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

Fri Sep 17 10:13:41 PDT 2021

If we’re suggesting supporting this sort of thing with a pragma, rather than the #pragma gcc optimize approach, I’d suggest something more akin to the C standard’s #pragma STDC FP_CONTRACT or #pragma STDC CX_LIMITED_RANGE support.

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Mehdi AMINI via llvm-dev
Sent: Friday, September 17, 2021 12:17
To: Serge Pavlov <sepavloff at gmail.com>
Cc: llvm-dev at lists.llvm.org; cfe-dev at lists.llvm.org
Subject: Re: [llvm-dev] [cfe-dev] Should isnan be optimized out in fast-math mode?

On Thu, Sep 16, 2021 at 11:19 PM Serge Pavlov <sepavloff at gmail.com<mailto:sepavloff at gmail.com>> wrote:
On Fri, Sep 17, 2021 at 10:53 AM Mehdi AMINI <joker.eph at gmail.com<mailto:joker.eph at gmail.com>> wrote:

On Thu, Sep 16, 2021 at 8:23 PM Serge Pavlov via cfe-dev <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:
On Fri, Sep 17, 2021 at 3:11 AM Chris Tetreault <ctetreau at quicinc.com<mailto:ctetreau at quicinc.com>> wrote:
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.

It is a common way to explain problems with -ffinite-math-only by user ignorance. However user misunderstandings and complaints may indicate a flaw in compiler implementation, which I believe we have in this case.

Using NaN as sentinels is a natural way when you cannot spend extra memory for keeping flags for each item, spend extra cycles to read that flag and do not want to pollute cache. It does not depend on reading documentation or writing the code from scratch. It is simply the best solution for storing data. If performance of the data processing is critical, -ffast-math is a good solution. This is a fairly legitimate use case. The fact that the compiler does not allow it is a compiler drawback.

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.

It is a workaround, it works in some cases but does not in others. ML kernel often is a single translation unit, there may be no such thing as linker for that processor. At the same time it is computation intensive and using fast-math in it may be very profitable.

Switching mode in a single TU seems valuable, but could this be handled with pragmas or function attributes instead?

GCC allows it by using `#pragma GCC optimize()`, but clang does not support it. No suitable function attribute exists for that.

Right, I know that clang does not support it, but it could :)
So since we're looking at what provides the best user-experience: isn't that it? Shouldn't we look into providing this level of granularity? (whether function-level or finer grain)

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.

ICC and MSVC do not remove `isnan` in fast math mode. If `isnan` is implemented in libc, it is also a real check. Actually removing `isnan` creates inconsistency.

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.

Removing `isnan` is only an optimization, it does not intend to change semantics. So it cannot create portability issues. Quite the contrary, it helps portability by making behavior consistent between compilers and libc implementations. The only possible issue is performance loss, this is discussed above, it is an unlikely case. Anyway, if such loss exists and it is absolutely intolerable for a user, a hack with redefinition of `isnan` restores the previous code generation.

If the behavior is confusing to users, that's because it's poorly explained.

What is confusing? That the explicitly written call to a function is not removed? According to user feedback it is the silent removal of `isnan` that confuses users.

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.

The documentation says about -ffinite-math-only:

 "Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs."

Is it clear whether `isnan` is arithmetic or not?

If the result of a floating point arithmetic is fed directly to `isnan()`, are we allowed to eliminate the computation and fold the check to none? (seems like it according to the sentence you're quoting).
Are we back to `isnan(x+0.0)` can be folded but not `isnan(x)`?

Initially there was such intention, but during the discussion it became clear that it is not profitable. For `isinf` there is a realistic use case when removing the call `isinf(a+b)` is an issue. Arthur O'Dwyer demonstrated an example for `isnan`. It is harder to provide guarantees about the result, than about the arguments.

--
Mehdi

Thanks,
   Chris Tetreault

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

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Am Mo., 13. Sept. 2021 um 11:46 Uhr schrieb Chris Tetreault via cfe-dev <cfe-dev at lists.llvm.org<mailto: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
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