There still are situations where we have stale LOG_LINKS. This causes
combine to try two-insn combinations I2->I3 where the register set by
I2 is used before I3 as well. Not good.
This patch fixes it by checking for this situation in can_combine_p
(similar to what we already do for three and four insn combinations).
Longer term we should make sure stale LOG_LINKS are removed (or moved),
or ideally we should use DF instead of LOG_LINKS.
I tested the patch on 31 architectures, no degradations seen. Zdenek
tested it on the original testcase (thanks!)
Committing to trunk.
2018-03-12 Segher Boessenkool <seg...@kernel.crashing.org>
* combine.c (can_combine_p): Check for a 2-insn combination whether
the destination register is used between the two insns, too.
gcc/combine.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/gcc/combine.c b/gcc/combine.c
index c9105ed..06a9ddd 100644
@@ -1990,6 +1990,7 @@ can_combine_p (rtx_insn *insn, rtx_insn *i3, rtx_insn
&& (reg_used_between_p (dest, succ2, i3)
|| reg_used_between_p (dest, succ, succ2)))
|| (!succ2 && succ && reg_used_between_p (dest, succ, i3))
+ || (!succ2 && !succ && reg_used_between_p (dest, insn, i3))
/* SUCC and SUCC2 can be split halves from a PARALLEL; in
that case SUCC is not in the insn stream, so use SUCC2