GEP closely resembles C array indexing and field selection.

Syntax

<result> = getelementptr <ty>, <ty>* <ptrval>{, [inrange] <ty> <idx>}*
<result> = getelementptr inbounds <ty>, <ty>* <ptrval>{, [inrange] <ty> <idx>}*
<result> = getelementptr <ty>, <ptr vector> <ptrval>, [inrange] <vector index type> <idx>

Parameter

• The first argument is always a type used as the basis for the calculations.
• The second argument is always a pointer or a vector of pointers, and is the base address to start from.
• The remaining arguments are indices that indicate which of the elements of the aggregate object are indexed.

Return value

• The address of a subelement of an aggregate data structure.

Attention: GEP can not be used without load or store instruction. The address calculated must be used afterwards.

Example

• Source code

struct munger_struct {
  int f1;
  int f2;
};
void munge(struct munger_struct *P) {
  P[0].f1 = P[1].f1 + P[2].f2;
}
...
struct munger_struct Array[3];
...
munge(Array);

• LLVM IR

P[1].f1 is translated and decomposed to two LLVM instructions

%tmp = getelementptr %struct.munger_struct, %struct.munger_struct* %P, i32 1, i32 0
%tmp1 = load i32, i32* %tmp

The bc file is as follows:

define void @munge(%struct.munger_struct* %P) {
entry:
  %tmp = getelementptr %struct.munger_struct, %struct.munger_struct* %P, i32 1, i32 0
  %tmp1 = load i32, i32* %tmp
  %tmp2 = getelementptr %struct.munger_struct, %struct.munger_struct* %P, i32 2, i32 1
  %tmp3 = load i32, i32* %tmp2
  %tmp4 = add i32 %tmp3, %tmp1
  %tmp5 = getelementptr %struct.munger_struct, %struct.munger_struct* %P, i32 0, i32 0
  store i32 %tmp4, i32* %tmp5
  ret void
}

Another example: DLL_meld.cc

Source code