This document is a reference manual for the LLVM assembly language. LLVM is an SSA based representation that provides type safety, low-level operations, flexibility, and the capability of representing 'all' high-level languages cleanly. It is the common code representation used throughout all phases of the LLVM compilation strategy.

The LLVM code representation is designed to be used in three different forms: as an in-memory compiler IR, as an on-disk bytecode representation (suitable for fast loading by a Just-In-Time compiler), and as a human readable assembly language representation. This allows LLVM to provide a powerful intermediate representation for efficient compiler transformations and analysis, while providing a natural means to debug and visualize the transformations. The three different forms of LLVM are all equivalent. This document describes the human readable representation and notation.

The LLVM representation aims to be light-weight and low-level while being expressive, typed, and extensible at the same time. It aims to be a "universal IR" of sorts, by being at a low enough level that high-level ideas may be cleanly mapped to it (similar to how microprocessors are "universal IR's", allowing many source languages to be mapped to them). By providing type information, LLVM can be used as the target of optimizations: for example, through pointer analysis, it can be proven that a C automatic variable is never accessed outside of the current function... allowing it to be promoted to a simple SSA value instead of a memory location.

It is important to note that this document describes 'well formed' LLVM assembly language. There is a difference between what the parser accepts and what is considered 'well formed'. For example, the following instruction is syntactically okay, but not well formed:

  %x = add int 1, %x

...because the definition of %x does not dominate all of its uses. The LLVM infrastructure provides a verification pass that may be used to verify that an LLVM module is well formed. This pass is automatically run by the parser after parsing input assembly and by the optimizer before it outputs bytecode. The violations pointed out by the verifier pass indicate bugs in transformation passes or input to the parser.

LLVM uses three different forms of identifiers, for different purposes:

1. Named values are represented as a string of characters with a '%' prefix. For example, %foo, %DivisionByZero, %a.really.long.identifier. The actual regular expression used is '%[a-zA-Z$._][a-zA-Z$._0-9]*'. Identifiers which require other characters in their names can be surrounded with quotes. In this way, anything except a " character can be used in a name.
2. Unnamed values are represented as an unsigned numeric value with a '%' prefix. For example, %12, %2, %44.
3. Constants, which are described in a section about constants, below.

LLVM requires that values start with a '%' sign for two reasons: Compilers don't need to worry about name clashes with reserved words, and the set of reserved words may be expanded in the future without penalty. Additionally, unnamed identifiers allow a compiler to quickly come up with a temporary variable without having to avoid symbol table conflicts.

Reserved words in LLVM are very similar to reserved words in other languages. There are keywords for different opcodes ('add', 'cast', 'ret', etc...), for primitive type names ('void', 'uint', etc...), and others. These reserved words cannot conflict with variable names, because none of them start with a '%' character.

Here is an example of LLVM code to multiply the integer variable '%X' by 8:

The easy way:

  %result = mul uint %X, 8

After strength reduction:

  %result = shl uint %X, ubyte 3

And the hard way:

  add uint %X, %X           ; yields {uint}:%0
  add uint %0, %0           ; yields {uint}:%1
  %result = add uint %1, %1

This last way of multiplying %X by 8 illustrates several important lexical features of LLVM:

1. Comments are delimited with a ';' and go until the end of line.
2. Unnamed temporaries are created when the result of a computation is not assigned to a named value.
3. Unnamed temporaries are numbered sequentially

...and it also shows a convention that we follow in this document. When demonstrating instructions, we will follow an instruction with a comment that defines the type and name of value produced. Comments are shown in italic text.

LLVM programs are composed of "Module"s, each of which is a translation unit of the input programs. Each module consists of functions, global variables, and symbol table entries. Modules may be combined together with the LLVM linker, which merges function (and global variable) definitions, resolves forward declarations, and merges symbol table entries. Here is an example of the "hello world" module:

; Declare the string constant as a global constant...
%.LC0 = internal constant [13 x sbyte] c"hello world\0A\00"          ; [13 x sbyte]*

; External declaration of the puts function
declare int %puts(sbyte*)                                            ; int(sbyte*)* 

; Global variable / Function body section separator
implementation

; Definition of main function
int %main() {                                                        ; int()* 
        ; Convert [13x sbyte]* to sbyte *...
        %cast210 = getelementptr [13 x sbyte]* %.LC0, long 0, long 0 ; sbyte*

        ; Call puts function to write out the string to stdout...
        call int %puts(sbyte* %cast210)                              ; int
        ret int 0
}

This example is made up of a global variable named ".LC0", an external declaration of the "puts" function, and a function definition for "main".

In general, a module is made up of a list of global values, where both functions and global variables are global values. Global values are represented by a pointer to a memory location (in this case, a pointer to an array of char, and a pointer to a function), and have one of the following linkage types.

Due to a limitation in the current LLVM assembly parser (it is limited by one-token lookahead), modules are split into two pieces by the "implementation" keyword. Global variable prototypes and definitions must occur before the keyword, and function definitions must occur after it. Function prototypes may occur either before or after it. In the future, the implementation keyword may become a noop, if the parser gets smarter.

All Global Variables and Functions have one of the following types of linkage:

internal

Global values with internal linkage are only directly accessible by objects in the current module. In particular, linking code into a module with an internal global value may cause the internal to be renamed as necessary to avoid collisions. Because the symbol is internal to the module, all references can be updated. This corresponds to the notion of the 'static' keyword in C, or the idea of "anonymous namespaces" in C++.

linkonce:

"linkonce" linkage is similar to internal linkage, with the twist that linking together two modules defining the same linkonce globals will cause one of the globals to be discarded. This is typically used to implement inline functions. Unreferenced linkonce globals are allowed to be discarded.

weak:

"weak" linkage is exactly the same as linkonce linkage, except that unreferenced weak globals may not be discarded. This is used to implement constructs in C such as "int X;" at global scope.

appending:

"appending" linkage may only be applied to global variables of pointer to array type. When two global variables with appending linkage are linked together, the two global arrays are appended together. This is the LLVM, typesafe, equivalent of having the system linker append together "sections" with identical names when .o files are linked.

externally visible:

If none of the above identifiers are used, the global is externally visible, meaning that it participates in linkage and can be used to resolve external symbol references.

extern_weak:

"extern_weak" TBD

The next two types of linkage are targeted for Microsoft Windows platform only. They are designed to support importing (exporting) symbols from (to) DLLs.

dllimport:

"dllimport" linkage causes the compiler to reference a function or variable via a global pointer to a pointer that is set up by the DLL exporting the symbol. On Microsoft Windows targets, the pointer name is formed by combining _imp__ and the function or variable name.

dllexport:

"dllexport" linkage causes the compiler to provide a global pointer to a pointer in a DLL, so that it can be referenced with the dllimport attribute. On Microsoft Windows targets, the pointer name is formed by combining _imp__ and the function or variable name.

