[LLVMdev] Expected behavior of calling bitcasted functions?

[Duncan Sands]

Hi Matthew,

On 30/05/13 02:40, Arsenault, Matthew wrote:
> Hi,
>
> I'm not sure what the expected behavior of calling a bitcasted function is. Suppose you have a case like this (which you get on the source level from attribute alias):
>
> @alias_f32 = alias bitcast (i32 (i32)* @func_i32 to float (float)*)
>
> define internal i32 @func_i32(i32 %v) noinline nounwind {
> entry:
>    ret i32 %v
> }
>
> define void @bitcast_alias_scalar(float* noalias %source, float* noalias %dest) nounwind {
> entry:
>    %arrayidx = getelementptr float* %source, i32 0
>    %tmp2 = load float* %arrayidx, align 8
>    %call = call float @alias_f32(float %tmp2) nounwind
>    %arrayidx8 = getelementptr float* %dest, i32 0
>    store float %call, float* %arrayidx8, align 8
>    ret void
> }
>
> If you run opt -instcombine on this, this transforms into
> define void @bitcast_alias_scalar(float* noalias %source, float* noalias %dest) nounwind {
> entry:
>    %tmp2 = load float* %source, align 8
>    %0 = fptoui float %tmp2 to i32
>    %call = call i32 @func_i32(i32 %0) nounwind
>    %1 = uitofp i32 %call to float
>    store float %1, float* %dest, align 8
>    ret void
> }
>
>
> Note the fptoui / uitofp conversions to the underlying function's argument / return type. I would expect this to bitcast the arguments and call the underlying function. This transformation happens in InstCombiner::transformConstExprCastCall. This kind of thing almost seems intentional though from some of the comments and tests, so I'm not sure what's supposed to be going on. A conversion that changes the bits doesn't make any sense to me.

I would also expect bitcast to be used here.  This may just be a bug.

Ciao, Duncan.