[LLVMdev] Assignment of large objects, optimization?

[Patrik Hägglund H]

Hi,

My fronted generates (bad) code, which I see that LLVM is unable to optimize. For example, code similar to:

%a = type [32 x i16]

declare void @set_obj(%a*)
declare void @use_obj(%a*)

define void @foo() {
entry:
  %a1 = alloca %a
  %a2 = alloca %a
  call void @set_obj(%a* %a2)
  %a3 = load %a* %a2
  store %a %a3, %a* %a1
  call void @use_obj(%a* %a1)
  ret void
}

(Or with load/store replaced with memcpy).

In C pseudo-code this is similar to:

a a1;
a a2 = set_obj();
a1 = a2;
use_obj(a1);

and the corresponding LLVM IR in foo() can be simplified to:

  %a1 = alloca %a
  call void @set_obj(%a* %a1)
  call void @use_obj(%a* %a1)

Is it unreasonable to expect LLVM to do this kind of simplifications?

On a side note: Why isn't there an assignment operator in the LLVM IR? Other compilers I have seen have some kind of assignment operator in the IR.

/Patrik Hägglund


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