For example, since the ".LC0" variable is defined to be internal, if another module defined a ".LC0" variable and was linked with this one, one of the two would be renamed, preventing a collision. Since "main" and "puts" are external (i.e., lacking any linkage declarations), they are accessible outside of the current module. It is illegal for a function declaration to have any linkage type other than "externally visible".

LLVM functions, calls and invokes can all have an optional calling convention specified for the call. The calling convention of any pair of dynamic caller/callee must match, or the behavior of the program is undefined. The following calling conventions are supported by LLVM, and more may be added in the future:

"ccc" - The C calling convention:

This calling convention (the default if no other calling convention is specified) matches the target C calling conventions. This calling convention supports varargs function calls and tolerates some mismatch in the declared prototype and implemented declaration of the function (as does normal C).

"csretcc" - The C struct return calling convention:

This calling convention matches the target C calling conventions, except that functions with this convention are required to take a pointer as their first argument, and the return type of the function must be void. This is used for C functions that return aggregates by-value. In this case, the function has been transformed to take a pointer to the struct as the first argument to the function. For targets where the ABI specifies specific behavior for structure-return calls, the calling convention can be used to distinguish between struct return functions and other functions that take a pointer to a struct as the first argument.

"fastcc" - The fast calling convention:

This calling convention attempts to make calls as fast as possible (e.g. by passing things in registers). This calling convention allows the target to use whatever tricks it wants to produce fast code for the target, without having to conform to an externally specified ABI. Implementations of this convention should allow arbitrary tail call optimization to be supported. This calling convention does not support varargs and requires the prototype of all callees to exactly match the prototype of the function definition.

"coldcc" - The cold calling convention:

This calling convention attempts to make code in the caller as efficient as possible under the assumption that the call is not commonly executed. As such, these calls often preserve all registers so that the call does not break any live ranges in the caller side. This calling convention does not support varargs and requires the prototype of all callees to exactly match the prototype of the function definition.

"cc <n>" - Numbered convention:

Any calling convention may be specified by number, allowing target-specific calling conventions to be used. Target specific calling conventions start at 64.

More calling conventions can be added/defined on an as-needed basis, to support pascal conventions or any other well-known target-independent convention.

Global variables define regions of memory allocated at compilation time instead of run-time. Global variables may optionally be initialized, may have an explicit section to be placed in, and may have an optional explicit alignment specified. A variable may be defined as a global "constant," which indicates that the contents of the variable will never be modified (enabling better optimization, allowing the global data to be placed in the read-only section of an executable, etc). Note that variables that need runtime initialization cannot be marked "constant" as there is a store to the variable.

LLVM explicitly allows declarations of global variables to be marked constant, even if the final definition of the global is not. This capability can be used to enable slightly better optimization of the program, but requires the language definition to guarantee that optimizations based on the 'constantness' are valid for the translation units that do not include the definition.

As SSA values, global variables define pointer values that are in scope (i.e. they dominate) all basic blocks in the program. Global variables always define a pointer to their "content" type because they describe a region of memory, and all memory objects in LLVM are accessed through pointers.

LLVM allows an explicit section to be specified for globals. If the target supports it, it will emit globals to the section specified.

An explicit alignment may be specified for a global. If not present, or if the alignment is set to zero, the alignment of the global is set by the target to whatever it feels convenient. If an explicit alignment is specified, the global is forced to have at least that much alignment. All alignments must be a power of 2.

LLVM function definitions consist of an optional linkage type, an optional calling convention, a return type, a function name, a (possibly empty) argument list, an optional section, an optional alignment, an opening curly brace, a list of basic blocks, and a closing curly brace. LLVM function declarations are defined with the "declare" keyword, an optional calling convention, a return type, a function name, a possibly empty list of arguments, and an optional alignment.

A function definition contains a list of basic blocks, forming the CFG for the function. Each basic block may optionally start with a label (giving the basic block a symbol table entry), contains a list of instructions, and ends with a terminator instruction (such as a branch or function return).

The first basic block in a program is special in two ways: it is immediately executed on entrance to the function, and it is not allowed to have predecessor basic blocks (i.e. there can not be any branches to the entry block of a function). Because the block can have no predecessors, it also cannot have any PHI nodes.

LLVM functions are identified by their name and type signature. Hence, two functions with the same name but different parameter lists or return values are considered different functions, and LLVM will resolve references to each appropriately.

LLVM allows an explicit section to be specified for functions. If the target supports it, it will emit functions to the section specified.

An explicit alignment may be specified for a function. If not present, or if the alignment is set to zero, the alignment of the function is set by the target to whatever it feels convenient. If an explicit alignment is specified, the function is forced to have at least that much alignment. All alignments must be a power of 2.

Modules may contain "module-level inline asm" blocks, which corresponds to the GCC "file scope inline asm" blocks. These blocks are internally concatenated by LLVM and treated as a single unit, but may be separated in the .ll file if desired. The syntax is very simple:

  module asm "inline asm code goes here"
  module asm "more can go here"

The strings can contain any character by escaping non-printable characters. The escape sequence used is simply "\xx" where "xx" is the two digit hex code for the number.

The inline asm code is simply printed to the machine code .s file when assembly code is generated.

The LLVM type system is one of the most important features of the intermediate representation. Being typed enables a number of optimizations to be performed on the IR directly, without having to do extra analyses on the side before the transformation. A strong type system makes it easier to read the generated code and enables novel analyses and transformations that are not feasible to perform on normal three address code representations.

The primitive types are the fundamental building blocks of the LLVM system. The current set of primitive types is as follows:

These different primitive types fall into a few useful classifications:

The first class types are perhaps the most important. Values of these types are the only ones which can be produced by instructions, passed as arguments, or used as operands to instructions. This means that all structures and arrays must be manipulated either by pointer or by component.

The real power in LLVM comes from the derived types in the system. This is what allows a programmer to represent arrays, functions, pointers, and other useful types. Note that these derived types may be recursive: For example, it is possible to have a two dimensional array.

Overview:

The array type is a very simple derived type that arranges elements sequentially in memory. The array type requires a size (number of elements) and an underlying data type.

Syntax:

  [<# elements> x <elementtype>]

The number of elements is a constant integer value; elementtype may be any type with a size.

Examples:

Here are some examples of multidimensional arrays:

Note that 'variable sized arrays' can be implemented in LLVM with a zero length array. Normally, accesses past the end of an array are undefined in LLVM (e.g. it is illegal to access the 5th element of a 3 element array). As a special case, however, zero length arrays are recognized to be variable length. This allows implementation of 'pascal style arrays' with the LLVM type "{ int, [0 x float]}", for example.

Overview:

The function type can be thought of as a function signature. It consists of a return type and a list of formal parameter types. Function types are usually used to build virtual function tables (which are structures of pointers to functions), for indirect function calls, and when defining a function.

The return type of a function type cannot be an aggregate type.

Syntax:

  <returntype> (<parameter list>)

...where '<parameter list>' is a comma-separated list of type specifiers. Optionally, the parameter list may include a type ..., which indicates that the function takes a variable number of arguments. Variable argument functions can access their arguments with the variable argument handling intrinsic functions.

