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  <>

        PR rtl-optimization/84169
        PR rtl-optimization/84780
        * 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
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -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
                  /* SUCC and SUCC2 can be split halves from a PARALLEL; in
                     that case SUCC is not in the insn stream, so use SUCC2

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