<div dir="ltr"><div dir="ltr">MSVC documentation says: “Special values (NaN, +infinity, -infinity, -0.0) may not be propagated or behave strictly according to the IEEE-754 standard”. Such exclusion is necessary to apply transformations that are suitable for real numbers only, like `x * 0 -> 0`. NaNs in arithmetic operations propagate from input to output, in most operations if an operand is NaN, the result is also NaN. `isnan` has nothing with NaN propagation, it just makes the check. The documentation does not provide justification for removal of `isnan`.</div><div dir="ltr"><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">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.</blockquote><div><br></div><div>Exactly. Leaving `isnan` in the code makes compiler behavior more consistent and convenient for users. Clang also can go this way.</div><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Do you have a concrete reason why a pragma is unsuitable?</blockquote><div><br></div><div>I described the concerns in the reply to Mehdi Amini's message. </div><div dir="ltr"><br clear="all"><div><div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature">Thanks,<br>--Serge<br></div></div><br></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, Sep 20, 2021 at 11:39 PM Chris Tetreault <<a href="mailto:ctetreau@quicinc.com">ctetreau@quicinc.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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<p class="MsoNormal">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.<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">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?
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<p class="MsoNormal"><b>From:</b> Serge Pavlov <<a href="mailto:sepavloff@gmail.com" target="_blank">sepavloff@gmail.com</a>> <br>
<b>Sent:</b> Monday, September 20, 2021 1:23 AM<br>
<b>To:</b> Mehdi AMINI <<a href="mailto:joker.eph@gmail.com" target="_blank">joker.eph@gmail.com</a>><br>
<b>Cc:</b> Chris Tetreault <<a href="mailto:ctetreau@quicinc.com" target="_blank">ctetreau@quicinc.com</a>>; <a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>; <a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a><br>
<b>Subject:</b> Re: [cfe-dev] [llvm-dev] Should isnan be optimized out in fast-math mode?<u></u><u></u></p>
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<p align="center" style="text-align:center"><strong><span style="font-size:10.5pt;font-family:Arial,sans-serif;color:black;background:yellow">WARNING:</span></strong><span style="font-size:10.5pt;font-family:Arial,sans-serif;color:black;background:yellow">
This email originated from outside of Qualcomm. Please be wary of any links or attachments, and do not enable macros.</span><u></u><u></u></p>
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<p class="MsoNormal">On Fri, Sep 17, 2021 at 11:17 PM Mehdi AMINI <<a href="mailto:joker.eph@gmail.com" target="_blank">joker.eph@gmail.com</a>> wrote:<u></u><u></u></p>
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<p class="MsoNormal">On Thu, Sep 16, 2021 at 11:19 PM Serge Pavlov <<a href="mailto:sepavloff@gmail.com" target="_blank">sepavloff@gmail.com</a>> wrote:<u></u><u></u></p>
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<p class="MsoNormal">On Fri, Sep 17, 2021 at 10:53 AM Mehdi AMINI <<a href="mailto:joker.eph@gmail.com" target="_blank">joker.eph@gmail.com</a>> wrote:<u></u><u></u></p>
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<p class="MsoNormal">On Thu, Sep 16, 2021 at 8:23 PM Serge Pavlov via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>> wrote:<u></u><u></u></p>
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<p class="MsoNormal">On Fri, Sep 17, 2021 at 3:11 AM Chris Tetreault <<a href="mailto:ctetreau@quicinc.com" target="_blank">ctetreau@quicinc.com</a>> wrote:<u></u><u></u></p>
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<p class="MsoNormal">The difference there is that doing pointer arithmetic on null pointers doesn't *usually* work, unless you turn on -ffast-pointers.<br>
<br>
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.<u></u><u></u></p>
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<p class="MsoNormal">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.<u></u><u></u></p>
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<p class="MsoNormal">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.<u></u><u></u></p>
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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.<u></u><u></u></p>
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<p class="MsoNormal">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.<u></u><u></u></p>
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<p class="MsoNormal">Switching mode in a single TU seems valuable, but could this be handled with pragmas or function attributes instead?<u></u><u></u></p>
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<p class="MsoNormal">GCC allows it by using `#pragma GCC optimize()`, but clang does not support it. No suitable function attribute exists for that.<u></u><u></u></p>
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<p class="MsoNormal">Right, I know that clang does not support it, but it could :)<u></u><u></u></p>
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<p class="MsoNormal">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)<u></u><u></u></p>
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<p class="MsoNormal">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.<u></u><u></u></p>
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<p class="MsoNormal">However, in general it does not solve the problem. The most important issue which remains unaddressed is inconsistency of the implementation.<u></u><u></u></p>
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<p class="MsoNormal">The handling of `isnan` in -ffinite-math-only by clang is not consistent because:<u></u><u></u></p>
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<p class="MsoNormal">- It differs from what other compilers do. Namely MSVC and Intel compiler do not throw away `isnan` in this mode: <a href="https://godbolt.org/z/qTaz47qhP" target="_blank">https://godbolt.org/z/qTaz47qhP</a>.<u></u><u></u></p>
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<p class="MsoNormal">- It depends on optimization options. With -O2 the check is removed but with -O0 remains: <a href="https://godbolt.org/z/cjYePv7s7" target="_blank">https://godbolt.org/z/cjYePv7s7</a>. 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).<u></u><u></u></p>
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<p class="MsoNormal">- It is inconsistent with libc implementations. If `isnan` is provided by libc, it is a real check, but the compiler may drop it.<u></u><u></u></p>
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<p class="MsoNormal">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.<u></u><u></u></p>
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<p class="MsoNormal">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 <a href="https://reviews.llvm.org/D69806" target="_blank">https://reviews.llvm.org/D69806</a>.<u></u><u></u></p>
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<p class="MsoNormal">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.<u></u><u></u></p>
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<p class="MsoNormal">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. <u></u><u></u></p>
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<p class="MsoNormal">Thanks,<u></u><u></u></p>
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<p class="MsoNormal">--Serge<u></u><u></u></p>
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