Examples:

Overview:

The structure type is used to represent a collection of data members together in memory. The packing of the field types is defined to match the ABI of the underlying processor. The elements of a structure may be any type that has a size.

Structures are accessed using 'load and 'store' by getting a pointer to a field with the 'getelementptr' instruction.

Syntax:

  { <type list> }

Examples:

Overview:

As in many languages, the pointer type represents a pointer or reference to another object, which must live in memory.

Syntax:

  <type> *

Examples:

Overview:

A packed type is a simple derived type that represents a vector of elements. Packed types are used when multiple primitive data are operated in parallel using a single instruction (SIMD). A packed type requires a size (number of elements) and an underlying primitive data type. Vectors must have a power of two length (1, 2, 4, 8, 16 ...). Packed types are considered first class.

Syntax:

  < <# elements> x <elementtype> >

The number of elements is a constant integer value; elementtype may be any integral or floating point type.

Examples:

Overview:

Opaque types are used to represent unknown types in the system. This corresponds (for example) to the C notion of a foward declared structure type. In LLVM, opaque types can eventually be resolved to any type (not just a structure type).

Syntax:

  opaque

Examples:

LLVM has several different basic types of constants. This section describes them all and their syntax.

Boolean constants

The two strings 'true' and 'false' are both valid constants of the bool type.

Integer constants

Standard integers (such as '4') are constants of the integer type. Negative numbers may be used with signed integer types.

Floating point constants

Floating point constants use standard decimal notation (e.g. 123.421), exponential notation (e.g. 1.23421e+2), or a more precise hexadecimal notation (see below). Floating point constants must have a floating point type.

Null pointer constants

The identifier 'null' is recognized as a null pointer constant and must be of pointer type.

The one non-intuitive notation for constants is the optional hexadecimal form of floating point constants. For example, the form 'double 0x432ff973cafa8000' is equivalent to (but harder to read than) 'double 4.5e+15'. The only time hexadecimal floating point constants are required (and the only time that they are generated by the disassembler) is when a floating point constant must be emitted but it cannot be represented as a decimal floating point number. For example, NaN's, infinities, and other special values are represented in their IEEE hexadecimal format so that assembly and disassembly do not cause any bits to change in the constants.

Aggregate constants arise from aggregation of simple constants and smaller aggregate constants.

Structure constants

Structure constants are represented with notation similar to structure type definitions (a comma separated list of elements, surrounded by braces ({})). For example: "{ int 4, float 17.0, int* %G }", where "%G" is declared as "%G = external global int". Structure constants must have structure type, and the number and types of elements must match those specified by the type.

Array constants

Array constants are represented with notation similar to array type definitions (a comma separated list of elements, surrounded by square brackets ([])). For example: "[ int 42, int 11, int 74 ]". Array constants must have array type, and the number and types of elements must match those specified by the type.

Packed constants

Packed constants are represented with notation similar to packed type definitions (a comma separated list of elements, surrounded by less-than/greater-than's (<>)). For example: "< int 42, int 11, int 74, int 100 >". Packed constants must have packed type, and the number and types of elements must match those specified by the type.

Zero initialization

The string 'zeroinitializer' can be used to zero initialize a value to zero of any type, including scalar and aggregate types. This is often used to avoid having to print large zero initializers (e.g. for large arrays) and is always exactly equivalent to using explicit zero initializers.

The addresses of global variables and functions are always implicitly valid (link-time) constants. These constants are explicitly referenced when the identifier for the global is used and always have pointer type. For example, the following is a legal LLVM file:

  %X = global int 17
  %Y = global int 42
  %Z = global [2 x int*] [ int* %X, int* %Y ]

The string 'undef' is recognized as a type-less constant that has no specific value. Undefined values may be of any type and be used anywhere a constant is permitted.

Undefined values indicate to the compiler that the program is well defined no matter what value is used, giving the compiler more freedom to optimize.

Constant expressions are used to allow expressions involving other constants to be used as constants. Constant expressions may be of any first class type and may involve any LLVM operation that does not have side effects (e.g. load and call are not supported). The following is the syntax for constant expressions:

trunc ( CST to TYPE )

Truncate a constant to another type. The bit size of CST must be larger than the bit size of TYPE. Both types must be integral.

zext ( CST to TYPE )

Zero extend a constant to another type. The bit size of CST must be smaller or equal to the bit size of TYPE. Both types must be integral.

sext ( CST to TYPE )

Sign extend a constant to another type. The bit size of CST must be smaller or equal to the bit size of TYPE. Both types must be integral.

fptrunc ( CST to TYPE )

Truncate a floating point constant to another floating point type. The size of CST must be larger than the size of TYPE. Both types must be floating point.

fpext ( CST to TYPE )

Floating point extend a constant to another type. The size of CST must be smaller or equal to the size of TYPE. Both types must be floating point.

fp2uint ( CST to TYPE )

Convert a floating point constant to the corresponding unsigned integer constant. TYPE must be an integer type. CST must be floating point. If the value won't fit in the integer type, the results are undefined.

fp2sint ( CST to TYPE )

Convert a floating point constant to the corresponding signed integer constant. TYPE must be an integer type. CST must be floating point. If the value won't fit in the integer type, the results are undefined.

uint2fp ( CST to TYPE )

Convert an unsigned integer constant to the corresponding floating point constant. TYPE must be floating point. CST must be of integer type. If the value won't fit in the floating point type, the results are undefined.

sint2fp ( CST to TYPE )

Convert a signed integer constant to the corresponding floating point constant. TYPE must be floating point. CST must be of integer type. If the value won't fit in the floating point type, the results are undefined.

bitconvert ( CST to TYPE )

Convert a constant, CST, to another TYPE. The size of CST and TYPE must be identical (same number of bits). The conversion is done as if the CST value was stored to memory and read back as TYPE. In other words, no bits change with this operator, just the type. This can be used for conversion of pointer and packed types to any other type, as long as they have the same bit width.

getelementptr ( CSTPTR, IDX0, IDX1, ... )

Perform the getelementptr operation on constants. As with the getelementptr instruction, the index list may have zero or more indexes, which are required to make sense for the type of "CSTPTR".

select ( COND, VAL1, VAL2 )

Perform the select operation on constants.

extractelement ( VAL, IDX )

Perform the extractelement operation on constants.

insertelement ( VAL, ELT, IDX )

Perform the insertelement operation on constants.

shufflevector ( VEC1, VEC2, IDXMASK )

Perform the shufflevector operation on constants.

OPCODE ( LHS, RHS )

Perform the specified operation of the LHS and RHS constants. OPCODE may be any of the binary or bitwise binary operations. The constraints on operands are the same as those for the corresponding instruction (e.g. no bitwise operations on floating point values are allowed).

