[PATCH] combine: Query can_change_dest_mode before changing dest mode

[Georg-Johann Lay]

On 24.11.2016 00:27, Segher Boessenkool wrote:
> 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.

Thanks for the swift fix!

>
> Segher
>
> 2016-11-23  Segher Boessenkool  <segher@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)));
>

IIUC, the will also avoid changing, for example, QI to HI if HI occupies 
more hard regs than QI?  Great!

Johann