+access for the first iteration, and then creating a new GEP instruction in the
+loop to increment the value by the appropriate amount.
+
+``-loop-rotate``: Rotate Loops
+------------------------------
+
+A simple loop rotation transformation.
+
+``-loop-simplify``: Canonicalize natural loops
+----------------------------------------------
+
+This pass performs several transformations to transform natural loops into a
+simpler form, which makes subsequent analyses and transformations simpler and
+more effective.
+
+Loop pre-header insertion guarantees that there is a single, non-critical entry
+edge from outside of the loop to the loop header. This simplifies a number of
+analyses and transformations, such as :ref:`LICM `.
+
+Loop exit-block insertion guarantees that all exit blocks from the loop (blocks
+which are outside of the loop that have predecessors inside of the loop) only
+have predecessors from inside of the loop (and are thus dominated by the loop
+header). This simplifies transformations such as store-sinking that are built
+into LICM.
+
+This pass also guarantees that loops will have exactly one backedge.
+
+Note that the :ref:`simplifycfg ` pass will clean up blocks
+which are split out but end up being unnecessary, so usage of this pass should
+not pessimize generated code.
+
+This pass obviously modifies the CFG, but updates loop information and
+dominator information.
+
+``-loop-unroll``: Unroll loops
+------------------------------
+
+This pass implements a simple loop unroller. It works best when loops have
+been canonicalized by the :ref:`indvars ` pass, allowing it to
+determine the trip counts of loops easily.
+
+``-loop-unswitch``: Unswitch loops
+----------------------------------
+
+This pass transforms loops that contain branches on loop-invariant conditions
+to have multiple loops. For example, it turns the left into the right code:
+
+.. code-block:: c++
+
+ for (...) if (lic)
+ A for (...)
+ if (lic) A; B; C
+ B else
+ C for (...)
+ A; C
+
+This can increase the size of the code exponentially (doubling it every time a
+loop is unswitched) so we only unswitch if the resultant code will be smaller
+than a threshold.
+
+This pass expects :ref:`LICM ` to be run before it to hoist
+invariant conditions out of the loop, to make the unswitching opportunity
+obvious.
+
+``-loweratomic``: Lower atomic intrinsics to non-atomic form
+------------------------------------------------------------
+
+This pass lowers atomic intrinsics to non-atomic form for use in a known
+non-preemptible environment.
+
+The pass does not verify that the environment is non-preemptible (in general
+this would require knowledge of the entire call graph of the program including
+any libraries which may not be available in bitcode form); it simply lowers
+every atomic intrinsic.
+
+``-lowerinvoke``: Lower invoke and unwind, for unwindless code generators
+-------------------------------------------------------------------------
+
+This transformation is designed for use by code generators which do not yet
+support stack unwinding. This pass supports two models of exception handling
+lowering, the "cheap" support and the "expensive" support.
+
+"Cheap" exception handling support gives the program the ability to execute any
+program which does not "throw an exception", by turning "``invoke``"
+instructions into calls and by turning "``unwind``" instructions into calls to
+``abort()``. If the program does dynamically use the "``unwind``" instruction,
+the program will print a message then abort.
+
+"Expensive" exception handling support gives the full exception handling
+support to the program at the cost of making the "``invoke``" instruction
+really expensive. It basically inserts ``setjmp``/``longjmp`` calls to emulate
+the exception handling as necessary.
+
+Because the "expensive" support slows down programs a lot, and EH is only used
+for a subset of the programs, it must be specifically enabled by the
+``-enable-correct-eh-support`` option.
+
+Note that after this pass runs the CFG is not entirely accurate (exceptional
+control flow edges are not correct anymore) so only very simple things should
+be done after the ``lowerinvoke`` pass has run (like generation of native
+code). This should not be used as a general purpose "my LLVM-to-LLVM pass
+doesn't support the ``invoke`` instruction yet" lowering pass.
+
+``-lowerswitch``: Lower ``SwitchInst``\ s to branches
+-----------------------------------------------------
+
+Rewrites switch instructions with a sequence of branches, which allows targets
+to get away with not implementing the switch instruction until it is
+convenient.
