[cfe-dev] [llvm-dev] Should isnan be optimized out in fast-math mode?

[Michael Kruse via cfe-dev]

Am Mo., 13. Sept. 2021 um 11:46 Uhr schrieb Chris Tetreault via
cfe-dev <cfe-dev at lists.llvm.org>:
> As a user, if I read that:
>
>
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> ```
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> if (isnan(x)) {
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> ```
>
>
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> … is guaranteed to work, and I read that fast-math enables the compiler to reason about constructs like `x + 0` being equal to `x`, then I’m going to be very confused when:
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>
>
> ```
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> if (isnan(x + 0)) {
>
> ```
>
>
>
> … does not also work. I’m going to open a bug and complain, and the slide down the slippery slope will continue. You and I understand the difference, and the technical reason why `isnan(x)` is supported but `isnan(x + 0)` isn’t, but Joe Coder just trying to figure out why he’s got NaN in his matrices despite his careful NaN handling code. Joe is not a compiler expert, and on the face of it, it seems like a silly limitation. This will never end until fast-math is gutted.

C/C++ already has cases like this. Pointer arithmetic on null pointers
is undefined behaviour, even if adding[1,2]/subtracting[3] zero. I
don't think it is too far fetched to expect from users to know that an
operation is undefined behaviour even if one of the operands is zero.

Michael

[1] https://github.com/llvm/llvm-project/blob/main/clang/test/Sema/pointer-addition.c
[2] https://github.com/llvm/llvm-project/blob/main/compiler-rt/test/ubsan/TestCases/Pointer/nullptr-and-nonzero-offset-constants.cpp
[3] https://github.com/llvm/llvm-project/blob/main/clang/test/Sema/pointer-subtraction.c


Michael