LLVM supports inline assembler expressions (as opposed to Module-Level Inline Assembly) through the use of a special value. This value represents the inline assembler as a string (containing the instructions to emit), a list of operand constraints (stored as a string), and a flag that indicates whether or not the inline asm expression has side effects. An example inline assembler expression is:

  int(int) asm "bswap $0", "=r,r"

Inline assembler expressions may only be used as the callee operand of a call instruction. Thus, typically we have:

  %X = call int asm "bswap $0", "=r,r"(int %Y)

Inline asms with side effects not visible in the constraint list must be marked as having side effects. This is done through the use of the 'sideeffect' keyword, like so:

  call void asm sideeffect "eieio", ""()

TODO: The format of the asm and constraints string still need to be documented here. Constraints on what can be done (e.g. duplication, moving, etc need to be documented).

The LLVM instruction set consists of several different classifications of instructions: terminator instructions, binary instructions, bitwise binary instructions, memory instructions, and other instructions.

As mentioned previously, every basic block in a program ends with a "Terminator" instruction, which indicates which block should be executed after the current block is finished. These terminator instructions typically yield a 'void' value: they produce control flow, not values (the one exception being the 'invoke' instruction).

There are six different terminator instructions: the 'ret' instruction, the 'br' instruction, the 'switch' instruction, the 'invoke' instruction, the 'unwind' instruction, and the 'unreachable' instruction.

Syntax:

  ret <type> <value>       ; Return a value from a non-void function
  ret void                 ; Return from void function

Overview:

The 'ret' instruction is used to return control flow (and a value) from a function back to the caller.

There are two forms of the 'ret' instruction: one that returns a value and then causes control flow, and one that just causes control flow to occur.

Arguments:

The 'ret' instruction may return any 'first class' type. Notice that a function is not well formed if there exists a 'ret' instruction inside of the function that returns a value that does not match the return type of the function.

Semantics:

When the 'ret' instruction is executed, control flow returns back to the calling function's context. If the caller is a "call" instruction, execution continues at the instruction after the call. If the caller was an "invoke" instruction, execution continues at the beginning of the "normal" destination block. If the instruction returns a value, that value shall set the call or invoke instruction's return value.

Example:

  ret int 5                       ; Return an integer value of 5
  ret void                        ; Return from a void function

Syntax:

  br bool <cond>, label <iftrue>, label <iffalse>
  br label <dest>          ; Unconditional branch

Overview:

The 'br' instruction is used to cause control flow to transfer to a different basic block in the current function. There are two forms of this instruction, corresponding to a conditional branch and an unconditional branch.

Arguments:

The conditional branch form of the 'br' instruction takes a single 'bool' value and two 'label' values. The unconditional form of the 'br' instruction takes a single 'label' value as a target.

Semantics:

Upon execution of a conditional 'br' instruction, the 'bool' argument is evaluated. If the value is true, control flows to the 'iftrue' label argument. If "cond" is false, control flows to the 'iffalse' label argument.

Example:

Test:
  %cond = seteq int %a, %b
  br bool %cond, label %IfEqual, label %IfUnequal
IfEqual:
  ret int 1
IfUnequal:
  ret int 0

Syntax:

  switch <intty> <value>, label <defaultdest> [ <intty> <val>, label <dest> ... ]

Overview:

The 'switch' instruction is used to transfer control flow to one of several different places. It is a generalization of the 'br' instruction, allowing a branch to occur to one of many possible destinations.

Arguments:

The 'switch' instruction uses three parameters: an integer comparison value 'value', a default 'label' destination, and an array of pairs of comparison value constants and 'label's. The table is not allowed to contain duplicate constant entries.

Semantics:

The switch instruction specifies a table of values and destinations. When the 'switch' instruction is executed, this table is searched for the given value. If the value is found, control flow is transfered to the corresponding destination; otherwise, control flow is transfered to the default destination.

Implementation:

Depending on properties of the target machine and the particular switch instruction, this instruction may be code generated in different ways. For example, it could be generated as a series of chained conditional branches or with a lookup table.

Example:

 ; Emulate a conditional br instruction
 %Val = zext bool %value to int
 switch int %Val, label %truedest [int 0, label %falsedest ]

 ; Emulate an unconditional br instruction
 switch uint 0, label %dest [ ]

 ; Implement a jump table:
 switch uint %val, label %otherwise [ uint 0, label %onzero 
                                      uint 1, label %onone 
                                      uint 2, label %ontwo ]

Syntax:

  <result> = invoke [cconv] <ptr to function ty> %<function ptr val>(<function args>) 
                to label <normal label> unwind label <exception label>

Overview:

The 'invoke' instruction causes control to transfer to a specified function, with the possibility of control flow transfer to either the 'normal' label or the 'exception' label. If the callee function returns with the "ret" instruction, control flow will return to the "normal" label. If the callee (or any indirect callees) returns with the "unwind" instruction, control is interrupted and continued at the dynamically nearest "exception" label.

Arguments:

This instruction requires several arguments:

1. The optional "cconv" marker indicates which calling convention the call should use. If none is specified, the call defaults to using C calling conventions.
2. 'ptr to function ty': shall be the signature of the pointer to function value being invoked. In most cases, this is a direct function invocation, but indirect invokes are just as possible, branching off an arbitrary pointer to function value.
3. 'function ptr val': An LLVM value containing a pointer to a function to be invoked.
4. 'function args': argument list whose types match the function signature argument types. If the function signature indicates the function accepts a variable number of arguments, the extra arguments can be specified.
5. 'normal label': the label reached when the called function executes a 'ret' instruction.
6. 'exception label': the label reached when a callee returns with the unwind instruction.

Semantics:

This instruction is designed to operate as a standard 'call' instruction in most regards. The primary difference is that it establishes an association with a label, which is used by the runtime library to unwind the stack.

This instruction is used in languages with destructors to ensure that proper cleanup is performed in the case of either a longjmp or a thrown exception. Additionally, this is important for implementation of 'catch' clauses in high-level languages that support them.

Example:

  %retval = invoke int %Test(int 15)             to label %Continue
              unwind label %TestCleanup     ; {int}:retval set
  %retval = invoke coldcc int %Test(int 15)             to label %Continue
              unwind label %TestCleanup     ; {int}:retval set

Syntax:

  unwind

Overview:

The 'unwind' instruction unwinds the stack, continuing control flow at the first callee in the dynamic call stack which used an invoke instruction to perform the call. This is primarily used to implement exception handling.

Semantics:

The 'unwind' intrinsic causes execution of the current function to immediately halt. The dynamic call stack is then searched for the first invoke instruction on the call stack. Once found, execution continues at the "exceptional" destination block specified by the invoke instruction. If there is no invoke instruction in the dynamic call chain, undefined behavior results.

Syntax:

  unreachable

Overview:

The 'unreachable' instruction has no defined semantics. This instruction is used to inform the optimizer that a particular portion of the code is not reachable. This can be used to indicate that the code after a no-return function cannot be reached, and other facts.

Semantics:

The 'unreachable' instruction has no defined semantics.

Binary operators are used to do most of the computation in a program. They require two operands, execute an operation on them, and produce a single value. The operands might represent multiple data, as is the case with the packed data type. The result value of a binary operator is not necessarily the same type as its operands.

