[PATCH] D116952: [ConstantFolding] Respect denormal handling mode attributes when folding instructions

[ChenZheng via Phabricator via llvm-commits]

shchenz added a comment.

Hi @nlopes, thanks for providing the useful info. However I am still not very clear about how to deal with our internal failure after this patch.

  define i1 @foo() denormal-fp-math=positive-zero,positive-zero {
    %_add = fadd double 0.000000, 0.000000, exceptions=ignore
    %_res = fcmp une double %_add, 0.000000
    ret i1 %_res
  }
  =>
  define i1 @foo() noread nowrite nofree willreturn denormal-fp-math=positive-zero,positive-zero {
    ret i1 1
  }

The alive result is very confusing. I don't understand why `0.000000 + 0.000000` != `0x000000` when  `denormal-fp-math=positive-zero`, could you help to explain?
I know you said online Alive2 is outdated, but seems the online Alive2 gets opposite result for the above 0.000000 case, it verifies `ret i1 0` as the valid transformation. https://alive2.llvm.org/ce/z/PjhR3U

There is a C case too:

  int main(void)
  {
    double a = 1.264810e-321;
    double b = 3.789480e-321;
  
    return (a + b != 5.054290e-321);
  }

`clang 1.c -Ofast -fdenormal-fp-math=positive-zero` without this patch, it gets 0 and with this patch, it gets 1. Our internal test expects 0 here.

I also tested above case with XLC(`-Ofast -qnostrict`)/GCC(`-Ofast -funsafe-math-optimizations`) on PowerPC, they both get 0.

Could you please tell me what's wrong with our internal failure? Thanks in advance. @nlopes @dcandler


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https://reviews.llvm.org/D116952