Re: [PATCH] i386: Enable small loop unrolling for O2

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

On Fri, Oct 28, 2022 at 10:08 AM Hongyu Wang <wwwhhhyyy333@gmail.com> wrote:
>
> > Ugh, that's all quite ugly and unmaintainable, no?
> Agreed, I have the same feeling.
>
> > I'm quite sure that if this works it's not by intention.  Doesn't this
> > also disable
> > register renaming and web when the user explicitely specifies -funroll-loops?
> >
> > Doesn't this change -funroll-loops behavior everywhere, only unrolling small
> > loops?
>
> The ugly part ensures that -funroll-loops would not be affected at all
> by -munroll-only-small-loops.
>
> >
> > I'd like to see a -munroll-only-small-loops addition that doesn't have any such
> > effects.  Note RTL unrolling could also
> > conditionally enabled on a new -funroll-small-loops which wouldn't enable
> > register renaming or web.
>
> Did you mean something like
>
> index b9e07973dd6..b707d4afb84 100644
> --- a/gcc/loop-init.cc
> +++ b/gcc/loop-init.cc
> @@ -567,7 +567,8 @@ public:
>    /* opt_pass methods: */
>    bool gate (function *) final override
>      {
> -      return (flag_unroll_loops || flag_unroll_all_loops || cfun->has_unroll);
> +      return (flag_unroll_loops || flag_unroll_all_loops || cfun->has_unroll
> +             || flag_unroll_only_small_loops);
>      }
>
> then the backend can turn it on by default in O2?
> I don't know if there is a way to turn on middle-end pass by
> target-specific flags.

There isn't, it would need to be a target hook.  Currently only i386, rs6000
and s390 have loop_unroll_adjust.  We could enable the pass conditional
on implementing that hook (and optimize >= 2, hopefully the pass only
unrolls loops that are optimized for speed)?

>
> Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> 于2022年10月28日周五 15:33写道：
> >
> > On Wed, Oct 26, 2022 at 7:53 AM Hongyu Wang <hongyu.wang@intel.com> wrote:
> > >
> > > Hi,
> > >
> > > Inspired by rs6000 and s390 port changes, this patch
> > > enables loop unrolling for small size loop at O2 by default.
> > > The default behavior is to unroll loop with unknown trip-count and
> > > less than 4 insns by 1 time.
> > >
> > > This improves 548.exchange2 by 3.5% on icelake and 6% on zen3 with
> > > 1.2% codesize increment. For other benchmarks the variants are minor
> > > and overall codesize increased by 0.2%.
> > >
> > > The kernel image size increased by 0.06%, and no impact on eembc.
> > >
> > > Bootstrapped & regrtested on x86_64-pc-linux-gnu.
> > >
> > > Ok for trunk?
> > >
> > > gcc/ChangeLog:
> > >
> > >         * common/config/i386/i386-common.cc (ix86_optimization_table):
> > >         Enable loop unroll and small loop unroll at O2 by default.
> > >         * config/i386/i386-options.cc
> > >         (ix86_override_options_after_change):
> > >         Disable small loop unroll when funroll-loops enabled, reset
> > >         cunroll_grow_size when it is not explicitly enabled.
> > >         (ix86_option_override_internal): Call
> > >         ix86_override_options_after_change instead of calling
> > >         ix86_recompute_optlev_based_flags and ix86_default_align
> > >         separately.
> > >         * config/i386/i386.cc (ix86_loop_unroll_adjust): Adjust unroll
> > >         factor if -munroll-only-small-loops enabled.
> > >         * config/i386/i386.opt: Add -munroll-only-small-loops,
> > >         -param=x86-small-unroll-ninsns= for loop insn limit,
> > >         -param=x86-small-unroll-factor= for unroll factor.
> > >         * doc/invoke.texi: Document -munroll-only-small-loops,
> > >         x86-small-unroll-ninsns and x86-small-unroll-factor.
