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

[Kyrill Tkachov <kyrylo dot tkachov at foss dot arm dot com>]

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.

> > 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)

Thanks,
Kyrill

> 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.