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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 (albeit 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: r255142
This commit is contained in:
Segher Boessenkool 2017-11-24 18:00:57 +01:00 committed by Segher Boessenkool
parent 0dc933f4f5
commit 6b856e6c31
2 changed files with 11 additions and 1 deletions
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9
gcc/ChangeLog
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@ -1,3 +1,12 @@
2017-11-24 Segher Boessenkool <segher@kernel.crashing.org>
Backport from mainline
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.
2017-11-23 Oleg Endo <olegendo@gcc.gnu.org>
Backport from mainline

3
gcc/combine.c
View File

@ -2964,7 +2964,8 @@ 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;