> > >
> > > gcc/testsuite/ChangeLog:
> > >
> > >         * gcc.target/i386/pr86270.c: Add -mno-unroll-only-small-loops.
> > >         * gcc.target/i386/pr93002.c: Likewise.
> > > ---
> > >  gcc/common/config/i386/i386-common.cc   |  6 ++++
> > >  gcc/config/i386/i386-options.cc         | 40 ++++++++++++++++++++++---
> > >  gcc/config/i386/i386.cc                 | 13 ++++++++
> > >  gcc/config/i386/i386.opt                | 13 ++++++++
> > >  gcc/doc/invoke.texi                     | 14 +++++++++
> > >  gcc/testsuite/gcc.target/i386/pr86270.c |  2 +-
> > >  gcc/testsuite/gcc.target/i386/pr93002.c |  2 +-
> > >  7 files changed, 84 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/gcc/common/config/i386/i386-common.cc b/gcc/common/config/i386/i386-common.cc
> > > index d6a68dc9b1d..0e580b39d14 100644
> > > --- a/gcc/common/config/i386/i386-common.cc
> > > +++ b/gcc/common/config/i386/i386-common.cc
> > > @@ -1686,6 +1686,12 @@ static const struct default_options ix86_option_optimization_table[] =
> > >      /* The STC algorithm produces the smallest code at -Os, for x86.  */
> > >      { OPT_LEVELS_2_PLUS, OPT_freorder_blocks_algorithm_, NULL,
> > >        REORDER_BLOCKS_ALGORITHM_STC },
> > > +    { OPT_LEVELS_2_PLUS_SPEED_ONLY, OPT_funroll_loops, NULL, 1 },
> > > +    { OPT_LEVELS_2_PLUS_SPEED_ONLY, OPT_munroll_only_small_loops, NULL, 1 },
> > > +    /* Turns off -frename-registers and -fweb which are enabled by
> > > +       funroll-loops.  */
> > > +    { OPT_LEVELS_ALL, OPT_frename_registers, NULL, 0 },
> > > +    { OPT_LEVELS_ALL, OPT_fweb, NULL, 0 },
> >
> > I'm quite sure that if this works it's not by intention.  Doesn't this
> > also disable
> > register renaming and web when the user explicitely specifies -funroll-loops?
> >
> > Doesn't this change -funroll-loops behavior everywhere, only unrolling small
> > loops?
> >
> > I'd like to see a -munroll-only-small-loops addition that doesn't have any such
> > effects.  Note RTL unrolling could also
> > conditionally enabled on a new -funroll-small-loops which wouldn't enable
> > register renaming or web.
> >
> > >      /* Turn off -fschedule-insns by default.  It tends to make the
> > >         problem with not enough registers even worse.  */
> > >      { OPT_LEVELS_ALL, OPT_fschedule_insns, NULL, 0 },
> > > diff --git a/gcc/config/i386/i386-options.cc b/gcc/config/i386/i386-options.cc
> > > index acb2291e70f..6ea347c32e1 100644
> > > --- a/gcc/config/i386/i386-options.cc
> > > +++ b/gcc/config/i386/i386-options.cc
> > > @@ -1819,8 +1819,43 @@ ix86_recompute_optlev_based_flags (struct gcc_options *opts,
> > >  void
> > >  ix86_override_options_after_change (void)
> > >  {
> > > +  /* Default align_* from the processor table.  */
> > >    ix86_default_align (&global_options);
> > > +
> > >    ix86_recompute_optlev_based_flags (&global_options, &global_options_set);
> > > +
> > > +  /* Disable unrolling small loops when there's explicit
> > > +     -f{,no}unroll-loop.  */
> > > +  if ((OPTION_SET_P (flag_unroll_loops))
> > > +     || (OPTION_SET_P (flag_unroll_all_loops)
> > > +        && flag_unroll_all_loops))
> > > +    {
> > > +      if (!OPTION_SET_P (ix86_unroll_only_small_loops))
> > > +       ix86_unroll_only_small_loops = 0;
> > > +      /* Re-enable -frename-registers and -fweb if funroll-loops
