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

Aaron Ballman via llvm-dev
Thu, 16 Sep 2021 17:56:13 -0000

On Thu, Sep 16, 2021 at 1:31 PM Serge Pavlov via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
>
> On Thu, Sep 16, 2021 at 10:02 PM Cranmer, Joshua <joshua.cranmer at
intel.com> wrote:
>>
>> I think you are not adequately summing up the risks of your approach;
there are three other issues I see.
>>
>>
>>
>> First, redefining `isnan` as a macro is expressly undefined behavior in
C (see section 7.1.3, clauses 2 and 3—it’s undefined behavior to define a macro
as a same name as reserved identifier in a standard library header).
Conditionally redefining an `isnan` macro is therefore not a permissible
solution.
>
>
> Defining things like `isnan` is the job of libc, which often is not a part
of the compiler.

The standard makes no real distinction between what's the job of the
compiler and what's the job of the library; it's all "the
implementation" as far as the standard is concerned. FWIW, there are
plenty of libc things which are produced by the compiler (see
https://github.com/llvm/llvm-project/tree/main/clang/lib/Headers for
all the standard library interfaces provided by Clang).

> It does not cause undefined behavior by itself.

A user-defined macro named `isnan` is UB per 7.1.3p1 if the user
includes <math.h> in the TU.

> Redefining macro defined in system headers may be harmful if the new macro
is inconsistent with other libc implementation (`errno` comes to mind). So this
looks like a kind of legal disclaimer.

I would hope so; the standard says explicitly that redefining that
macro is UB. :-)

~Aaron

> As with `-ffinite-math-only` the redefinition is used at your own risk. For
our task to remove the call of pure function the risk is negligible. And it is
needed only to reproduce the old behavior.
>
>>
>>
>>
>> The second thing that has been repeatedly brought up that is missing is
the fact that `isnan` may still be inconsistently optimized out. `isnan(x)`
would only be retained in the program if the compiler cannot deduce that `x` is
the result of a nnan arithmetic operation. If it can deduce that—the simplest
case being the somewhat questionable `isnan(x + 0)` example, but it’s also
possible that, e.g., you’re calling `isnan(sum)` on the result of a summation,
which would be the result of an arithmetic expression post-mem2reg/SROA—then the
compiler would still elide it. It could be that this is less surprising to users
than unconditionally optimizing array `isnan(x)`, but it should still be
admitted that there is a potential for surprise here.
>
>
> Regarding your example, this thread already contains consideration of this
case:
>
> On Tue, Sep 14, 2021 at 12:50 AM Serge Pavlov <sepavloff at
gmail.com> wrote:
>>
>> On Mon, Sep 13, 2021 at 11:46 PM Chris Tetreault <ctetreau at
quicinc.com> wrote:
>>>
>>> … is guaranteed to work, and I read that fast-math enables the
compiler to reason about constructs like `x + 0` being equal to `x`, then I’m
going to be very confused when:
>>
>> You are right, this was a bad idea. Compiler may optimize out `isnan`
but only when it deduces that the value cannot be NaN, but not due to the
user's promise. It is especially important for `isinf`. Addition of two
finite values may produce infinity and there is no universal way to predict it.
It is probably not an issue for types like float or double, but ML cores use
halfs or even minifloats, where overflow is much more probable. If in the code:
>> ```
>> float r = a + b;
>> if (isinf(r)) {...
>> ```
>> `isinf` were optimized out just because -ffinite-math-only is in
effect, the user cannot check if overflow did not occur.
>
>
> Rules proposed by Richard are also formulated using arguments, not results.
Now there is no intention to optimize such a case.
>
>>
>>
>> A final point is that the potential optimization benefits of eliding
`isnan` are not limited to the cost of running the function itself (which are
likely to be negligible), but also include the benefits of deleting any
subsequent code that is attempting to handle NaN values, which may be fairly
large blocks. A salient example is complex multiplication and division, where
the actual expansion of  the multiplication and division code itself is dwarfed
by the recalculation code if the result turns out to be a NaN.
>
>
> The code intended for handling NaNs won't be executed in
-ffinite-math-only mode, if the mode is used correctly. So expenses are only the
check itself and the associated jump. For the code that does intensive
calculation they must be negligible. Anyway, the user can redefine `isnan`, it
is as safe as `-ffinite-math-only` itself.
>
>>
>>
>> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of
Serge Pavlov via llvm-dev
>> Sent: Thursday, September 16, 2021 1:37
>> To: Chris Tetreault <ctetreau at quicinc.com>
>> Cc: Arthur O'Dwyer <arthur.j.odwyer at gmail.com>; 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?
>>
>>
>>
>> Let me make some summary. I omit references for brevity, they are
spread in the thread.
>>
>> Treatment of `isnan` with `-ffinite-math-only` has issues:
>> - There are many users' complaints and disagreement expressed in
GCC bug tracker and forums about the treatment.
>> - There are legitimate use cases when `isnan` needs to be called in
`-ffinite-math-only` mode.
>> - Users have to invent workarounds to get functionality of `isnan`,
which results in portability and performance loss.
>> - There is inconsistency with the behavior of libc, which always does a
real check, and the compiler, which omits it.
>> Preserving `isnan` in the code would solve all of them.
>>
>> What is the risk?
>>
>> `-ffinite-math-only` is an optimization option, so preserving `isnan`
cannot break the behavior of correct programs. The only possible negative impact
is some loss of performance. It is unlikely that a real program spends so much
time in `isnan` calls that it has noticeable effect, but if it does, a user can
conditionally redefine `isnan` macro.
>>
>> Preserving `isnan` in `-ffinite-math-only` mode is safe and makes the
compiler more reliable and user-friendly.
>>
>>
>>
>> Thanks,
>> --Serge
>
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev