[LLVMdev] Expected behavior of calling bitcasted functions?

[Arsenault, Matthew]

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.