1. Abstract
2. Introduction
3. Identifiers
4. Type System
   1. Primitive Types
      1. Type Classifications
   2. Derived Types
      1. Array Type
      2. Function Type
      3. Pointer Type
      4. Structure Type
5. High Level Structure
   1. Module Structure
   2. Global Variables
   3. Function Structure
6. Instruction Reference
   1. Terminator Instructions
      1. 'ret' Instruction
      2. 'br' Instruction
      3. 'switch' Instruction
      4. 'invoke' Instruction
   2. Binary Operations
      1. 'add' Instruction
      2. 'sub' Instruction
      3. 'mul' Instruction
      4. 'div' Instruction
      5. 'rem' Instruction
      6. 'setcc' Instructions
   3. Bitwise Binary Operations
      1. 'and' Instruction
      2. 'or' Instruction
      3. 'xor' Instruction
      4. 'shl' Instruction
      5. 'shr' Instruction
   4. Memory Access Operations
      1. 'malloc' Instruction
      2. 'free' Instruction
      3. 'alloca' Instruction
      4. 'load' Instruction
      5. 'store' Instruction
      6. 'getelementptr' Instruction
   5. Other Operations
      1. 'phi' Instruction
      2. 'cast .. to' Instruction
      3. 'call' Instruction

   Written by Chris Lattner and Vikram Adve

Abstract

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 dynamic 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 a 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. Numeric constants are represented as you would expect: 12, -3 123.421, etc. Floating point constants have an optional hexidecimal notation.
2. 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]*'.
3. Unnamed values are represented as an unsigned numeric value with a '%' prefix. For example, %12, %2, %44.

LLVM requires the 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 show 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.

The one unintuitive notation for constants is the optional hexidecimal form of floating point constants. For example, the form 'double 0x432ff973cafa8000' is equivalent to (but harder to read than) 'double 4.5e+15' which is also supported by the parser. The only time hexadecimal floating point constants are useful (and the only time that they are generated by the disassembler) is when an FP constant has to be emitted that is not representable as a decimal floating point number exactly. For example, NaN's, infinities, and other special cases are represented in their IEEE hexadecimal format so that assembly and disassembly do not cause any bits to change in the constants.

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

void No value ubyte Unsigned 8 bit value ushort Unsigned 16 bit value uint Unsigned 32 bit value ulong Unsigned 64 bit value float 32 bit floating point value label Branch destination

bool True or False value sbyte Signed 8 bit value short Signed 16 bit value int Signed 32 bit value long Signed 64 bit value double 64 bit floating point value

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:

{ int, int, int }	: a triple of three int values
{ float, int (int) * }	: A pair, where the first element is a float and the second element is a pointer to a function that takes an int, returning an int.

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]*

; Forward declaration of puts
declare int "puts"(sbyte*)                                           ; int(sbyte*)* 

; Definition of main function
int "main"() {                                                       ; int()* 
        ; Convert [13x sbyte]* to sbyte *...
        %cast210 = getelementptr [13 x sbyte]* %.LC0, uint 0, uint 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 can be either "internal" or externally accessible (which corresponds to the static keyword in C, when used at global scope).

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 "internal" declarations), they are accessible outside of the current module. It is illegal for a function declaration to be "internal".

Global variables define regions of memory allocated at compilation time instead of run-time. Global variables may optionally be initialized. A variable may be defined as a global "constant", which indicates that the contents of the variable will never be modified (opening options for optimization). Constants must always have an initial value.

As SSA values, global variables define pointer values that are in scope (i.e. they dominate) for 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 functions definitions are composed of a (possibly empty) argument list, an opening curly brace, a list of basic blocks, and a closing curly brace. LLVM function declarations are defined with the "declare" keyword, a function name and a function signature.

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 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).

The LLVM instruction set consists of several different classifications of instructions: terminator instructions, 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 four different terminator instructions: the 'ret' instruction, the 'br' instruction, the 'switch' instruction, and the 'invoke' 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' instructruction: 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 instruction returns a value, that value shall be propagated into the calling function's data space.

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:

  ; Definitions for lookup indirect branch
  %switchtype = type [<anysize> x { uint, label }]

  ; Lookup indirect branch
  switch uint <value>, label <defaultdest>, %switchtype <switchtable>

Overview:

NOTE: The switch instruction may go away in the future. It is not very well supported in LLVM anyway, so don't go to great lengths to support it. Talk to Chris for more info if this concerns you.

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.

The 'switch' statement supports two different styles of indirect branching: lookup branching and indexed branching. Lookup branching is generally useful if the values to switch on are spread far appart, where index branching is useful if the values to switch on are generally dense.

The two different forms of the 'switch' statement are simple hints to the underlying implementation. For example, the compiler may choose to implement a small indirect branch table as a series of predicated comparisons: if it is faster for the target architecture.

Arguments:

The lookup form of the 'switch' instruction uses three parameters: a 'uint' comparison value 'value', a default 'label' destination, and an array of pairs of comparison value constants and 'label's. The sized array must be a constant value.

The indexed form of the 'switch' instruction uses three parameters: an 'uint' index value, a default 'label' and a sized array of 'label's. The 'dests' array must be a constant array.

Semantics:

The lookup style switch statement 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, the corresponding destination is branched to.

The index branch form simply looks up a label element directly in a table and branches to it.

In either case, the compiler knows the static size of the array, because it is provided as part of the constant values type.