+
+.. _passes-mem2reg:
+
+``-mem2reg``: Promote Memory to Register
+----------------------------------------
+
+This file promotes memory references to be register references. It promotes
+alloca instructions which only have loads and stores as uses. An ``alloca`` is
+transformed by using dominator frontiers to place phi nodes, then traversing
+the function in depth-first order to rewrite loads and stores as appropriate.
+This is just the standard SSA construction algorithm to construct "pruned" SSA
+form.
+
+``-memcpyopt``: MemCpy Optimization
+-----------------------------------
+
+This pass performs various transformations related to eliminating ``memcpy``
+calls, or transforming sets of stores into ``memset``\ s.
+
+``-mergefunc``: Merge Functions
+-------------------------------
+
+This pass looks for equivalent functions that are mergable and folds them.
+
+A hash is computed from the function, based on its type and number of basic
+blocks.
+
+Once all hashes are computed, we perform an expensive equality comparison on
+each function pair. This takes n^2/2 comparisons per bucket, so it's important
+that the hash function be high quality. The equality comparison iterates
+through each instruction in each basic block.
+
+When a match is found the functions are folded. If both functions are
+overridable, we move the functionality into a new internal function and leave
+two overridable thunks to it.
+
+``-mergereturn``: Unify function exit nodes
+-------------------------------------------
+
+Ensure that functions have at most one ``ret`` instruction in them.
+Additionally, it keeps track of which node is the new exit node of the CFG.
+
+``-partial-inliner``: Partial Inliner
+-------------------------------------
+
+This pass performs partial inlining, typically by inlining an ``if`` statement
+that surrounds the body of the function.
+
+``-prune-eh``: Remove unused exception handling info
+----------------------------------------------------
+
+This file implements a simple interprocedural pass which walks the call-graph,
+turning invoke instructions into call instructions if and only if the callee
+cannot throw an exception. It implements this as a bottom-up traversal of the
+call-graph.
+
+``-reassociate``: Reassociate expressions
+-----------------------------------------
+
+This pass reassociates commutative expressions in an order that is designed to
+promote better constant propagation, GCSE, :ref:`LICM `, PRE, etc.
+
+For example: 4 + (x + 5) ⇒ x + (4 + 5)
+
+In the implementation of this algorithm, constants are assigned rank = 0,
+function arguments are rank = 1, and other values are assigned ranks
+corresponding to the reverse post order traversal of current function (starting
+at 2), which effectively gives values in deep loops higher rank than values not
+in loops.
+
+``-reg2mem``: Demote all values to stack slots
+----------------------------------------------
+
+This file demotes all registers to memory references. It is intended to be the
+inverse of :ref:`mem2reg `. By converting to ``load``
+instructions, the only values live across basic blocks are ``alloca``
+instructions and ``load`` instructions before ``phi`` nodes. It is intended
+that this should make CFG hacking much easier. To make later hacking easier,
+the entry block is split into two, such that all introduced ``alloca``
+instructions (and nothing else) are in the entry block.
+
+``-scalarrepl``: Scalar Replacement of Aggregates (DT)
+------------------------------------------------------
+
+The well-known scalar replacement of aggregates transformation. This transform
+breaks up ``alloca`` instructions of aggregate type (structure or array) into
+individual ``alloca`` instructions for each member if possible. Then, if
+possible, it transforms the individual ``alloca`` instructions into nice clean
+scalar SSA form.
+
+This combines a simple scalar replacement of aggregates algorithm with the
+:ref:`mem2reg ` algorithm because often interact, especially
+for C++ programs. As such, iterating between ``scalarrepl``, then
+:ref:`mem2reg ` until we run out of things to promote works
+well.
+
+.. _passes-sccp:
+
+``-sccp``: Sparse Conditional Constant Propagation
+--------------------------------------------------
+
+Sparse conditional constant propagation and merging, which can be summarized
+as:
+
+* Assumes values are constant unless proven otherwise
+* Assumes BasicBlocks are dead unless proven otherwise
+* Proves values to be constant, and replaces them with constants
+* Proves conditional branches to be unconditional
+
+Note that this pass has a habit of making definitions be dead. It is a good
+idea to to run a :ref:`DCE ` pass sometime after running this pass.