> > > +        enabled.  */
> > > +      if (!OPTION_SET_P (flag_web))
> > > +       flag_web = flag_unroll_loops;
> > > +      if (!OPTION_SET_P (flag_rename_registers))
> > > +       flag_rename_registers = flag_unroll_loops;
> > > +      if (!OPTION_SET_P (flag_cunroll_grow_size))
> > > +       flag_cunroll_grow_size = flag_unroll_loops
> > > +                                || flag_peel_loops
> > > +                                || optimize >= 3;
> > > +    }
> > > +  else
> > > +    {
> > > +      if (!OPTION_SET_P (flag_cunroll_grow_size))
> > > +       flag_cunroll_grow_size = flag_peel_loops || optimize >= 3;
> > > +      /* Disables loop unrolling if -mno-unroll-only-small-loops is
> > > +        explicitly set and -funroll-loops is not enabled.  */
> > > +      if (OPTION_SET_P (ix86_unroll_only_small_loops)
> > > +         && !ix86_unroll_only_small_loops
> > > +         && !(OPTION_SET_P (flag_unroll_loops)
> > > +              || OPTION_SET_P (flag_unroll_all_loops)))
> > > +       flag_unroll_loops = flag_unroll_all_loops = 0;
> > > +    }
> >
> > Ugh, that's all quite ugly and unmaintainable, no?
> >
> > > +
> > >  }
> > >
> > >  /* Clear stack slot assignments remembered from previous functions.
> > > @@ -2332,7 +2367,7 @@ ix86_option_override_internal (bool main_args_p,
> > >
> > >    set_ix86_tune_features (opts, ix86_tune, opts->x_ix86_dump_tunes);
> > >
> > > -  ix86_recompute_optlev_based_flags (opts, opts_set);
> > > +  ix86_override_options_after_change ();
> > >
> > >    ix86_tune_cost = processor_cost_table[ix86_tune];
> > >    /* TODO: ix86_cost should be chosen at instruction or function granuality
> > > @@ -2363,9 +2398,6 @@ ix86_option_override_internal (bool main_args_p,
> > >        || TARGET_64BIT_P (opts->x_ix86_isa_flags))
> > >      opts->x_ix86_regparm = REGPARM_MAX;
> > >
> > > -  /* Default align_* from the processor table.  */
> > > -  ix86_default_align (opts);
> > > -
> > >    /* Provide default for -mbranch-cost= value.  */
> > >    SET_OPTION_IF_UNSET (opts, opts_set, ix86_branch_cost,
> > >                        ix86_tune_cost->branch_cost);
> > > diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
> > > index 480db35f6cd..75829a5d0f4 100644
> > > --- a/gcc/config/i386/i386.cc
> > > +++ b/gcc/config/i386/i386.cc
> > > @@ -23820,6 +23820,19 @@ ix86_loop_unroll_adjust (unsigned nunroll, class loop *loop)
> > >    unsigned i;
> > >    unsigned mem_count = 0;
> > >
> > > +  /* Unroll small size loop when unroll factor is not explicitly
> > > +     specified.  */
> > > +  if (ix86_unroll_only_small_loops && !loop->unroll)
> > > +    {
> > > +      int small_unroll = 0;
> > > +      if (loop->ninsns <= (unsigned) ix86_small_unroll_ninsns)
> > > +       small_unroll = MIN ((unsigned) ix86_small_unroll_factor,
> > > +                           nunroll);
> > > +      else
> > > +       small_unroll = 1;
> > > +      return small_unroll;
> > > +    }
> > > +
> > >    if (!TARGET_ADJUST_UNROLL)
> > >       return nunroll;
> > >
> > > diff --git a/gcc/config/i386/i386.opt b/gcc/config/i386/i386.opt
> > > index 0dbaacb57ed..a724c73c0c4 100644
> > > --- a/gcc/config/i386/i386.opt
> > > +++ b/gcc/config/i386/i386.opt
> > > @@ -1214,3 +1214,16 @@ Do not use GOT to access external symbols.