There are several different binary operators:

Syntax:

  <result> = add <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'add' instruction returns the sum of its two operands.

Arguments:

The two arguments to the 'add' instruction must be either integer or floating point values. This instruction can also take packed versions of the values. Both arguments must have identical types.

Semantics:

The value produced is the integer or floating point sum of the two operands.

Example:

  <result> = add int 4, %var          ; yields {int}:result = 4 + %var

Syntax:

  <result> = sub <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'sub' instruction returns the difference of its two operands.

Note that the 'sub' instruction is used to represent the 'neg' instruction present in most other intermediate representations.

Arguments:

The two arguments to the 'sub' instruction must be either integer or floating point values. This instruction can also take packed versions of the values. Both arguments must have identical types.

Semantics:

The value produced is the integer or floating point difference of the two operands.

Example:

  <result> = sub int 4, %var          ; yields {int}:result = 4 - %var
  <result> = sub int 0, %val          ; yields {int}:result = -%var

Syntax:

  <result> = mul <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'mul' instruction returns the product of its two operands.

Arguments:

The two arguments to the 'mul' instruction must be either integer or floating point values. This instruction can also take packed versions of the values. Both arguments must have identical types.

Semantics:

The value produced is the integer or floating point product of the two operands.

There is no signed vs unsigned multiplication. The appropriate action is taken based on the type of the operand.

Example:

  <result> = mul int 4, %var          ; yields {int}:result = 4 * %var

Syntax:

  <result> = udiv <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'udiv' instruction returns the quotient of its two operands.

Arguments:

The two arguments to the 'udiv' instruction must be integer values. Both arguments must have identical types. This instruction can also take packed versions of the values in which case the elements must be integers.

Semantics:

The value produced is the unsigned integer quotient of the two operands. This instruction always performs an unsigned division operation, regardless of whether the arguments are unsigned or not.

Example:

  <result> = udiv uint 4, %var          ; yields {uint}:result = 4 / %var

Syntax:

  <result> = sdiv <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'sdiv' instruction returns the quotient of its two operands.

Arguments:

The two arguments to the 'sdiv' instruction must be integer values. Both arguments must have identical types. This instruction can also take packed versions of the values in which case the elements must be integers.

Semantics:

The value produced is the signed integer quotient of the two operands. This instruction always performs a signed division operation, regardless of whether the arguments are signed or not.

Example:

  <result> = sdiv int 4, %var          ; yields {int}:result = 4 / %var

Syntax:

  <result> = fdiv <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'fdiv' instruction returns the quotient of its two operands.

Arguments:

The two arguments to the 'div' instruction must be floating point values. Both arguments must have identical types. This instruction can also take packed versions of the values in which case the elements must be floating point.

Semantics:

The value produced is the floating point quotient of the two operands.

Example:

  <result> = fdiv float 4.0, %var          ; yields {float}:result = 4.0 / %var

Syntax:

  <result> = urem <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'urem' instruction returns the remainder from the unsigned division of its two arguments.

Arguments:

The two arguments to the 'urem' instruction must be integer values. Both arguments must have identical types.

Semantics:

This instruction returns the unsigned integer remainder of a division. This instruction always performs an unsigned division to get the remainder, regardless of whether the arguments are unsigned or not.

Example:

  <result> = urem uint 4, %var          ; yields {uint}:result = 4 % %var

Syntax:

  <result> = srem <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'srem' instruction returns the remainder from the signed division of its two operands.

Arguments:

The two arguments to the 'srem' instruction must be integer values. Both arguments must have identical types.

Semantics:

This instruction returns the remainder of a division (where the result has the same sign as the divisor), not the modulus (where the result has the same sign as the dividend) of a value. For more information about the difference, see The Math Forum.

Example:

  <result> = srem int 4, %var          ; yields {int}:result = 4 % %var

Syntax:

  <result> = frem <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'frem' instruction returns the remainder from the division of its two operands.

Arguments:

The two arguments to the 'frem' instruction must be floating point values. Both arguments must have identical types.

Semantics:

This instruction returns the remainder of a division.

Example:

  <result> = frem float 4.0, %var          ; yields {float}:result = 4.0 % %var

Syntax:

  <result> = seteq <ty> <var1>, <var2>   ; yields {bool}:result
  <result> = setne <ty> <var1>, <var2>   ; yields {bool}:result
  <result> = setlt <ty> <var1>, <var2>   ; yields {bool}:result
  <result> = setgt <ty> <var1>, <var2>   ; yields {bool}:result
  <result> = setle <ty> <var1>, <var2>   ; yields {bool}:result
  <result> = setge <ty> <var1>, <var2>   ; yields {bool}:result

Overview:

The 'setcc' family of instructions returns a boolean value based on a comparison of their two operands.

Arguments:

The two arguments to the 'setcc' instructions must be of first class type (it is not possible to compare 'label's, 'array's, 'structure' or 'void' values, etc...). Both arguments must have identical types.

Semantics:

The 'seteq' instruction yields a true 'bool' value if both operands are equal.
The 'setne' instruction yields a true 'bool' value if both operands are unequal.
The 'setlt' instruction yields a true 'bool' value if the first operand is less than the second operand.
The 'setgt' instruction yields a true 'bool' value if the first operand is greater than the second operand.
The 'setle' instruction yields a true 'bool' value if the first operand is less than or equal to the second operand.
The 'setge' instruction yields a true 'bool' value if the first operand is greater than or equal to the second operand.

Example:

  <result> = seteq int   4, 5        ; yields {bool}:result = false
  <result> = setne float 4, 5        ; yields {bool}:result = true
  <result> = setlt uint  4, 5        ; yields {bool}:result = true
  <result> = setgt sbyte 4, 5        ; yields {bool}:result = false
  <result> = setle sbyte 4, 5        ; yields {bool}:result = true
  <result> = setge sbyte 4, 5        ; yields {bool}:result = false

Bitwise binary operators are used to do various forms of bit-twiddling in a program. They are generally very efficient instructions and can commonly be strength reduced from other instructions. They require two operands, execute an operation on them, and produce a single value. The resulting value of the bitwise binary operators is always the same type as its first operand.

Syntax:

  <result> = and <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'and' instruction returns the bitwise logical and of its two operands.

Arguments:

The two arguments to the 'and' instruction must be integral values. Both arguments must have identical types.

Semantics:

The truth table used for the 'and' instruction is:

Example:

  <result> = and int 4, %var         ; yields {int}:result = 4 & %var
  <result> = and int 15, 40          ; yields {int}:result = 8
  <result> = and int 4, 8            ; yields {int}:result = 0

Syntax:

  <result> = or <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'or' instruction returns the bitwise logical inclusive or of its two operands.

Arguments:

The two arguments to the 'or' instruction must be integral values. Both arguments must have identical types.