+
+``-simplify-libcalls``: Simplify well-known library calls
+---------------------------------------------------------
+
+Applies a variety of small optimizations for calls to specific well-known
+function calls (e.g. runtime library functions). For example, a call
+``exit(3)`` that occurs within the ``main()`` function can be transformed into
+simply ``return 3``.
+
+.. _passes-simplifycfg:
+
+``-simplifycfg``: Simplify the CFG
+----------------------------------
+
+Performs dead code elimination and basic block merging. Specifically:
+
+* Removes basic blocks with no predecessors.
+* Merges a basic block into its predecessor if there is only one and the
+ predecessor only has one successor.
+* Eliminates PHI nodes for basic blocks with a single predecessor.
+* Eliminates a basic block that only contains an unconditional branch.
+
+``-sink``: Code sinking
+-----------------------
+
+This pass moves instructions into successor blocks, when possible, so that they
+aren't executed on paths where their results aren't needed.
+
+``-strip``: Strip all symbols from a module
+-------------------------------------------
+
+Performs code stripping. This transformation can delete:
+
+* names for virtual registers
+* symbols for internal globals and functions
+* debug information
+
+Note that this transformation makes code much less readable, so it should only
+be used in situations where the strip utility would be used, such as reducing
+code size or making it harder to reverse engineer code.
+
+``-strip-dead-debug-info``: Strip debug info for unused symbols
+---------------------------------------------------------------
+
+.. FIXME: this description is the same as for -strip
+
+performs code stripping. this transformation can delete:
+
+* names for virtual registers
+* symbols for internal globals and functions
+* debug information
+
+note that this transformation makes code much less readable, so it should only
+be used in situations where the strip utility would be used, such as reducing
+code size or making it harder to reverse engineer code.
+
+``-strip-dead-prototypes``: Strip Unused Function Prototypes
+------------------------------------------------------------
+
+This pass loops over all of the functions in the input module, looking for dead
+declarations and removes them. Dead declarations are declarations of functions
+for which no implementation is available (i.e., declarations for unused library
+functions).
+
+``-strip-debug-declare``: Strip all ``llvm.dbg.declare`` intrinsics
+-------------------------------------------------------------------
+
+.. FIXME: this description is the same as for -strip
+
+This pass implements code stripping. Specifically, it can delete:
+
+#. names for virtual registers
+#. symbols for internal globals and functions
+#. debug information
+
+Note that this transformation makes code much less readable, so it should only
+be used in situations where the 'strip' utility would be used, such as reducing
+code size or making it harder to reverse engineer code.
+
+``-strip-nondebug``: Strip all symbols, except dbg symbols, from a module
+-------------------------------------------------------------------------
+
+.. FIXME: this description is the same as for -strip
+
+This pass implements code stripping. Specifically, it can delete:
+
+#. names for virtual registers
+#. symbols for internal globals and functions
+#. debug information
+
+Note that this transformation makes code much less readable, so it should only
+be used in situations where the 'strip' utility would be used, such as reducing
+code size or making it harder to reverse engineer code.
+
+``-tailcallelim``: Tail Call Elimination
+----------------------------------------
+
+This file transforms calls of the current function (self recursion) followed by
+a return instruction with a branch to the entry of the function, creating a
+loop. This pass also implements the following extensions to the basic
+algorithm:
+
+#. Trivial instructions between the call and return do not prevent the
+ transformation from taking place, though currently the analysis cannot
+ support moving any really useful instructions (only dead ones).
+#. This pass transforms functions that are prevented from being tail recursive
+ by an associative expression to use an accumulator variable, thus compiling
+ the typical naive factorial or fib implementation into efficient code.
+#. TRE is performed if the function returns void, if the return returns the
+ result returned by the call, or if the function returns a run-time constant
+ on all exits from the function. It is possible, though unlikely, that the
+ return returns something else (like constant 0), and can still be TRE'd. It
+ can be TRE'd if *all other* return instructions in the function return the
+ exact same value.
