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

Chris Tetreault via llvm-dev
Thu, 09 Sep 2021 18:03:04 -0000

In this case, I think it’s perfectly reasonable to reinterpret_cast the floats
to uint32_t, and then inspect the bit pattern. Since NaN is being used as a
sentinel value, I assume it’s a known bit pattern, and not just any old NaN.

I think it’s fine that fast-math renders isnan useless. As far as I know, the
C++ standard wasn’t written to account for compilers providing fast-math flags.
fast-math is itself a workaround for “IEEE floats do not behave like actual real
numbers”, so working around a workaround seems reasonable to me.

From: Serge Pavlov <sepavloff at gmail.com>
Sent: Thursday, September 9, 2021 10:34 AM
To: Chris Tetreault <ctetreau at quicinc.com>
Cc: Krzysztof Parzyszek <kparzysz at quicinc.com>; cfe-dev at
lists.llvm.org; llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] [cfe-dev] Should isnan be optimized out in fast-math
mode?


WARNING: This email originated from outside of Qualcomm. Please be wary of any
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Let me describe a real life example.

There is a realtime program that processes float values from a huge array.
Calculations do not produce NaNs and do not expect them. Using
-ffinite-math-only substantially speeds up the program, so it is highly
desirable to use it. The problem is that the array contains NaNs, they mark
elements that should not be processed.

An obvious solution is to check an element for NaN, and if it is not, process
it. Now there is no clean way to do so. Only workarounds, like using integer
arithmetics. The function 'isnan' became useless. And there are many
cases when users complain of this optimization.

Thanks,
--Serge


On Fri, Sep 10, 2021 at 12:09 AM Chris Tetreault <ctetreau at
quicinc.com<mailto:ctetreau at quicinc.com>> wrote:
If the issue is that users want their asserts to fire, then they should be
encouraged to only enable fast math in release builds.

From: llvm-dev <llvm-dev-bounces at lists.llvm.org<mailto:llvm-dev-bounces
at lists.llvm.org>> On Behalf Of Serge Pavlov via llvm-dev
Sent: Thursday, September 9, 2021 9:53 AM
To: Krzysztof Parzyszek <kparzysz at quicinc.com<mailto:kparzysz at
quicinc.com>>
Cc: LLVM Developers <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: [llvm-dev] [cfe-dev] Should isnan be optimized out in fast-math
mode?


WARNING: This email originated from outside of Qualcomm. Please be wary of any
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On Thu, Sep 9, 2021 at 11:29 PM Krzysztof Parzyszek <kparzysz at
quicinc.com<mailto:kparzysz at quicinc.com>> wrote:
This goes back to what these options actually imply.  The interpretation that I
favor is “this code will never see a NaN”, or “the program can assume that no
floating point expression will evaluate to a NaN”.  The benefit of that is that
it’s intuitively clear.  In that case “isnan(x)” is false, because x cannot be a
NaN.  There is no distinction between “isnan(x+x)” and “isnan(x)”.  If the user
wants to preserve “isnan(x)”, they can apply some pragma (which clang may
actually have already).

It is apparent simplicity. As the discussion in gcc mail list demonstrated
(https://gcc.gnu.org/pipermail/gcc-patches/2020-April/544641.html) this is
actually an impromissing way. From a practical viewpoint it is also a bad
solution as users cannot even check the assertions.


To be honest, I’m not sure that I understand your argument.  Are you saying that
under your interpretation we could optimize “isnan(x+x) -> false”, but not
“isnan(x) -> false”?

Argument of `isnan(x+x)` is a result of arithmetic operation. According to the
meaning of -ffinite-math-only it cannot produce NaN. So this call can be
optimized out. In the general case `isnan(x)` value may be, say, loaded from
memory. Load is not an arithmetic operation, so nothing prevents from loading
NaN. Optimizing the call out is dangerous in this case.



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

From: Serge Pavlov <sepavloff at gmail.com<mailto:sepavloff at
gmail.com>>
Sent: Thursday, September 9, 2021 11:10 AM
To: Krzysztof Parzyszek <kparzysz at quicinc.com<mailto:kparzysz at
quicinc.com>>
Cc: Chris Lattner <clattner at nondot.org<mailto:clattner at
nondot.org>>; James Y Knight <jyknight at google.com<mailto:jyknight
at google.com>>; LLVM Developers <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?


WARNING: This email originated from outside of Qualcomm. Please be wary of any
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On Thu, Sep 9, 2021 at 8:30 PM Krzysztof Parzyszek via cfe-dev <cfe-dev at
lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:
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.

Transformation 'x * 0 -> 0' is an optimization allowed in the absence
of nans as arguments, because it produces a program that behaves identically
under the given restrictions. Replacement of `isnan(x + x)` is also an
optimization under the same restrictions. Replacement of `isnan(x)` in general
case is not, because we cannot assume that x cannot be a NaN.


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.

GCC allows using `#pragma GCC optimize ("finite-math-only")` or
`#pragma GCC optimize ("no-finite-math-only")` to enable/disable
optimization per function basis. Clang could support this pragmf or maybe
`#pragma clang fp` can be extended to support similar functionality.



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

From: cfe-dev <cfe-dev-bounces at lists.llvm.org<mailto: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<mailto:jyknight at
google.com>>
Cc: LLVM Developers <llvm-dev at lists.llvm.org<mailto:llvm-dev at
lists.llvm.org>>; Clang Dev <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?


WARNING: This email originated from outside of Qualcomm. Please be wary of any
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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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