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

[Richard Biener <richard.guenther@gmail.com>]

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).

> 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.

> FWIW, this patch does improve performance on 521.wrf from SPEC2017 on aarch64.

You said that, yes.  Even without -ffast-math?

Richard.

> Thanks,
> Kyrill
>
> >
> > Richard.
> >
> >> midend and optimise. The previous approach of generating the open-coded version of that
> >> is used when we don't have an appropriate __builtin_fmin/max available.
> >> For example, for a configuration of x86_64-unknown-linux-gnu that I tested there was no
> >> 128-bit __built_fminl available.
> >>
> >> With this patch I'm seeing more than 7000 FMINNM/FMAXNM instructions being generated at -O3
> >> on aarch64 for 521.wrf from fprate SPEC2017 where none before were generated
> >> (we were generating explicit comparisons and NaN checks). This gave a 2.4% improvement
> >> in performance on a Cortex-A72.
> >>
> >> Bootstrapped and tested on aarch64-none-linux-gnu and x86_64-unknown-linux-gnu.
> >>
> >> Ok for trunk?
> >> Thanks,
> >> Kyrill
> >>
> >> 2018-07-17  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
> >>
> >>       * f95-lang.c (gfc_init_builtin_functions): Define __builtin_fmin,
> >>       __builtin_fminf, __builtin_fminl, __builtin_fmax, __builtin_fmaxf,
> >>       __builtin_fmaxl.
> >>       * trans-intrinsic.c: Include builtins.h.
> >>       (gfc_conv_intrinsic_minmax): Emit __builtin_fmin/max or MIN/MAX_EXPR
> >>       functions to calculate the min/max.
> >>
> >> 2018-07-17  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
> >>
> >>       * gfortran.dg/max_fmaxf.f90: New test.
> >>       * gfortran.dg/min_fminf.f90: Likewise.
> >>       * gfortran.dg/minmax_integer.f90: Likewise.
> >>       * gfortran.dg/max_fmaxl_aarch64.f90: Likewise.
> >>       * gfortran.dg/min_fminl_aarch64.f90: Likewise.
>