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

[Chris Tetreault via cfe-dev]

The difference there is that doing pointer arithmetic on null pointers doesn't *usually* work, unless you turn on -ffast-pointers.

It seems to me that  most confusion related to -ffast-math is likely caused by people who are transitioning to using it. I have some codebase, and I turn on fast math, and then a few months down the road I notice a strangeness that I did not catch during the initial transition period. If you're writing new code with fast-math, you don't do things like try to use NaN as a sentinel value in a TU with fast math turned on. This is the sort of thing you catch when you try to transition an existing codebase. Forgive me for the uncharitable interpretation, but it's much easier to ask the compiler to change to accommodate your use case than it is to refactor your code.

To me, I think Mehdi had the best solution: The algorithm that is the bottleneck, and experiences the huge speedup using fast-math should be separated into its own source file. This source file, and only this source file should be compiled with fast-math. The outer driver loop should not be compiled with fast math. This solution is clean, (probably) easy, and doesn't require a change in the compiler. Changing the compiler is hard, affects everybody who uses the compiler, and creates inconsistency in behavior between clang and gcc (and msvc with /fp:fast), and clang and old versions of clang. The behavior of fast-math with respect to NaN is consistent across the mainstream c/c++ compilers: no promises are made, users should not assume that they can use it for anything. Changing it now would create a major portability issue for user codebases, which in and of itself is a very strong reason to not make this change.

If the behavior is confusing to users, that's because it's poorly explained. Honestly, I think the docs are pretty clear, but "It's clear, you just need to learn to read" is never an acceptable answer so it could certainly be improved. This is the only thing that needs to be fixed in my opinion.

Thanks,
   Chris Tetreault

-----Original Message-----
From: Michael Kruse <cfe-dev at meinersbur.de> 
Sent: Thursday, September 16, 2021 12:29 PM
To: Chris Tetreault <ctetreau at quicinc.com>
Cc: Serge Pavlov <sepavloff at gmail.com>; llvm-dev at lists.llvm.org; cfe-dev at lists.llvm.org
Subject: Re: [cfe-dev] [llvm-dev] Should isnan be optimized out in fast-math mode?

WARNING: This email originated from outside of Qualcomm. Please be wary of any links or attachments, and do not enable macros.

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:
>
>
>
> ```
>
> if (isnan(x)) {
>
> ```
>
>
>
> … 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:
>
>
>
> ```
>
> 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