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

[Nuno Lopes via Phabricator via llvm-commits]

nlopes added a comment.

In D116952#3601819 <https://reviews.llvm.org/D116952#3601819>, @shchenz wrote:

> In D116952#3601629 <https://reviews.llvm.org/D116952#3601629>, @nlopes wrote:
>
>> In D116952#3600716 <https://reviews.llvm.org/D116952#3600716>, @shchenz wrote:
>>
>>>   define zeroext i1 @foo() #0 {
>>>     %_add = fadd fast double 1.264810e-321, 3.789480e-321
>>>     %_res = fcmp fast une double %_add, 5.054290e-321
>>>     ret i1 %_res
>>>   }
>>>   
>>>   attributes #0 = { "denormal-fp-math"="positive-zero" }
>>>
>>>   llc 1.ll -mtriple=powerpc64le-unknown-linux-gnu
>>>
>>> Hi, this patch causes mis-compile for above case, now `%_res` is true while before this patch it is false. We can not handle the denormal constantFP in fcmp? Will the denormal constantFP be in other opcodes as well?
>>
>> Alive says that LLVM is correct:
>>
>>   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
>>   }
>>   Transformation seems to be correct!
>
> Alive 2 says "ERROR: Couldn't prove the correctness of the transformation"...
>
> https://alive2.llvm.org/ce/z/GPLj4Z

The online version is outdated, sorry.

> And from the semantic of the case, `%_add` not equal to `5.054290e-321` should be wrong, (1.264810e-321 + 3.789480e-321 == 5.054290e-321), so we should expect `false` here?

%_add = #x00000000000003ff
Which is a subnormal, so per the function attribute it is changed to +0.0.


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