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

Serge Pavlov via llvm-dev
Thu, 09 Sep 2021 17:34:24 -0000

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>
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> *On Behalf Of
*Serge
> Pavlov via llvm-dev
> *Sent:* Thursday, September 9, 2021 9:53 AM
> *To:* Krzysztof Parzyszek <kparzysz at quicinc.com>
> *Cc:* LLVM Developers <llvm-dev at lists.llvm.org>; 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 links or attachments, and do not enable macros.
>
> On Thu, Sep 9, 2021 at 11:29 PM Krzysztof Parzyszek <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   AI tools development
>
>
>
> *From:* Serge Pavlov <sepavloff at gmail.com>
> *Sent:* Thursday, September 9, 2021 11:10 AM
> *To:* Krzysztof Parzyszek <kparzysz at quicinc.com>
> *Cc:* Chris Lattner <clattner at nondot.org>; James Y Knight <
> jyknight at google.com>; LLVM Developers <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?
>
>
>
> *WARNING:* This email originated from outside of Qualcomm. Please be wary
> of any links or attachments, and do not enable macros.
>
> On Thu, Sep 9, 2021 at 8:30 PM Krzysztof Parzyszek via cfe-dev <
> 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   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 links or attachments, and do not enable macros.
>
> 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)
> -- 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> 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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