Hi!
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.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2023-12-05 Jakub Jelinek <ja...@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.
--- gcc/config/i386/i386-passes.def.jj 2023-01-16 11:52:15.960735877 +0100
+++ gcc/config/i386/i386-passes.def 2023-12-05 19:15:01.748279329 +0100
@@ -24,7 +24,7 @@ along with GCC; see the file COPYING3.
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. */
--- gcc/config/i386/i386-features.cc.jj 2023-11-02 07:49:15.029894060 +0100
+++ gcc/config/i386/i386-features.cc 2023-12-05 19:15:48.658620698 +0100
@@ -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;
--- gcc/testsuite/gcc.dg/pr112760.c.jj 2023-12-01 13:46:57.444746529 +0100
+++ gcc/testsuite/gcc.dg/pr112760.c 2023-12-01 13:46:36.729036971 +0100
@@ -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 ();
+}
Jakub
