From 58b46683bd70c20c2543d60ca28af4dec4ec0443 Mon Sep 17 00:00:00 2001
From: Segher Boessenkool <segher@kernel.crashing.org>
Date: Fri, 17 Nov 2017 15:46:04 +0100
Subject: [PATCH] combine: Don't split insns if half is unused (PR82621)
If we have a PARALLEL of two SETs, and one half is unused, we currently
happily split that into two instructions (although the unused one is
useless). Worse, as PR82621 shows, combine will happily merge this
insn into I3 even if some intervening insn sets the same register
again, which is wrong.
This fixes it by not splitting PARALLELs with REG_UNUSED notes. It
all is handled fine by combine in that case: just the "single set
that is unused" case isn't handled properly.
This also results in better code: combine will now actually throw
away the unused SET. (It still won't do that in an I3).
PR rtl-optimization/82621
* combine.c (try_combine): Do not split PARALLELs of two SETs if the
dest of one of those SETs is unused.
From-SVN: r254874
---
gcc/ChangeLog | 6 ++++++
gcc/combine.c | 3 ++-
2 files changed, 8 insertions(+), 1 deletion(-)
diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 993ac9bba249..c0249e81aa5f 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,9 @@
+2017-11-17 Segher Boessenkool <segher@kernel.crashing.org>
+
+ PR rtl-optimization/82621
+ * combine.c (try_combine): Do not split PARALLELs of two SETs if the
+ dest of one of those SETs is unused.
+
2017-11-17 Richard Biener <rguenther@suse.de>
PR fortran/83017
diff --git a/gcc/combine.c b/gcc/combine.c
index eab10c599a61..404cf33e9d06 100644
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -3010,7 +3010,8 @@ try_combine (rtx_insn *i3, rtx_insn *i2, rtx_insn *i1, rtx_insn *i0,
&& is_parallel_of_n_reg_sets (PATTERN (i2), 2)
&& 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))
+ && !reg_used_between_p (SET_DEST (XVECEXP (PATTERN (i2), 0, 1)), i2, i3)
+ && !find_reg_note (i2, REG_UNUSED, 0))
{
/* If there is no I1, there is no I0 either. */
i0 = i1;
--
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