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

Serge Pavlov via cfe-dev cfe-dev at lists.llvm.org
Thu Sep 16 20:23:03 PDT 2021

On Fri, Sep 17, 2021 at 3:11 AM Chris Tetreault <ctetreau at quicinc.com>

> 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.

> 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?

> 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
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