Re: [PATCH]Use MIN/MAX_EXPR for intrinsics or __builtin_fmin/max when appropriate

[Richard Sandiford <richard dot sandiford at arm dot com>]

Richard Biener <richard.guenther@gmail.com> writes:
> On Wed, Jul 18, 2018 at 11:50 AM Kyrill Tkachov
> <kyrylo.tkachov@foss.arm.com> wrote:
>>
>>
>> On 18/07/18 10:44, Richard Biener wrote:
>> > On Tue, Jul 17, 2018 at 3:46 PM Kyrill Tkachov
>> > <kyrylo.tkachov@foss.arm.com> wrote:
>> >> Hi Richard,
>> >>
>> >> On 17/07/18 14:27, Richard Biener wrote:
>> >>> On Tue, Jul 17, 2018 at 2:35 PM Kyrill Tkachov
>> >>> <kyrylo.tkachov@foss.arm.com> wrote:
>> >>>> Hi all,
>> >>>>
>> >>>> This is my first Fortran patch, so apologies if I'm missing something.
>> >>>> The current expansion of the min and max intrinsics explicitly expands
>> >>>> the comparisons between each argument to calculate the global min/max.
>> >>>> Some targets, like aarch64, have instructions that can calculate
>> >>>> the min/max
>> >>>> of two real (floating-point) numbers with the proper NaN-handling
>> >>>> semantics
>> >>>> (if both inputs are NaN, return Nan. If one is NaN, return the
>> >>>> other) and those
>> >>>> are the semantics provided by the __builtin_fmin/max family of
>> >>>> functions that expand
>> >>>> to these instructions.
>> >>>>
>> >>>> This patch makes the frontend emit __builtin_fmin/max directly to
>> >>>> compare each
>> >>>> pair of numbers when the numbers are floating-point, and use
>> >>>> MIN_EXPR/MAX_EXPR otherwise
>> >>>> (integral types and -ffast-math) which should hopefully be easier
>> >>>> to recognise in the
>> >>> What is Fortrans requirement on min/max intrinsics?  Doesn't it only
>> >>> require things that
>> >>> are guaranteed by MIN/MAX_EXPR anyways?  The only restriction here is
>> >> The current implementation expands to:
>> >>       mvar = a1;
>> >>       if (a2 .op. mvar || isnan (mvar))
>> >>         mvar = a2;
>> >>       if (a3 .op. mvar || isnan (mvar))
>> >>         mvar = a3;
>> >>       ...
>> >>       return mvar;
>> >>
>> >> That is, if one of the operands is a NaN it will return the other argument.
>> >> If both (all) are NaNs, it will return NaN. This is the same as the semantics of fmin/max
>> >> as far as I can tell.
>> >>
>> >>> /* Minimum and maximum values.  When used with floating point, if both
>> >>>      operands are zeros, or if either operand is NaN, then it is
>> >>> unspecified
>> >>>      which of the two operands is returned as the result.  */
>> >>>
>> >>> which means MIN/MAX_EXPR are not strictly IEEE compliant with signed
>> >>> zeros or NaNs.
>> >>> Thus the correct test would be !HONOR_SIGNED_ZEROS && !HONOR_NANS
>> >>> if singed
>> >>> zeros are significant.
>> >> True, MIN/MAX_EXPR would not be appropriate in that condition. I
>> >> guarded their use
>> >> on !HONOR_NANS (type) only. I'll update it to !HONOR_SIGNED_ZEROS
>> >> (type) && !HONOR_NANS (type).
>> >>
>> >>
>> >>> I'm not sure if using fmin/max calls when we cannot use MIN/MAX_EXPR
>> >>> is a good idea,
>> >>> this may both generate bigger code and be slower.
>> >> The patch will generate fmin/fmax calls (or the fminf,fminl
>> >> variants) when mathfn_built_in advertises
>> >> them as available (does that mean they'll have a fast inline
>> >> implementation?)
>> > This doesn't mean anything given you make them available with your
>> > patch ;)  So I expect it may
>> > cause issues for !c99_runtime targets (and long double at least).
>>
>> Urgh, that can cause headaches...
>>
>> >> If the above doesn't hold and we can't use either MIN/MAX_EXPR of
>> >> fmin/fmax then the patch falls back
>> >> to the existing expansion.
>> > As said I would not use fmin/fmax calls here at all.
>>
>> ... Given the comments from Thomas and Janne, maybe we should just
>> emit MIN/MAX_EXPRs here
>> since there is no language requirement on NaN/signed zero handling on
>> these intrinsics?
>> That should make it simpler and more portable.
>
> That's fortran maintainers call.
>
>> >> FWIW, this patch does improve performance on 521.wrf from SPEC2017
>> >> on aarch64.
>> > You said that, yes.  Even without -ffast-math?
>>
>> It improves at -O3 without -ffast-math in particular. With -ffast-math
>> phiopt optimisation
>> is more aggressive and merges the conditionals into MIN/MAX_EXPRs
>> (minmax_replacement in tree-ssa-phiopt.c)
>
> The question is will it be slower without -ffast-math, that is, when
> fmin/max() calls are emitted rather
> than inline conditionals.
>
> I think a patch just using MAX/MIN_EXPR within the existing
> constraints and otherwise falling back to
> the current code would be more obvious and other changes should be
> mande independently.

If going to MIN_EXPR and MAX_EXPR unconditionally isn't acceptable,
maybe an alternative would be to go straight to internal functions,
under the usual:

  direct_internal_fn_supported_p (IFN_F{MIN,MAX}, type, OPTIMIZE_FOR_SPEED)

condition.

Thanks,
Richard