[PATCH] i386: Enable small loop unrolling for O2

[Hongyu Wang <hongyu.wang@intel.com>]

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 },
     /* 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;
+    }
+
 }
 
 /* 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