Semantics:

The truth table used for the 'or' instruction is:

Example:

  <result> = or int 4, %var         ; yields {int}:result = 4 | %var
  <result> = or int 15, 40          ; yields {int}:result = 47
  <result> = or int 4, 8            ; yields {int}:result = 12

Syntax:

  <result> = xor <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'xor' instruction returns the bitwise logical exclusive or of its two operands. The xor is used to implement the "one's complement" operation, which is the "~" operator in C.

Arguments:

The two arguments to the 'xor' instruction must be integral values. Both arguments must have identical types.

Semantics:

The truth table used for the 'xor' instruction is:

Example:

  <result> = xor int 4, %var         ; yields {int}:result = 4 ^ %var
  <result> = xor int 15, 40          ; yields {int}:result = 39
  <result> = xor int 4, 8            ; yields {int}:result = 12
  <result> = xor int %V, -1          ; yields {int}:result = ~%V

Syntax:

  <result> = shl <ty> <var1>, ubyte <var2>   ; yields {ty}:result

Overview:

The 'shl' instruction returns the first operand shifted to the left a specified number of bits.

Arguments:

The first argument to the 'shl' instruction must be an integer type. The second argument must be an 'ubyte' type.

Semantics:

The value produced is var1 * 2^var2.

Example:

  <result> = shl int 4, ubyte %var   ; yields {int}:result = 4 << %var
  <result> = shl int 4, ubyte 2      ; yields {int}:result = 16
  <result> = shl int 1, ubyte 10     ; yields {int}:result = 1024

Syntax:

  <result> = shr <ty> <var1>, ubyte <var2>   ; yields {ty}:result

Overview:

The 'shr' instruction returns the first operand shifted to the right a specified number of bits.

Arguments:

The first argument to the 'shr' instruction must be an integer type. The second argument must be an 'ubyte' type.

Semantics:

If the first argument is a signed type, the most significant bit is duplicated in the newly free'd bit positions. If the first argument is unsigned, zero bits shall fill the empty positions.

Example:

  <result> = shr int 4, ubyte %var   ; yields {int}:result = 4 >> %var
  <result> = shr uint 4, ubyte 1     ; yields {uint}:result = 2
  <result> = shr int 4, ubyte 2      ; yields {int}:result = 1
  <result> = shr sbyte 4, ubyte 3    ; yields {sbyte}:result = 0
  <result> = shr sbyte -2, ubyte 1   ; yields {sbyte}:result = -1

LLVM supports several instructions to represent vector operations in a target-independent manner. This instructions cover the element-access and vector-specific operations needed to process vectors effectively. While LLVM does directly support these vector operations, many sophisticated algorithms will want to use target-specific intrinsics to take full advantage of a specific target.

Syntax:

  <result> = extractelement <n x <ty>> <val>, uint <idx>    ; yields <ty>

Overview:

The 'extractelement' instruction extracts a single scalar element from a packed vector at a specified index.

Arguments:

The first operand of an 'extractelement' instruction is a value of packed type. The second operand is an index indicating the position from which to extract the element. The index may be a variable.

Semantics:

The result is a scalar of the same type as the element type of val. Its value is the value at position idx of val. If idx exceeds the length of val, the results are undefined.

Example:

  %result = extractelement <4 x int> %vec, uint 0    ; yields int

Syntax:

  <result> = insertelement <n x <ty>> <val>, <ty> <elt>, uint <idx>    ; yields <n x <ty>>

Overview:

The 'insertelement' instruction inserts a scalar element into a packed vector at a specified index.

Arguments:

The first operand of an 'insertelement' instruction is a value of packed type. The second operand is a scalar value whose type must equal the element type of the first operand. The third operand is an index indicating the position at which to insert the value. The index may be a variable.

Semantics:

The result is a packed vector of the same type as val. Its element values are those of val except at position idx, where it gets the value elt. If idx exceeds the length of val, the results are undefined.

Example:

  %result = insertelement <4 x int> %vec, int 1, uint 0    ; yields <4 x int>

Syntax:

  <result> = shufflevector <n x <ty>> <v1>, <n x <ty>> <v2>, <n x uint> <mask>    ; yields <n x <ty>>

Overview:

The 'shufflevector' instruction constructs a permutation of elements from two input vectors, returning a vector of the same type.

Arguments:

The first two operands of a 'shufflevector' instruction are vectors with types that match each other and types that match the result of the instruction. The third argument is a shuffle mask, which has the same number of elements as the other vector type, but whose element type is always 'uint'.

The shuffle mask operand is required to be a constant vector with either constant integer or undef values.

Semantics:

The elements of the two input vectors are numbered from left to right across both of the vectors. The shuffle mask operand specifies, for each element of the result vector, which element of the two input registers the result element gets. The element selector may be undef (meaning "don't care") and the second operand may be undef if performing a shuffle from only one vector.

Example:

  %result = shufflevector <4 x int> %v1, <4 x int> %v2, 
                          <4 x uint> <uint 0, uint 4, uint 1, uint 5>    ; yields <4 x int>
  %result = shufflevector <4 x int> %v1, <4 x int> undef, 
                          <4 x uint> <uint 0, uint 1, uint 2, uint 3>  ; yields <4 x int> - Identity shuffle.

Syntax:

<result> = vsetint <op>, <n x <ty>> <var1>, <var2>   ; yields <n x bool>

Overview:

The 'vsetint' instruction takes two integer vectors and returns a vector of boolean values representing, at each position, the result of the comparison between the values at that position in the two operands.

Arguments:

The arguments to a 'vsetint' instruction are a comparison operation and two value arguments. The value arguments must be of integral packed type, and they must have identical types. The operation argument must be one of eq, ne, slt, sgt, sle, sge, ult, ugt, ule, uge, true, and false. The result is a packed bool value with the same length as each operand.

Semantics:

The following table shows the semantics of 'vsetint'. For each position of the result, the comparison is done on the corresponding positions of the two value arguments. Note that the signedness of the comparison depends on the comparison opcode and not on the signedness of the value operands. E.g., vsetint slt <4 x unsigned> %x, %y does an elementwise signed comparison of %x and %y.

Example:

  <result> = vsetint eq <2 x int> <int 0, int 1>, <int 1, int 0>      ; yields {<2 x bool>}:result = false, false
  <result> = vsetint ne <2 x int> <int 0, int 1>, <int 1, int 0>      ; yields {<2 x bool>}:result = true, true
  <result> = vsetint slt <2 x int> <int 0, int 1>, <int 1, int 0>      ; yields {<2 x bool>}:result = true, false
  <result> = vsetint sgt <2 x int> <int 0, int 1>, <int 1, int 0>      ; yields {<2 x bool>}:result = false, true
  <result> = vsetint sle <2 x int> <int 0, int 1>, <int 1, int 0>      ; yields {<2 x bool>}:result = true, false
  <result> = vsetint sge <2 x int> <int 0, int 1>, <int 1, int 0>      ; yields {<2 x bool>}:result = false, true

Syntax:

<result> = vsetfp <op>, <n x <ty>> <var1>, <var2>   ; yields <n x bool>

Overview:

The 'vsetfp' instruction takes two floating point vector arguments and returns a vector of boolean values representing, at each position, the result of the comparison between the values at that position in the two operands.

