[LLVMdev] Chatty C++API code generation

[Reid Kleckner]

Yes, the C++ API LLVM IR "backend" is pretty verbose. It doesn't use
IRBuilder, which is the preferred IR generation interface these days.

However, the "load" in your example is required. All LLVM based compilers
model user variables as objects in memory. In order to pass a variable to a
function, you have to load it from memory first.

On Tue, Mar 10, 2015 at 11:33 AM, Alexander Poddey <alexander.poddey at gmx.net
> wrote:

> Hi all,
>
> when I have c code like
> --- c code -------------
>  struct stest {         /* deklariert den Strukturtyp person */
>     int   age;
>     float weight;
>  } foo={44,67.2};            /* deklariert Variable des Typs person */
>
>
>
> int main() {
>
>    callAFunction(foo.weight);
>
> ------------------------
>
> The generated c++API code to me seems to be too chatty in the sense that
> the foo.weight's data is copied to a local variable using a load
> instruction.
> The call instruction is then using the local copy.
>
> --- API code ---------------
> ...
> std::vector<Constant*> const_ptr_113_indices;
> const_ptr_113_indices.push_back(const_int32_103);
> const_ptr_113_indices.push_back(const_int32_104);
> Constant* const_ptr_113 = ConstantExpr::getGetElementPtr(gvar_struct_adam,
> const_ptr_113_indices);
>
> //here we have const_ptr_113; why not use it in theCallInst?
>
>  LoadInst* float_130 = new LoadInst(const_ptr_113, "", false,
> label_entry_121);
>  float_130->setAlignment(4);
>
>  CallInst* float_call = CallInst::Create(func__Z4fib3f, float_130, "call",
> label_entry_121);
> ------------------------------------------------
>
>
> To me, this ll code seems to relate to something like
> int main() {
>    float localfloat=foo.weight;
>    callAFunction(localfloat);
>
> This is distinct from the original code...
>
> Do I have a missunderstanding?
> Thanks
> Alexander
>
>
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