[llvm-dev] [cfe-dev] Do we need intrinsics for floating-point classification functions?

[James Y Knight via llvm-dev]

On Thu, Sep 2, 2021 at 3:07 PM Sanjay Patel via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> On Thu, Sep 2, 2021 at 2:34 PM Serge Pavlov <sepavloff at gmail.com> wrote:
>
>> Optimization is not the motivation for this intrinsic. If FP exceptions
>> are ignored, unordered comparison can be used to implement `isnan`. If not,
>> we need to make the test without affecting FP exceptions. Without this
>> intrinsic there is no way to represent such a test.
>>
>
> Here's a way to represent such a test without an intrinsic:
> declare i1 @isnan(double)
> ...
> %r = call i1 isnan(double x) #1
> ...
> attributes #1 = { nounwind }
>

No -- there is no "isnan" standard library call. C "isnan" is a macro which
expands to whatever each libc desires. There is effectively no
standardization as to what library function -- if any -- they actually end
up calling. Therefore, the compiler's __builtin_isnan (and friends) cannot
reasonably rely on generating library calls for these.

Probably the best you could say is that we don't support
__builtin_isnan/etc in strict-fp mode. But that's not a good solution,
either.
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