combine: Do not throw away unneeded arms of parallels (PR83156)

The fix for PR82621 makes us not split an I2 if one of the results of
those SETs is unused, since combine does not handle that properly.  But
this results in degradation for i386 (or more in general, for any
target that does not have patterns for parallels with an unused result
as a CLOBBER instead of a SET for that result).

This patch instead makes us not split only if one of the results is set
again before I3.  That fixes PR83156 and also fixes PR82621.

Unfortunately it undoes the nice optimisations that the previous patch
did on powerpc.

	PR rtl-optimization/83156
	PR rtl-optimization/82621
	* combine.c (try_combine): Don't split an I2 if one of the dests is
	set again before I3.  Allow unused dests.

From-SVN: r255260

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 62ca3dca421513e6ccfb301475f2fb4fc9400e04..c13f67e76a6e724773144d69d41c673861832b73 100644 (file)
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,10 @@
+2017-11-29  Segher Boessenkool  <segher@kernel.crashing.org>
+
+       PR rtl-optimization/83156
+       PR rtl-optimization/82621
+       * combine.c (try_combine): Don't split an I2 if one of the dests is
+       set again before I3.  Allow unused dests.
+
 2017-11-29  Segher Boessenkool  <segher@kernel.crashing.org>
 
        * config/rs6000/rs6000.md (*add<mode>3_carry_in_internal2): New.

diff --git a/gcc/combine.c b/gcc/combine.c
index 0ce07511a3e351fd01d8f80ad6fe55298defc227..cfe24f42633840957f46c57d063b7723f63b721e 100644 (file)
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -3016,7 +3016,8 @@ try_combine (rtx_insn *i3, rtx_insn *i2, rtx_insn *i1, rtx_insn *i0,
       && can_split_parallel_of_n_reg_sets (i2, 2)
       && !reg_used_between_p (SET_DEST (XVECEXP (PATTERN (i2), 0, 0)), i2, i3)
       && !reg_used_between_p (SET_DEST (XVECEXP (PATTERN (i2), 0, 1)), i2, i3)
-      && !find_reg_note (i2, REG_UNUSED, 0))
+      && !reg_set_between_p  (SET_DEST (XVECEXP (PATTERN (i2), 0, 0)), i2, i3)
+      && !reg_set_between_p  (SET_DEST (XVECEXP (PATTERN (i2), 0, 1)), i2, i3))
     {
       /* If there is no I1, there is no I0 either.  */
       i0 = i1;