i386: Move vzeroupper pass from after reload pass to after postreload_cse [PR112760]

Regardless of the outcome of the REG_UNUSED discussions, I think
it is a good idea to move the vzeroupper pass one pass later.
As can be seen in the multiple PRs and as postreload.cc documents,
reload/LRA is known to create dead statements quite often, which
is the reason why we have postreload_cse pass at all.
Doing vzeroupper pass before such cleanup means the pass including
df_analyze for it needs to process more instructions than needed
and because mode switching adds note problem, also higher chance of
having stale REG_UNUSED notes.
And, I really don't see why vzeroupper can't wait until those cleanups
are done.

2023-12-06  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/112760
	* config/i386/i386-passes.def (pass_insert_vzeroupper): Insert
	after pass_postreload_cse rather than pass_reload.
	* config/i386/i386-features.cc (rest_of_handle_insert_vzeroupper):
	Adjust comment for it.

	* gcc.dg/pr112760.c: New test.

3 files changed, 28 insertions, 5 deletions

diff --git a/gcc/config/i386/i386-features.cc b/gcc/config/i386/i386-features.cc
index 6fac67e..e6fc135 100644
--- a/gcc/config/i386/i386-features.cc
+++ b/gcc/config/i386/i386-features.cc
@@ -2627,10 +2627,11 @@ convert_scalars_to_vector (bool timode_p)
 static unsigned int
 rest_of_handle_insert_vzeroupper (void)
 {
-  /* vzeroupper instructions are inserted immediately after reload to
-     account for possible spills from 256bit or 512bit registers.  The pass
-     reuses mode switching infrastructure by re-running mode insertion
-     pass, so disable entities that have already been processed.  */
+  /* vzeroupper instructions are inserted immediately after reload and
+     postreload_cse to clean up after it a little bit to account for possible
+     spills from 256bit or 512bit registers.  The pass reuses mode switching
+     infrastructure by re-running mode insertion pass, so disable entities
+     that have already been processed.  */
   for (int i = 0; i < MAX_386_ENTITIES; i++)
     ix86_optimize_mode_switching[i] = 0;
 
diff --git a/gcc/config/i386/i386-passes.def b/gcc/config/i386/i386-passes.def
index 90f2234..2d18981 100644
--- a/gcc/config/i386/i386-passes.def
+++ b/gcc/config/i386/i386-passes.def
@@ -24,7 +24,7 @@ along with GCC; see the file COPYING3.  If not see
    REPLACE_PASS (PASS, INSTANCE, TGT_PASS)
  */
 
-  INSERT_PASS_AFTER (pass_reload, 1, pass_insert_vzeroupper);
+  INSERT_PASS_AFTER (pass_postreload_cse, 1, pass_insert_vzeroupper);
   INSERT_PASS_AFTER (pass_combine, 1, pass_stv, false /* timode_p */);
   /* Run the 64-bit STV pass before the CSE pass so that CONST0_RTX and
      CONSTM1_RTX generated by the STV pass can be CSEed.  */
diff --git a/gcc/testsuite/gcc.dg/pr112760.c b/gcc/testsuite/gcc.dg/pr112760.c
new file mode 100644
index 0000000..b4ec70e
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr112760.c
@@ -0,0 +1,22 @@
+/* PR rtl-optimization/112760 */
+/* { dg-do run } */
+/* { dg-options "-O2 -fno-dce -fno-guess-branch-probability --param=max-cse-insns=0" } */
+/* { dg-additional-options "-m8bit-idiv -mavx" { target i?86-*-* x86_64-*-* } } */
+
+unsigned g;
+
+__attribute__((__noipa__)) unsigned short
+foo (unsigned short a, unsigned short b)
+{
+  unsigned short x = __builtin_add_overflow_p (a, g, (unsigned short) 0);
+  g -= g / b;
+  return x;
+}
+
+int
+main ()
+{
+  unsigned short x = foo (40, 6);
+  if (x != 0)
+    __builtin_abort ();
+}