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

[ChenZheng via Phabricator via llvm-commits]

shchenz added a comment.

In D116952#3604095 <https://reviews.llvm.org/D116952#3604095>, @nlopes wrote:

> In D116952#3603700 <https://reviews.llvm.org/D116952#3603700>, @shchenz wrote:
>
>> 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?
>
> True, the output isn't great (floats are truncated, hence the 0.000000, which is not what's underneath). But what matters is the final result.
>
>> I know you said online Alive2 is outdated, but seems the online Alive2 gets opposite result for the above 0.000000 case, it verifies `ret i1 0` as the valid transformation.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
>
> The online version if Alive2 doesn't implement the denormal-fp-math attribute.
>
>> 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.
>
> Look at the generated assembly with -O0 without and without -fdenormal-fp-math. There's no difference. So it seems that this flag doesn't guarantee anything (it's best effort) or it's not fully implemented yet.
> Nevertheless, your internal test is wrong. Check the math here: https://en.wikipedia.org/wiki/Double-precision_floating-point_format#Exponent_encoding

Thanks. I need some time to have a better understanding.

So GCC/XLC both returning 0 for the C case is caused by `-fdenormal-fp-math=positive-zero` not implemented or not used in the command line? I tested with clang, without `-fdenormal-fp-math=positive-zero`, it also returns 0 with this patch.


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