Re: [PATCH] i386: Only enable small loop unrolling in backend [PR 107602]

[Hongyu Wang <wwwhhhyyy333@gmail.com>]

> It's not necessarily right. unroll_factor will be set as 1 when
> -fno-unroll-loops, which is exactly -fno-unroll-loops means.

Not that exactly, -fno-unroll-loops previously will prevent the pass
from running, and on the current trunk the pass still runs.
Actually I think the implementation on trunk is a bit tricky since we
use a target hook without return value to execute the pass. Also the
logic !OPTION_SET_P looks quite like a workaround. IMHO this is also
not good for maintenance.

Hongtao Liu via Gcc-patches <gcc-patches@gcc.gnu.org> 于2022年11月21日周一 09:33写道：
>
> On Mon, Nov 21, 2022 at 9:01 AM Liu, Hongtao via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> >
> >
> >
> > > -----Original Message-----
> > > From: Wang, Hongyu <hongyu.wang@intel.com>
> > > Sent: Saturday, November 19, 2022 2:26 PM
> > > To: gcc-patches@gcc.gnu.org
> > > Cc: richard.guenther@gmail.com; ubizjak@gmail.com; Liu, Hongtao
> > > <hongtao.liu@intel.com>
> > > Subject: [PATCH] i386: Only enable small loop unrolling in backend [PR 107602]
> > >
> > > Hi,
> > >
> > > Followed by the discussion in pr107602, -munroll-only-small-loops Does not
> > PR107692?
> > > turns on/off -funroll-loops, and current check in pass_rtl_unroll_loops::gate
> > > would cause -funroll-loops do not take effect. Revert the change about
> It's not necessarily right. unroll_factor will be set as 1 when
> -fno-unroll-loops, which is exactly -fno-unroll-loops means.
> > > targetm.loop_unroll_adjust and apply the backend option change to strictly
> > > follow the rule that -funroll-loops takes full control of loop unrolling, and
> > > munroll-only-small-loops just change its behavior to unroll small size loops.
> > >
> > > Bootstrapped and regtested on x86-64-pc-linux-gnu.
> > >
> > > Ok for trunk?
> > >
> > > gcc/ChangeLog:
> > >
> > >       PR target/107602
> > >       * common/config/i386/i386-common.cc (ix86_optimization_table):
> > >       Enable loop unroll O2, disable -fweb and -frename-registers
> > >       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.
> > >       * loop-init.cc (pass_rtl_unroll_loops::gate): Do not enable
> > >       loop unrolling for -O2-speed.
> > >       (pass_rtl_unroll_loops::execute): Rmove
> > >       targetm.loop_unroll_adjust check.
> > >
> > > gcc/testsuite/ChangeLog:
> > >
> > >       PR target/107602
> > >       * gcc.target/i386/pr86270.c: Add -fno-unroll-loops.
> > >       * gcc.target/i386/pr93002.c: Likewise.
> > > ---
> > >  gcc/common/config/i386/i386-common.cc   |  8 ++++++
> > >  gcc/config/i386/i386-options.cc         | 34 ++++++++++++++++++++++---
> > >  gcc/config/i386/i386.cc                 | 18 ++++---------
> > >  gcc/loop-init.cc                        | 11 +++-----
> > >  gcc/testsuite/gcc.target/i386/pr86270.c |  2 +-
> > > gcc/testsuite/gcc.target/i386/pr93002.c |  2 +-
> > >  6 files changed, 49 insertions(+), 26 deletions(-)
> > >
> > > diff --git a/gcc/common/config/i386/i386-common.cc
> > > b/gcc/common/config/i386/i386-common.cc
> > > index 6ce2a588adc..660a977b68b 100644
> > > --- a/gcc/common/config/i386/i386-common.cc
> > > +++ b/gcc/common/config/i386/i386-common.cc
> > > @@ -1808,7 +1808,15 @@ 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 },
> > > +
> > > +    /* Turn on -funroll-loops with -munroll-only-small-loops to enable small
> > > +       loop unrolling at -O2.  */
> > > +    { 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 },
> > >      /* 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
