llvm dev - Sep 2021 - [cfe-dev] Should isnan be optimized out in fast-math mode?

On Sep 8, 2021, at 3:27 PM, James Y Knight via llvm-dev <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
<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
<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
<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
<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
<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
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84949> or
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50724
<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
<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
<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
<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
<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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> cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>
> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
<https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev>
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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?


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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