LLVM's Analysis and Transform Passes

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 ome utility but don't otherwise fit categorization. For example passes to extract functions to bytecode or write a module to bytecode are neither analysis nor transform passes.

The table below provides a quick summary of each pass and links to the more complete pass description later in the document.

ANALYSIS PASSES
Option	Name	Directory
-aa-eval	Exhaustive Alias Analysis Precision Evaluator
-anders-aa	Andersen's Interprocedural Alias Analysis
-basicaa	Basic Alias Analysis (default AA impl)
-basiccg	Basic CallGraph Construction
-basicvn	Basic Value Numbering (default GVN impl)
-callgraph	Print a call graph
-callscc	Print SCCs of the Call Graph
-cfgscc	Print SCCs of each function CFG
-count-aa	Count Alias Analysis Query Responses
-debug-aa	AA use debugger
-domfrontier	Dominance Frontier Construction
-domset	Dominator Set Construction
-domtree	Dominator Tree Construction
-etforest	ET Forest Construction
-externalfnconstants	Print external fn callsites passed constants
-globalsmodref-aa	Simple mod/ref analysis for globals
-idom	Immediate Dominators Construction
-instcount	Counts the various types of Instructions
-intervals	Interval Partition Construction
-load-vn	Load Value Numbering
-loops	Natural Loop Construction
-no-aa	No Alias Analysis (always returns 'may' alias)
-no-profile	No Profile Information
-postdomfrontier	Post-Dominance Frontier Construction
-postdomset	Post-Dominator Set Construction
-postdomtree	Post-Dominator Tree Construction
-postetforest	Post-ET-Forest Construction
-postidom	Immediate Post-Dominators Construction
-print	Print function to stderr
-print-alias-sets	Alias Set Printer
-print-callgraph	Print Call Graph to 'dot' file
-print-cfg	Print CFG of function to 'dot' file
-print-cfg-only	Print CFG of function to 'dot' file (with no function bodies)
-printm	Print module to stderr
-printusedtypes	Find Used Types
-profile-loader	Load profile information from llvmprof.out
-scalar-evolution	Scalar Evolution Analysis
-targetdata	Target Data Layout
TRANSFORM PASSES
Option	Name	Directory
-adce	Aggressive Dead Code Elimination
-argpromotion	Promote 'by reference' arguments to scalars
-block-placement	Profile Guided Basic Block Placement
-break-crit-edges	Break Critical Edges in CFG
-cee	Correlated Expression Elimination
-condprop	Conditional Propagation
-constmerge	Merge Duplicate Global Constants
-constprop	Simple constant propagation
-dce	Dead Code Elimination
-deadargelim	Dead Argument Elimination
-deadtypeelim	Dead Type Elimination
-die	Dead Instruction Elimination
-dse	Dead Store Elimination
-gcse	Global Common Subexpression Elimination
-globaldce	Dead Global Elimination
-globalopt	Global Variable Optimizer
-indmemrem	Indirect Malloc and Free Removal
-indvars	Canonicalize Induction Variables
-inline	Function Integration/Inlining
-insert-block-profiling	Insert instrumentation for block profiling
-insert-edge-profiling	Insert instrumentation for edge profiling
-insert-function-profiling	Insert instrumentation for function profiling
-insert-null-profiling-rs	Measure profiling framework overhead
-insert-rs-profiling-framework	Insert random sampling instrumentation framework
-instcombine	Combine redundant instructions
-internalize	Internalize Global Symbols
-ipconstprop	Interprocedural constant propagation
-ipsccp	Interprocedural Sparse Conditional Constant Propagation
-lcssa	Loop-Closed SSA Form Pass
-licm	Loop Invariant Code Motion
-loop-extract	Extract loops into new functions
-loop-extract-single	Extract at most one loop into a new function
-loop-reduce	Loop Strength Reduction
-loop-unroll	Unroll Loops
-loop-unswitch	Unswitch Loops
-loopsimplify	Canonicalize Natural Loops
-lower-packed	Lower Packed Operations
-lowerallocs	Lower allocations from instructions to calls
-lowergc	Lower GC intrinsics, for GCless code generators
-lowerinvoke	Lower Invoke and Unwind
-lowerselect	Lower Selects To Branches
-lowersetjmp	Lower Set Jump
-lowerswitch	Lower SwitchInst's to branches
-mem2reg	Promote Memory to Register
-mergereturn	Unify Function Exit Nodes
-predsimplify	Predicate Simplifier
-prune-eh	Remove unused exception handling info
-raiseallocs	Raise allocations from calls to instructions
-reassociate	Reassociate Expressions
-reg2mem	Demote Values to Memory
-scalarrepl	Scalar Replacement of Aggregates
-sccp	Sparse Conditional Constant Propagation
-simplify-libcalls	Simplify well-known library calls
-simplifycfg	Simplify the CFG
-strip	Strip all symbols from a module
-tailcallelim	Tail Call Elimination
-tailduplicate	Tail Duplication
UTILITY PASSES
Option	Name	Directory
-deadarghaX0r	Dead Argument Hacking (BUGPOINT ONLY)
-extract-blocks	Extract Basic Blocks From Module (BUGPOINT ONLY)
-emitbytecode	Bytecode Writer
-verify	Module Verifier

This section describes the LLVM Analysis Passes.

Yet to be written.

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.

Yet to be written.

This pass implements 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.

The algorithm implemented here is basically "Algo1" from "Profile Guided Code Positioning" by Pettis and Hansen, except that it uses basic block counts instead of edge counts. This could be improved in many ways, but is very simple for now.

Basically we "place" the entry block, then loop over all successors in a DFO, placing the most frequently executed successor until we run out of blocks. Did we mention that this was extremely simplistic? This is also much slower than it could be. When it becomes important, this pass will be rewritten to use a better algorithm, and then we can worry about efficiency.

Yet to be written.

Correlated Expression Elimination propagates information from conditional branches to blocks dominated by destinations of the branch. It propagates information from the condition check itself into the body of the branch, allowing transformations like these for example:

    if (i == 7)
      ... 4*i;  // constant propagation

    M = i+1; N = j+1;
    if (i == j)
      X = M-N;  // = M-M == 0;

This is called Correlated Expression Elimination because we eliminate or simplify expressions that are correlated with the direction of a branch. In this way we use static information to give us some information about the dynamic value of a variable.

This pass propagates information about conditional expressions through the program, allowing it to eliminate conditional branches in some cases.

Yet to be written.

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.

Yet to be written.

This section describes the LLVM Utility Passes.

Yet to be written.