combine: Avoid split_i2i3 search if i2 is unchanged [PR116398]

When combining a single-set i2 into a multi-set i3, combine
first tries to match the new multi-set in-place.  If that fails,
combine considers splitting the multi-set so that one set goes in
i2 and the other set stays in i3.  That moves a destination from i3
to i2 and so combine needs to update any associated log link for that
destination to point to i2 rather than i3.

However, that kind of split can also occur for 2->2 combinations.
For a 2-instruction combination in which i2 doesn't die in i3, combine
tries a 2->1 combination by turning i3 into a parallel of the original
i2 and the combined i3.  If that fails, combine will split the parallel
as above, so that the first set goes in i2 and the second set goes in i3.
But that can often leave i2 unchanged, meaning that no destinations have
moved and so no search is necessary.

gcc/
	PR testsuite/116398
	* combine.cc (try_combine): Shortcut the split_i2i3 handling if
	i2 is unchanged.

1 files changed, 6 insertions, 0 deletions

diff --git a/gcc/combine.cc b/gcc/combine.cc
index 65a87a4..e29cff7 100644
--- a/gcc/combine.cc
+++ b/gcc/combine.cc
@@ -4212,6 +4212,12 @@ try_combine (rtx_insn *i3, rtx_insn *i2, rtx_insn *i1, rtx_insn *i0,
 
   bool only_i3_changed = !i0 && !i1 && rtx_equal_p (newi2pat, PATTERN (i2));
 
+  /* If only i3 has changed, any split of the combined instruction just
+     restored i2 to its original state.  No destinations moved from i3
+     to i2.  */
+  if (only_i3_changed)
+    split_i2i3 = false;
+
   /* We now know that we can do this combination.  Merge the insns and
      update the status of registers and LOG_LINKS.  */