[LLVMdev] Generating GetElementPtr inlined in a function argument list programmatically

[Tim Northover]

Hi Tommy,

> %3 = call i32 @_Z4funcPKc(i8* getelementptr inbounds ([5 x i8]* @.str2, i32
> 0, i32 0))

> - What is this phenomenon called, is it called "inlined/expanded function
> argument"?

This is the syntax used when the getelementptr is a
GetElementPtrConstantExpr rather than an Instruction, and that's
generally true for the inlined forms: if you can make an expression
out of Constants in some way then it'll be "inlined".

> - I'm now generating the aforementioned instructions (non-inlined) with the
> following code snippet with LLVM 3.2. Is there a way to generate "inlined"
> GetElementPtr instructions when calling a function just like the first
> example programmatically?

Yep, the function you want is GetElementPtrConstantExpr::Create.
Incidentally, in these cases the "cpp" backend can be very useful. If
you can write LLVM IR to do what you want it will give you back some
C++ code that emits the IR.

For example:
$ cat simple.ll
@var = global [5 x i32] zeroinitializer
define i32* @foo() {
  ret i32* getelementptr(i32* @var, i32 0, i32 1)
}
$ llc simple.ll -o - -march=cpp
    [...]
    GlobalVariable* gvar_array_var = new GlobalVariable(/*Module=*/*mod, [...]);
    [...]
    std::vector<Constant*> const_ptr_5_indices;
    ConstantInt* const_int32_6 = ConstantInt::get(mod->getContext(),
APInt(32, StringRef("0"), 10));
    const_ptr_5_indices.push_back(const_int32_6);
    ConstantInt* const_int32_7 = ConstantInt::get(mod->getContext(),
APInt(32, StringRef("1"), 10));
    const_ptr_5_indices.push_back(const_int32_7);
    Constant* const_ptr_5 =
ConstantExpr::getGetElementPtr(gvar_array_var, const_ptr_5_indices);
    [...]
    ReturnInst::Create(mod->getContext(), const_ptr_5, label_8);

Obviously the code is auto-generated and usually not the best, but it
usually gives you enough information about what you need to do.

Cheers.

Tim.