Example:

  ; Emulate a conditional br instruction
  %Val = cast bool %value to uint
  switch uint %Val, label %truedest, [1 x label] [label %falsedest ]

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

  ; Implement a jump table:
  switch uint %val, label %otherwise, [3 x label] [ label %onzero, 
                                                    label %onone, 
                                                    label %ontwo ]

Syntax:

  <result> = invoke <ptr to function ty> %<function ptr val>(<function args>)
                 to label <normal label> except label <exception label>

Overview:

The 'invoke' instruction is used to cause control flow to transfer to a specified function, with the possibility of control flow transfer to either the 'normal label' label or the 'exception label'. The 'call' instruction is closely related, but guarantees that control flow either never returns from the called function, or that it returns to the instruction following the 'call' instruction.

Arguments:

This instruction requires several arguments:
1. '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.

2. 'function ptr val': An LLVM value containing a pointer to a function to be invoked.
3. '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.
4. 'normal label': the label reached when the called function executes a 'ret' instruction.
5. 'exception label': the label reached when an exception is thrown.

Semantics:

This instruction is designed to operate as a standard 'call' instruction in most regards. The primary difference is that it associates a label with the function invocation that may be accessed via the runtime library provided by the execution environment. 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 except label %TestCleanup     ; {int}:retval set

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 result value of a binary operator is not neccesarily 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. 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. 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. 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> = div <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

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

Arguments:

The two arguments to the 'div' instruction must be either integer or floating point values. Both arguments must have identical types.

Semantics:

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

Example:

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

Syntax:

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

Overview:

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

Arguments:

The two arguments to the 'rem' instruction must be either integer or floating point values. Both arguments must have identical types.

Semantics:

This 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> = rem int 4, %var          ; yields {int}:result = 4 % %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 or pointer type (it is not possible to compare 'label's, 'array's, 'structure' or 'void' values, etc...). Both arguments must have identical types.

The 'setlt', 'setgt', 'setle', and 'setge' instructions do not operate on 'bool' typed arguments.

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:

In0	In1	Out
0	0	0
0	1	0
1	0	0
1	1	1

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:

In0	In1	Out
0	0	0
0	1	1
1	0	1
1	1	1

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.

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:

In0	In1	Out
0	0	0
0	1	1
1	0	1
1	1	0

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

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 int 4, ubyte 1      ; yields {int}:result = 2
  <result> = shr int 4, ubyte 2      ; yields {int}:result = 1
  <result> = shr int 4, ubyte 3      ; yields {int}:result = 0

Accessing memory in SSA form is, well, sticky at best. This section describes how to read, write, allocate and free memory in LLVM.

Syntax:

  <result> = malloc <type>, uint <NumElements>     ; yields {type*}:result
  <result> = malloc <type>                         ; yields {type*}:result

Overview:

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

Arguments:

The 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. The second form of the instruction is a shorter version of the first instruction that defaults to allocating one element.

'type' must be a sized type

Semantics:

Memory is allocated, 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

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 not longer defined after this instruction executes.

Example:

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

Syntax:

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

Overview:

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

Arguments:

The argument to the 'load' instruction specifies the memory address to load from. The pointer must point to a first class type.

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:

  store <ty> <value>, <ty>* <pointer>                   ; yields {void}

Overview:

The 'store' instruction is used to write to memory.

Arguments:

There are two arguments to the 'store' instruction: a value to store and an address to store it into. The type of the '<pointer>' operand must be a pointer to the type of the '<value>' operand.

Semantics:

The contents of memory are updated to contain '<value>' at the location specified by the '<pointer>' operand.

Example:

  %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>{, uint <aidx>|, ubyte <sidx>}*

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 uint values and ubyte constants that indicate what form of addressing to perform. 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.

For example, lets 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 }

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

Semantics:

The index types specified for the 'getelementptr' instruction depend on the pointer type that is being index into. Pointer and array types require 'uint' values, and structure types require 'ubyte' 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 return a pointer to this element, thus yielding a '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 , uint 1                        ; yields %ST*:%t1
  %t2 = getelementptr %ST* %t1, uint 0, ubyte 2               ; yields %RT*:%t2
  %t3 = getelementptr %RT* %t2, uint 0, ubyte 1               ; yields [10 x [20 x int]]*:%t3
  %t4 = getelementptr [10 x [20 x int]]* %t3, uint 0, uint 5  ; yields [20 x int]*:%t4
  %t5 = getelementptr [20 x int]* %t4, uint 0, uint 13        ; yields int*:%t5
  ret int* %t5
}

Example:

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

The instructions in this catagory are the "miscellaneous" functions, that 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.

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> = call <ty>* <fnptrval>(<param list>)

Overview:

The 'call' instruction represents a simple function call.

Arguments:

This instruction requires several arguments:
1. 'ty': shall be the signature of the pointer to function value being invoked. The argument types must match the types implied by this signature.

2. 'fnptrval': An LLVM value containing a pointer to a function to be invoked. In most cases, this is a direct function invocation, but indirect calls are just as possible, calling an arbitrary pointer to function values.

3. '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.

Semantics:

The 'call' instruction is used to cause control flow to transfer to a specified function, with its incoming arguments bound to the specified values. Upon a 'ret' instruction in the called function, control flow continues with the instruction after the function call, and the return value of the function is bound to the result argument. This is a simpler case of the invoke instruction.

Example:

  %retval = call int %test(int %argc)
  call int(sbyte*, ...) *%printf(sbyte* %msg, int 12, sbyte 42);