[llvm-dev] Is it a valid fp transformation?

[Sanjay Patel via llvm-dev]

Looks broken to me; I don't think there's UB in the original program.

The fold in visitFAdd() should check if the sitofp is guaranteed to produce
an exact result? Ie, if the int value input to the sitofp could possibly be
different when converted back using fptosi, then the transform does not
work.

define float @test(i32 %x) {
  %mul = mul i32 %x, 58
  %conv = sitofp i32 %mul to float
  %add = fadd float %conv, 1.0
  ret float %add
}


On Mon, Mar 20, 2017 at 4:25 AM, Artur Pilipenko <apilipenko at azul.com>
wrote:

> This C program produces different results with -O0 and -O3 optimization
> levels.
>
> #include <stdio.h>
> float test(unsigned int arg) {
>     return (float)((int)(arg * 58)) + 1;
> }
> int main() {
>     printf("%d\n", (int)test((unsigned int)-831710640));
> }
>
> O0 result is -994576896
> O3 result is -994576832
>
> It happens because LLVM (specifically instcombine) does the following
> transformation:
> (float)x + 1.0 => (float)(x + 1)
>
> For some values the expression before and after yield different results:
>                   x = -994576864
>          (float)x = -994576896.000000
> (float)x + 1.0 = -994576896.000000
> (float)(x + 1) = -994576832.000000
>
> I’m curious if this is a correct transformation and why.
>
> Artur
>
>
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