> > >  -param=x86-stlf-window-ninsns=
> > >  Target Joined UInteger Var(x86_stlf_window_ninsns) Init(64) Param
> > >  Instructions number above which STFL stall penalty can be compensated.
> > > +
> > > +munroll-only-small-loops
> > > +Target Var(ix86_unroll_only_small_loops) Init(0) Save
> > > +Enable conservative small loop unrolling.
> > > +
> > > +-param=x86-small-unroll-ninsns=
> > > +Target Joined UInteger Var(ix86_small_unroll_ninsns) Init(4) Param
> > > +Insturctions number limit for loop to be unrolled under
> > > +-munroll-only-small-loops.
> > > +
> > > +-param=x86-small-unroll-factor=
> > > +Target Joined UInteger Var(ix86_small_unroll_factor) Init(2) Param
> > > +Unroll factor for -munroll-only-small-loops.
> > > diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> > > index cd4d3c1d72c..b6fa79eccc3 100644
> > > --- a/gcc/doc/invoke.texi
> > > +++ b/gcc/doc/invoke.texi
> > > @@ -15779,6 +15779,14 @@ The following choices of @var{name} are available on i386 and x86_64 targets:
> > >  @item x86-stlf-window-ninsns
> > >  Instructions number above which STFL stall penalty can be compensated.
> > >
> > > +@item x86-small-unroll-ninsns
> > > +If -munroll-only-small-loops is enabled, only unroll loops with instruction
> > > +count less than this parameter. The default value is 4.
> > > +
> > > +@item x86-small-unroll-factor
> > > +If -munroll-only-small-loops is enabled, reset the unroll factor with this
> > > +value. The default value is 2 which means the loop will be unrolled once.
> > > +
> > >  @end table
> > >
> > >  @end table
> > > @@ -25186,6 +25194,12 @@ environments where no dynamic link is performed, like firmwares, OS
> > >  kernels, executables linked with @option{-static} or @option{-static-pie}.
> > >  @option{-mdirect-extern-access} is not compatible with @option{-fPIC} or
> > >  @option{-fpic}.
> > > +
> > > +@item -munroll-only-small-loops
> > > +@itemx -mno-unroll-only-small-loops
> > > +@opindex munroll-only-small-loops
> > > +Controls conservative small loop unrolling. It is default enbaled by
> > > +O2, and unrolls loop with less than 4 insns by 1 time.
> > >  @end table
> > >
> > >  @node M32C Options
> > > diff --git a/gcc/testsuite/gcc.target/i386/pr86270.c b/gcc/testsuite/gcc.target/i386/pr86270.c
> > > index 81841ef5bd7..cbc9fbb0450 100644
> > > --- a/gcc/testsuite/gcc.target/i386/pr86270.c
> > > +++ b/gcc/testsuite/gcc.target/i386/pr86270.c
> > > @@ -1,5 +1,5 @@
> > >  /* { dg-do compile } */
> > > -/* { dg-options "-O2" } */
> > > +/* { dg-options "-O2 -mno-unroll-only-small-loops" } */
> > >
> > >  int *a;
> > >  long len;
> > > diff --git a/gcc/testsuite/gcc.target/i386/pr93002.c b/gcc/testsuite/gcc.target/i386/pr93002.c
> > > index 0248fcc00a5..f75a847f75d 100644
> > > --- a/gcc/testsuite/gcc.target/i386/pr93002.c
> > > +++ b/gcc/testsuite/gcc.target/i386/pr93002.c
> > > @@ -1,6 +1,6 @@
> > >  /* PR target/93002 */
> > >  /* { dg-do compile } */
> > > -/* { dg-options "-O2" } */
> > > +/* { dg-options "-O2 -mno-unroll-only-small-loops" } */
> > >  /* { dg-final { scan-assembler-not "cmp\[^\n\r]*-1" } } */
> > >
> > >  volatile int sink;
> > > --
> > > 2.18.1
> > >