+#. If it can prove that callees do not access theier caller stack frame, they
+ are marked as eligible for tail call elimination (by the code generator).
+
+Utility Passes
+==============
+
+This section describes the LLVM Utility Passes.
+
+``-deadarghaX0r``: Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)
+------------------------------------------------------------------------
+
+Same as dead argument elimination, but deletes arguments to functions which are
+external. This is only for use by :doc:`bugpoint `.
+
+``-extract-blocks``: Extract Basic Blocks From Module (for bugpoint use)
+------------------------------------------------------------------------
+
+This pass is used by bugpoint to extract all blocks from the module into their
+own functions.
+
+``-instnamer``: Assign names to anonymous instructions
+------------------------------------------------------
+
+This is a little utility pass that gives instructions names, this is mostly
+useful when diffing the effect of an optimization because deleting an unnamed
+instruction can change all other instruction numbering, making the diff very
+noisy.
+
+``-preverify``: Preliminary module verification
+-----------------------------------------------
+
+Ensures that the module is in the form required by the :ref:`Module Verifier
+` pass. Running the verifier runs this pass automatically, so
+there should be no need to use it directly.
+
+.. _passes-verify:
+
+``-verify``: Module Verifier
+----------------------------
+
+Verifies an LLVM IR code. This is useful to run after an optimization which is
+undergoing testing. Note that llvm-as verifies its input before emitting
+bitcode, and also that malformed bitcode is likely to make LLVM crash. All
+language front-ends are therefore encouraged to verify their output before
+performing optimizing transformations.
+
+#. Both of a binary operator's parameters are of the same type.
+#. Verify that the indices of mem access instructions match other operands.
+#. Verify that arithmetic and other things are only performed on first-class
+ types. Verify that shifts and logicals only happen on integrals f.e.
+#. All of the constants in a switch statement are of the correct type.
+#. The code is in valid SSA form.
+#. It is illegal to put a label into any other type (like a structure) or to
+ return one.
+#. Only phi nodes can be self referential: ``%x = add i32 %x``, ``%x`` is
+ invalid.
+#. PHI nodes must have an entry for each predecessor, with no extras.
+#. PHI nodes must be the first thing in a basic block, all grouped together.
+#. PHI nodes must have at least one entry.
+#. All basic blocks should only end with terminator insts, not contain them.
+#. The entry node to a function must not have predecessors.
+#. All Instructions must be embedded into a basic block.
+#. Functions cannot take a void-typed parameter.
+#. Verify that a function's argument list agrees with its declared type.
+#. It is illegal to specify a name for a void value.
+#. It is illegal to have an internal global value with no initializer.
+#. It is illegal to have a ``ret`` instruction that returns a value that does
+ not agree with the function return value type.
+#. Function call argument types match the function prototype.
+#. All other things that are tested by asserts spread about the code.
+
+Note that this does not provide full security verification (like Java), but
+instead just tries to ensure that code is well-formed.
+
+``-view-cfg``: View CFG of function
+-----------------------------------
+
+Displays the control flow graph using the GraphViz tool.
+
+``-view-cfg-only``: View CFG of function (with no function bodies)
+------------------------------------------------------------------
+
+Displays the control flow graph using the GraphViz tool, but omitting function
+bodies.
+
+``-view-dom``: View dominance tree of function
+----------------------------------------------
+
+Displays the dominator tree using the GraphViz tool.
+
+``-view-dom-only``: View dominance tree of function (with no function bodies)
+-----------------------------------------------------------------------------
+
+Displays the dominator tree using the GraphViz tool, but omitting function
+bodies.
+
+``-view-postdom``: View postdominance tree of function
+------------------------------------------------------
+
+Displays the post dominator tree using the GraphViz tool.
+
+``-view-postdom-only``: View postdominance tree of function (with no function bodies)
+-------------------------------------------------------------------------------------
+
+Displays the post dominator tree using the GraphViz tool, but omitting function
+bodies.
+