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

Thu Sep 9 06:29:59 PDT 2021

If we say that the fast-math flags are “enabling optimizations that the presence of nans otherwise prohibits”, then there is no reason for clang to keep calls to “isnan” around, or to keep checks like “fpclassify(x) == it’s_a_nan” unfolded.  These are exactly the types of optimizations that the presence of NaNs would prohibit.

I understand the need for having some NaN-handling preserved in an otherwise finite-math code.  We already have fast-math-related attributes attached to each function in the LLVM IR, so we could introduce a source-level attribute for enabling/disabling these flags per function.

--
Krzysztof Parzyszek  kparzysz at quicinc.com<mailto:kparzysz at quicinc.com>   AI tools development

From: cfe-dev <cfe-dev-bounces at lists.llvm.org> On Behalf Of Chris Lattner via cfe-dev
Sent: Wednesday, September 8, 2021 5:51 PM
To: James Y Knight <jyknight at google.com>
Cc: LLVM Developers <llvm-dev at lists.llvm.org>; Clang Dev <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 Sep 8, 2021, at 3:27 PM, James Y Knight via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

I expressed my strong support for this on the previous thread, but I'll just repost the most important piece...

I believe the proposed semantics from the Clang level ought to be:
  The -ffinite-math-only and -fno-signed-zeros options do not impact the ability to accurately load, store, copy, or pass or return such values from general function calls. They also do not impact any of the "non-computational" and "quiet-computational" IEEE-754 operations, which includes classification functions (fpclassify, signbit, isinf/isnan/etc), sign-modification (copysign, fabs, and negation `-(x)`), as well as the totalorder and totalordermag functions. Those correctly handle NaN, Inf, and signed zeros even when the flags are in effect. These flags do affect the behavior of other expressions and math standard-library calls, as well as comparison operations.

FWIW, I completely agree - these flags are about enabling optimizations that the presence of nans otherwise prohibits.  We shouldn’t take a literal interpretation of an old GCC manual, as that would not be useful.

If we converge on this definition, I think it should be documented.  This is a source of confusion that comes up periodically.

-Chris

I would not expect this to have an actual negative impact on the performance benefit of those flags, since the optimization benefits mainly arise from comparisons and the general computation instructions which are unchanged.

In further support of this position, I note that the previous thread uncovered at least one vendor -- Apple (https://opensource.apple.com/source/Libm/Libm-2026/Source/Intel/math.h.auto.html) -- going out of their way to cause isnan and friends to function properly with -ffast-math enabled.

On Wed, Sep 8, 2021 at 1:02 PM Serge Pavlov via cfe-dev <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:
Hi all,

One of the purposes of `llvm::isnan` was to help preserve the check made by `isnan` if fast-math mode is
specified (https://reviews.llvm.org/D104854). I'd like to describe reason for that and propose to use the behavior
implemented in that patch.

The option `-ffast-math` is often used when performance is important, as it allows a compiler to generate faster code.
This option itself is a collection of different optimization techniques, each having its own option. For this topic only the
option `-ffinite-math-only` is of interest. With it the compiler treats floating point numbers as mathematical real numbers,
so transformations like `0 * x -> 0` become valid.

In clang documentation (https://clang.llvm.org/docs/UsersManual.html#cmdoption-ffast-math) this option is described as:

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

GCC documentation (https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html) is a bit more concrete:

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

**What is the issue?**

C standard defines a macro `isnan`, which can be mapped to an intrinsic function provided by the compiler. For both
clang and gcc it is `__builtin_isnan`. How should this function behave if `-ffinite-math-only` is specified? Should it make a
real check or the compiler can assume that it always returns false?

GCC optimizes out `isnan`. It follows from the viewpoint that (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50724#c1):

    "With -ffinite-math-only you are telling that there are no NaNs and thus GCC optimizes isnan (x) to 0."

Such treatment of `-ffinite-math-only` has sufficient drawbacks. In particular it makes it impossible to check validity of
data: a user cannot write

assert(!isnan(x));

because the compiler replaces the actual function call with its expected value. There are many complaints in GCC bug
tracker (for instance https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84949 or https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50724)
as well as in forums (https://stackoverflow.com/questions/47703436/isnan-does-not-work-correctly-with-ofast-flags or
https://stackoverflow.com/questions/22931147/stdisinf-does-not-work-with-ffast-math-how-to-check-for-infinity). Proposed
solutions are using integer operations to make the check, to turn off `-ffinite-math-only` in some parts of the code or to
ensure that libc function is called. It clearly demonstrates that `isnan` in this case is useless, but users need its functionality
and do not have a proper tool to make required checks. The similar direction was criticized in llvm as well (https://reviews.llvm.org/D18513#387418).

**Why imposing restrictions on floating types is bad?**

If `-ffinite-math-only` modifies properties of `double` type, several issues arise, for instance:
- What should return `std::numeric_limits<double>::has_quiet_NaN()`?
- What body should have this function if it is used in a program where some functions are compiled with `fast-math` and some without?
- Should inlining of a function compiled with `fast-math` to a function compiled without it be prohibited in inliner?
- Should `std::isnan(std::numeric_limits<float>::quiet_NaN())` be true?

If the type `double` cannot have NaN value, it means that `double` and `double` under `-ffinite-math-only` are different types
(https://gcc.gnu.org/pipermail/gcc-patches/2020-April/544641.html). Such a way can solve these problems but it is so expensive
that hardly it has a chance to be realized.

**The solution**

Instead of modifying properties of floating point types, the effect of `-ffinite-math-only` can be expressed as a restriction on
operation usage.  Actually clang and gcc documentation already follows this way. Fast-math flags in llvm IR also are attributes
of instructions. The only question is whether `isnan` and similar functions are floating-point arithmetic.

From a practical viewpoint, treating non-computational functions as arithmetic does not add any advantage. If a code extensively
uses `isnan` (so could profit by their removal), it is likely it is not suitable for -ffinite-math-only. This interpretation however creates
the problems described above. So it is profitable to consider `isnan` and similar functions as non-arithmetical.

**Why is it safe to leave `isnan`?**

The probable concern of this solution is deviation from gcc behavior. There are several reasons why this is not an issue.

1. -ffinite-math-only is an optimization option. A correct program compiled with -ffinite-math-only and without it should behave
   identically, if conditions for using -ffinite-math-only are fulfilled. So making the check cannot break functionality.
2. `isnan` is implemented by libc, which can map it to a compiler builtin or use its own implementation, depending on
   configuration options. `isnan` implemented in libc obviously always does the real check.
3. ICC and MSVC preserve `isnan` in fast-math mode.

The proposal is to not consider `isnan` and other such functions as arithmetic operations and do not optimize them out
just because -ffinite-math-only is specified. Of course, there are cases when `isnan` may be optimized out, for instance,
`isnan(a + b)` may be optimized if -ffinite-math-only is in effect due to the assumption (result of arithmetic operation is not NaN).

What are your opinions?
Thanks,
--Serge
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