https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84032
--- Comment #4 from Roman Zhuykov <zhroma at ispras dot ru> ---
There is the following mistake in logic behind the code.
We want to schedule the branch instructions only as a last instruction in a
row. But when branch was scheduled and we add other instructions into partial
schedule, we sometimes allow them to be in same row after the branch.
The issue happens later when we try to reschedule branch into another row,
algorithm there works like this:
(1) Remove branch from the row where it is (say, “previous row”)
(2) Try insert into the needed row
(3) If success – OK, continue scheduling other instructions
(4) But when inserting (2) was not done – insert it back into “previous row”
and this insertion must be certainly successful, which is checked by assertion.
But when on step (1) branch in not last in a row there is no guarantee, that on
step (4) we could insert it back, because there we will try only last-in-a-row
position for it.
Proposed patch solves this totally preventing other instructions to be
scheduled after branch in the same row.
Patch was successfully bootstrapped and regtested with few other patches on
x86_64. In cross-compiler mode to s390, spu, aarch64, arm, ia64, ppc and ppc64
patch was regtested, and also with -fmodulo-sched enabled by default.
All same testing also done on 8 branch. No new failures introduced.
diff --git a/gcc/modulo-sched.c b/gcc/modulo-sched.c
--- a/gcc/modulo-sched.c
+++ b/gcc/modulo-sched.c
@@ -2996,9 +2996,7 @@ ps_insn_find_column (partial_schedule_ptr ps, ps_insn_ptr
ps_i,
last_must_precede = next_ps_i;
}
/* The closing branch must be the last in the row. */
- if (must_precede
- && bitmap_bit_p (must_precede, next_ps_i->id)
- && JUMP_P (ps_rtl_insn (ps, next_ps_i->id)))
+ if (JUMP_P (ps_rtl_insn (ps, next_ps_i->id)))
return false;
last_in_row = next_ps_i;
