[LLVMdev] Expected behavior of calling bitcasted functions?

[Pete Couperus]

Hello,

This is an interesting example.  Whenever I see strange things like this, I
use opt's -lint.
In this case, opt -lint reports:
Undefined behavior: Call return type mismatches callee return type
  %call = call float @alias_f32(float %tmp2) #1

You'll get a similar report when the parameter types mismatch.

Pete



On Wed, May 29, 2013 at 5:40 PM, Arsenault, Matthew <
Matthew.Arsenault at amd.com> 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.
>
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