[PATCH] combine: Query can_change_dest_mode before changing dest mode

[Segher Boessenkool]

As reported in https://gcc.gnu.org/ml/gcc-patches/2016-11/msg02388.html .

Changing the mode of a hard register can lead to problems, or at least
it can make worse code if the result will need reloads.

Tested on avr-elf on the test in that email, and bootstrapped and
regression checked on powerpc64-linux {-m32,-m64}.  Committing to trunk.


Segher


2016-11-23  Segher Boessenkool  <seg...@kernel.crashing.org>

        * combine.c (change_zero_ext): Only change the mode of a hard register
        destination if can_change_dest_mode holds for that.

---
 gcc/combine.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/gcc/combine.c b/gcc/combine.c
index eef917a..c8a71eb 100644
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -11287,7 +11287,8 @@ change_zero_ext (rtx pat)
       else if (GET_CODE (x) == ZERO_EXTEND
               && SCALAR_INT_MODE_P (mode)
               && REG_P (XEXP (x, 0))
-              && HARD_REGISTER_P (XEXP (x, 0)))
+              && HARD_REGISTER_P (XEXP (x, 0))
+              && can_change_dest_mode (XEXP (x, 0), 0, mode))
        {
          size = GET_MODE_PRECISION (GET_MODE (XEXP (x, 0)));
          x = gen_rtx_REG (mode, REGNO (XEXP (x, 0)));
-- 
1.9.3