> > > e5c77f3a84d..bc1d36e36a8 100644
> > > --- a/gcc/config/i386/i386-options.cc
> > > +++ b/gcc/config/i386/i386-options.cc
> > > @@ -1838,8 +1838,37 @@ 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;
> > > +      /* -fcunroll-grow-size default follws -f[no]-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;
> > > +    }
> > > +
> > >  }
> > >
> > >  /* Clear stack slot assignments remembered from previous functions.
> > > @@ -2351,7 +2380,7 @@ 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
> > > @@ -2382,9 +2411,6 @@ 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
> > > 292b32c5e99..10f8b4e91a5 100644
> > > --- a/gcc/config/i386/i386.cc
> > > +++ b/gcc/config/i386/i386.cc
> > > @@ -23829,20 +23829,12 @@ ix86_loop_unroll_adjust (unsigned nunroll,
> > > class loop *loop)
> > >
> > >    /* Unroll small size loop when unroll factor is not explicitly
> > >       specified.  */
> > > -  if (!(flag_unroll_loops
> > > -     || flag_unroll_all_loops
> > > -     || loop->unroll))
> > > +  if (ix86_unroll_only_small_loops && !loop->unroll)
> > >      {
> > > -      nunroll = 1;
> > > -
> > > -      /* Any explicit -f{no-}unroll-{all-}loops turns off
> > > -      -munroll-only-small-loops.  */
> > > -      if (ix86_unroll_only_small_loops
> > > -       && !OPTION_SET_P (flag_unroll_loops)
> > > -       && loop->ninsns <= ix86_cost->small_unroll_ninsns)
> > > -     nunroll = ix86_cost->small_unroll_factor;
> > > -
> > > -      return nunroll;
> > > +      if (loop->ninsns <= ix86_cost->small_unroll_ninsns)
> > > +     return MIN (nunroll, ix86_cost->small_unroll_factor);
> > > +      else
> > > +     return 1;
> > >      }
> > >
> > >    if (!TARGET_ADJUST_UNROLL)
> > > diff --git a/gcc/loop-init.cc b/gcc/loop-init.cc index 9789efa1e11..f03c077cae1
> > > 100644
> > > --- a/gcc/loop-init.cc
> > > +++ b/gcc/loop-init.cc
> > > @@ -565,12 +565,10 @@ public:
> > >    {}
> > >
> > >    /* opt_pass methods: */
> > > -  bool gate (function *fun) final override
> > > +  bool gate (function *) final override
> > >      {
> > > -      return (flag_unroll_loops || flag_unroll_all_loops || cfun->has_unroll
> > > -           || (targetm.loop_unroll_adjust
> > > -               && optimize >= 2
> > > -               && optimize_function_for_speed_p (fun)));
> > > +      return (flag_unroll_loops || flag_unroll_all_loops
> > > +           || cfun->has_unroll);
> > >      }
> > >
> > >    unsigned int execute (function *) final override; @@ -586,8 +584,7 @@
> > > pass_rtl_unroll_loops::execute (function *fun)
> > >        if (dump_file)
> > >       df_dump (dump_file);
> > >
> > > -      if (flag_unroll_loops
> > > -       || targetm.loop_unroll_adjust)
> > > +      if (flag_unroll_loops)
> > >       flags |= UAP_UNROLL;
> > >        if (flag_unroll_all_loops)
> > >       flags |= UAP_UNROLL_ALL;
> > > diff --git a/gcc/testsuite/gcc.target/i386/pr86270.c
> > > b/gcc/testsuite/gcc.target/i386/pr86270.c
> > > index cbc9fbb0450..98b012caf23 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 -mno-unroll-only-small-loops" } */
> > > +/* { dg-options "-O2 -fno-unroll-loops" } */
> > >
> > >  int *a;
> > >  long len;
> > > diff --git a/gcc/testsuite/gcc.target/i386/pr93002.c
> > > b/gcc/testsuite/gcc.target/i386/pr93002.c
> > > index f75a847f75d..7e2d869e17b 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 -mno-unroll-only-small-loops" } */
> > > +/* { dg-options "-O2 -fno-unroll-loops" } */
> > >  /* { dg-final { scan-assembler-not "cmp\[^\n\r]*-1" } } */
> > >
> > >  volatile int sink;
> > > --
> > > 2.18.1
> >
>
>
> --
> BR,
> Hongtao