Arguments:

The arguments to a 'vsetfp' instruction are a comparison operation and two value arguments. The value arguments must be of floating point packed type, and they must have identical types. The operation argument must be one of eq, ne, lt, gt, le, ge, oeq, one, olt, ogt, ole, oge, ueq, une, ult, ugt, ule, uge, o, u, true, and false. The result is a packed bool value with the same length as each operand.

Semantics:

The following table shows the semantics of 'vsetfp' for floating point types. If either operand is a floating point Not a Number (NaN) value, the operation is unordered, and the value in the first column below is produced at that position. Otherwise, the operation is ordered, and the value in the second column is produced.

Example:

  <result> = vsetfp eq <2 x float> <float 0.0, float 1.0>, <float 1.0, float 0.0>      ; yields {<2 x bool>}:result = false, false
  <result> = vsetfp ne <2 x float> <float 0.0, float 1.0>, <float 1.0, float 0.0>      ; yields {<2 x bool>}:result = true, true
  <result> = vsetfp lt <2 x float> <float 0.0, float 1.0>, <float 1.0, float 0.0>      ; yields {<2 x bool>}:result = true, false
  <result> = vsetfp gt <2 x float> <float 0.0, float 1.0>, <float 1.0, float 0.0>      ; yields {<2 x bool>}:result = false, true
  <result> = vsetfp le <2 x float> <float 0.0, float 1.0>, <float 1.0, float 0.0>      ; yields {<2 x bool>}:result = true, false
  <result> = vsetfp ge <2 x float> <float 0.0, float 1.0>, <float 1.0, float 0.0>      ; yields {<2 x bool>}:result = false, true

Syntax:

  <result> = vselect <n x bool> <cond>, <n x <ty>> <val1>, <n x <ty>> <val2> ; yields <n x <ty>>

Overview:

The 'vselect' instruction chooses one value at each position of a vector based on a condition.

Arguments:

The 'vselect' instruction requires a packed bool value indicating the condition at each vector position, and two values of the same packed type. All three operands must have the same length. The type of the result is the same as the type of the two value operands.

Semantics:

At each position where the bool vector is true, that position of the result gets its value from the first value argument; otherwise, it gets its value from the second value argument.

Example:

  %X = vselect bool <2 x bool> <bool true, bool false>, <2 x ubyte> <ubyte 17, ubyte 17>, 
    <2 x ubyte> <ubyte 42, ubyte 42>      ; yields <2 x ubyte>:17, 42

A key design point of an SSA-based representation is how it represents memory. In LLVM, no memory locations are in SSA form, which makes things very simple. This section describes how to read, write, allocate, and free memory in LLVM.

Syntax:

  <result> = malloc <type>[, uint <NumElements>][, align <alignment>]     ; yields {type*}:result

Overview:

The 'malloc' instruction allocates memory from the system heap and returns a pointer to it.

Arguments:

The 'malloc' instruction allocates sizeof(<type>)*NumElements bytes of memory from the operating system and returns a pointer of the appropriate type to the program. If "NumElements" is specified, it is the number of elements allocated. If an alignment is specified, the value result of the allocation is guaranteed to be aligned to at least that boundary. If not specified, or if zero, the target can choose to align the allocation on any convenient boundary.

'type' must be a sized type.

Semantics:

Memory is allocated using the system "malloc" function, and a pointer is returned.

Example:

  %array  = malloc [4 x ubyte ]                    ; yields {[%4 x ubyte]*}:array

  %size   = add uint 2, 2                          ; yields {uint}:size = uint 4
  %array1 = malloc ubyte, uint 4                   ; yields {ubyte*}:array1
  %array2 = malloc [12 x ubyte], uint %size        ; yields {[12 x ubyte]*}:array2
  %array3 = malloc int, uint 4, align 1024         ; yields {int*}:array3
  %array4 = malloc int, align 1024                 ; yields {int*}:array4

Syntax:

  free <type> <value>                              ; yields {void}

Overview:

The 'free' instruction returns memory back to the unused memory heap to be reallocated in the future.

Arguments:

'value' shall be a pointer value that points to a value that was allocated with the 'malloc' instruction.

Semantics:

Access to the memory pointed to by the pointer is no longer defined after this instruction executes.

Example:

  %array  = malloc [4 x ubyte]                    ; yields {[4 x ubyte]*}:array
            free   [4 x ubyte]* %array

Syntax:

  <result> = alloca <type>[, uint <NumElements>][, align <alignment>]     ; yields {type*}:result

Overview:

The 'alloca' instruction allocates memory on the current stack frame of the procedure that is live until the current function returns to its caller.

Arguments:

The 'alloca' instruction allocates sizeof(<type>)*NumElements bytes of memory on the runtime stack, returning a pointer of the appropriate type to the program. If "NumElements" is specified, it is the number of elements allocated. If an alignment is specified, the value result of the allocation is guaranteed to be aligned to at least that boundary. If not specified, or if zero, the target can choose to align the allocation on any convenient boundary.

'type' may be any sized type.

Semantics:

Memory is allocated; a pointer is returned. 'alloca'd memory is automatically released when the function returns. The 'alloca' instruction is commonly used to represent automatic variables that must have an address available. When the function returns (either with the ret or unwind instructions), the memory is reclaimed.

Example:

  %ptr = alloca int                              ; yields {int*}:ptr
  %ptr = alloca int, uint 4                      ; yields {int*}:ptr
  %ptr = alloca int, uint 4, align 1024          ; yields {int*}:ptr
  %ptr = alloca int, align 1024                  ; yields {int*}:ptr

Syntax:

  <result> = load <ty>* <pointer>
  <result> = volatile load <ty>* <pointer>

Overview:

The 'load' instruction is used to read from memory.

Arguments:

The argument to the 'load' instruction specifies the memory address from which to load. The pointer must point to a first class type. If the load is marked as volatile, then the optimizer is not allowed to modify the number or order of execution of this load with other volatile load and store instructions.

Semantics:

The location of memory pointed to is loaded.

Examples:

  %ptr = alloca int                               ; yields {int*}:ptr
  store int 3, int* %ptr                          ; yields {void}
  %val = load int* %ptr                           ; yields {int}:val = int 3

Syntax:

  <result> = getelementptr <ty>* <ptrval>{, <ty> <idx>}*

Overview:

The 'getelementptr' instruction is used to get the address of a subelement of an aggregate data structure.

Arguments:

This instruction takes a list of integer constants that indicate what elements of the aggregate object to index to. The actual types of the arguments provided depend on the type of the first pointer argument. The 'getelementptr' instruction is used to index down through the type levels of a structure or to a specific index in an array. When indexing into a structure, only uint integer constants are allowed. When indexing into an array or pointer, int and long indexes are allowed of any sign.

