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

[Chris Tetreault via llvm-dev]

You’re confusing implementation details (you have a Godbolt link that shows that MSVC just happens to not remove the isnan call) with documented behavior (I provided a link to the MSVC docs that shows that no promises are made with respect to NaN). The fact is that no compiler (Maybe ICC does, I don’t know, I haven’t checked. I bet their docs say something similar to MSVC, clang, and GCC though.) guarantees that isnan(x) will not be optimized out with fast-math enabled. There is no inconsistency: all the compilers document that they are free to optimize as if there were no NaNs, and they then do whatever is best for their implementation. If you think this is inconsistent, then let me tell you about that time I dereferenced a null pointer and it didn’t segfault.

Now, many people have suggested in this thread that a pragma be added. I personally fully support this proposal. I think it’s a very clean solution, and any non-trivial portable codebase probably already has a library of preprocessor macros that abstract this sort of thing. Do you have a concrete reason why a pragma is unsuitable?

From: Serge Pavlov <sepavloff at gmail.com>
Sent: Monday, September 20, 2021 1:23 AM
To: Mehdi AMINI <joker.eph at gmail.com>
Cc: Chris Tetreault <ctetreau at quicinc.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?


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On Fri, Sep 17, 2021 at 11:17 PM Mehdi AMINI <joker.eph at gmail.com<mailto:joker.eph at gmail.com>> wrote:
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)

It could mitigate the problem if it were implemented. A user who needs to handle NaNs in -ffinite-math-only compilation and writes the code from scratch could use this facility to get things working. I also think such pragma, implemented with enough degree of flexibility, could be useful irrespective of this topic.

However, in general it does not solve the problem. The most important issue which remains unaddressed is inconsistency of the implementation.

The handling of `isnan` in -ffinite-math-only by clang is not consistent because:
- It differs from what other compilers do. Namely MSVC and Intel compiler do not throw away `isnan` in this mode: https://godbolt.org/z/qTaz47qhP.
- It depends on optimization options. With -O2 the check is removed but with -O0 remains: https://godbolt.org/z/cjYePv7s7. Other options also can affect the behavior, for example with `-ffp-model=strict` the check is generated irrespective of the optimization mode (see the same link).
- It is inconsistent with libc implementations. If `isnan` is provided by libc, it is a real check, but the compiler may drop it.
It would not be an issue if `isnan` removal were just an optimization. It however changes semantics in the presence of NaNs, so such removal can break user code.

In the typical use case a user puts a call to `isnan` to ensure no operations on NaNs occur. The call can also be present in some header that implements some functionality for the general case. It may work because `isnan` is provided by libc. Later on when configuration changes or libc is updated the code may be broken, because implementation of `isnan` changes, as it happened after https://reviews.llvm.org/D69806.

If clang kept calls to `isnan`, it would be consistent with ICC and MSVC and with all libc implementations. The behavior would be different from gcc, but clang would be on the winning side, because the number of programs that work with clang would be larger.

Also if we agree that NaNs can appear in the code compiled with -ffinite-math-only, there must be a way to check if a number is a NaN.

Thanks,
--Serge
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