For example, let's consider a C code fragment and how it gets compiled to LLVM:

  struct RT {
    char A;
    int B[10][20];
    char C;
  };
  struct ST {
    int X;
    double Y;
    struct RT Z;
  };

  int *foo(struct ST *s) {
    return &s[1].Z.B[5][13];
  }

The LLVM code generated by the GCC frontend is:

  %RT = type { sbyte, [10 x [20 x int]], sbyte }
  %ST = type { int, double, %RT }

  implementation

  int* %foo(%ST* %s) {
  entry:
    %reg = getelementptr %ST* %s, int 1, uint 2, uint 1, int 5, int 13
    ret int* %reg
  }

Semantics:

The index types specified for the 'getelementptr' instruction depend on the pointer type that is being indexed into. Pointer and array types require uint, int, ulong, or long values, and structure types require uint constants.

In the example above, the first index is indexing into the '%ST*' type, which is a pointer, yielding a '%ST' = '{ int, double, %RT }' type, a structure. The second index indexes into the third element of the structure, yielding a '%RT' = '{ sbyte, [10 x [20 x int]], sbyte }' type, another structure. The third index indexes into the second element of the structure, yielding a '[10 x [20 x int]]' type, an array. The two dimensions of the array are subscripted into, yielding an 'int' type. The 'getelementptr' instruction returns a pointer to this element, thus computing a value of 'int*' type.

Note that it is perfectly legal to index partially through a structure, returning a pointer to an inner element. Because of this, the LLVM code for the given testcase is equivalent to:

  int* %foo(%ST* %s) {
    %t1 = getelementptr %ST* %s, int 1                        ; yields %ST*:%t1
    %t2 = getelementptr %ST* %t1, int 0, uint 2               ; yields %RT*:%t2
    %t3 = getelementptr %RT* %t2, int 0, uint 1               ; yields [10 x [20 x int]]*:%t3
    %t4 = getelementptr [10 x [20 x int]]* %t3, int 0, int 5  ; yields [20 x int]*:%t4
    %t5 = getelementptr [20 x int]* %t4, int 0, int 13        ; yields int*:%t5
    ret int* %t5
  }

Note that it is undefined to access an array out of bounds: array and pointer indexes must always be within the defined bounds of the array type. The one exception for this rules is zero length arrays. These arrays are defined to be accessible as variable length arrays, which requires access beyond the zero'th element.

The getelementptr instruction is often confusing. For some more insight into how it works, see the getelementptr FAQ.

Example:

    ; yields [12 x ubyte]*:aptr
    %aptr = getelementptr {int, [12 x ubyte]}* %sptr, long 0, uint 1

The instructions in this category are the "miscellaneous" instructions, which defy better classification.

Syntax:

  <result> = phi <ty> [ <val0>, <label0>], ...

Overview:

The 'phi' instruction is used to implement the φ node in the SSA graph representing the function.

Arguments:

The type of the incoming values are specified with the first type field. After this, the 'phi' instruction takes a list of pairs as arguments, with one pair for each predecessor basic block of the current block. Only values of first class type may be used as the value arguments to the PHI node. Only labels may be used as the label arguments.

There must be no non-phi instructions between the start of a basic block and the PHI instructions: i.e. PHI instructions must be first in a basic block.

Semantics:

At runtime, the 'phi' instruction logically takes on the value specified by the parameter, depending on which basic block we came from in the last terminator instruction.

Example:

Loop:       ; Infinite loop that counts from 0 on up...
  %indvar = phi uint [ 0, %LoopHeader ], [ %nextindvar, %Loop ]
  %nextindvar = add uint %indvar, 1
  br label %Loop

Syntax:

  <result> = trunc <ty> <value> to <ty2>             ; yields ty2

Overview:

The 'trunc' instruction truncates its operand to the type ty2.

Arguments:

The 'trunc' instruction takes a value to trunc, which must be an integer type, and a type that specifies the size and type of the result, which must be an integral type.

Semantics:

The 'trunc' instruction truncates the high order bits in value and converts the reamining bits to ty2. The bit size of value must be larger than the bit size of ty2. Equal sized types are not allowed. This implies that a trunc cannot be a no-op cast. It will always truncate bits.

When truncating to bool, the truncation is done as a comparison against zero. If the value was zero, the bool result will be false. If the value was non-zero, the bool result will be true.

Example:

  %X = trunc int 257 to ubyte              ; yields ubyte:1
  %Y = trunc int 123 to bool               ; yields bool:true

Syntax:

  <result> = zext <ty> <value> to <ty2>             ; yields ty2

Overview:

The 'zext' instruction zero extends its operand to type ty2.

Arguments:

The 'zext' instruction takes a value to cast, which must be of integral type, and a type to cast it to, which must also be of integral type. The bit size of the value must be smaller than or equal to the bit size of the destination type, ty2.

Semantics:

The zext fills the high order bits of the value with zero bits until it reaches the size of the destination type, ty2. When the the operand and the type are the same size, no bit filling is done and the cast is considered a no-op cast because no bits change (only the type changes).

When zero extending to bool, the extension is done as a comparison against zero. If the value was zero, the bool result will be false. If the value was non-zero, the bool result will be true.

Example:

  %X = zext int 257 to ulong              ; yields ulong:257
  %Y = zext bool true to int              ; yields int:1

Syntax:

  <result> = sext <ty> <value> to <ty2>             ; yields ty2

Overview:

The 'sext' sign extends value to the type ty2.

Arguments:

The 'sext' instruction takes a value to cast, which must be of integral type, and a type to cast it to, which must also be of integral type.

Semantics:

The 'sext' instruction performs a sign extension by copying the sign bit (highest order bit) of the value until it reaches the bit size of the type ty2. When the the operand and the type are the same size, no bit filling is done and the cast is considered a no-op cast because no bits change (only the type changes).

When sign extending to bool, the extension is done as a comparison against zero. If the value was zero, the bool result will be false. If the value was non-zero, the bool result will be true.

Example:

  %X = sext sbyte -1 to ushort           ; yields ushort:65535
  %Y = sext bool true to int             ; yields int:-1

Syntax:

  <result> = fpext <ty> <value> to <ty2>             ; yields ty2

Overview:

The 'fpext' extends a floating point value to a larger floating point value.

Arguments:

The 'fpext' instruction takes a floating point value to cast, and a floating point type to cast it to.

Semantics:

The 'fpext' instruction extends the value from one floating point type to another. If the type of the value and ty2 are the same, the instruction is considered a no-op cast because no bits change.

Example:

  %X = fpext float 3.1415 to double        ; yields double:3.1415
  %Y = fpext float 1.0 to float            ; yields float:1.0 (no-op)

Syntax:

  <result> = fptrunc <ty> <value> to <ty2>             ; yields ty2

Overview:

The 'fptrunc' instruction truncates value to type ty2.

Arguments:

The 'fptrunc' instruction takes a floating point value to cast and a floating point type to cast it to. The size of value must be larger than the size of ty2. This implies that fptrunc cannot be used to make a no-op cast.

  %X = fptrunc double 123.0 to float         ; yields float:123.0
  %Y = fptrunc double 1.0E+300 to float      ; yields undefined