Re: Fix pessimistic DImode handling in combine.c:make_field_assignment

[Richard Sandiford <richard.sandiford@linaro.org>]

Richard Sandiford <richard.sandiford@linaro.org> writes:
> The make_field_assignment code:
>
>       src = force_to_mode (src, mode,
> 		       GET_MODE_PRECISION (mode) >= HOST_BITS_PER_WIDE_INT
> 		       ? HOST_WIDE_INT_M1U
> 		       : (HOST_WIDE_INT_1U << len) - 1,
> 		       0);
>
> would ignore the field length len for DImode, even though DImode can be
> handled using HWIs.  I think the code should be testing len instead.
>
> The patch was originally part of the SVE machine_mode series.
> Retesting showed that it changed the asm output on powerpc for a few
> tests, so I thought it should go in separately.  Each test change
> seemed to be an improvement.
>
> Tested on aarch64-linux-gnu and x86_64-linux-gnu.  I no longer have
> access to the compile farm to test on the powerpc boxes there.

Now tested on powerpc64le-linux-gnu too (thanks to Segher for the access).

> Thanks,
> Richard
>
>
> 2017-05-24  Richard Sandiford  <richard.sandiford@linaro.org>
>
> gcc/
> 	* combine.c (make_field_assignment): Check len rather than the mode
> 	precision when calling force_to_mode.
>
> Index: gcc/combine.c
> ===================================================================
> --- gcc/combine.c	2017-05-03 08:46:32.777861592 +0100
> +++ gcc/combine.c	2017-05-24 09:25:25.170351268 +0100
> @@ -9634,7 +9634,7 @@ make_field_assignment (rtx x)
>  						     other, pos),
>  			       dest);
>    src = force_to_mode (src, mode,
> -		       GET_MODE_PRECISION (mode) >= HOST_BITS_PER_WIDE_INT
> +		       len >= HOST_BITS_PER_WIDE_INT
>  		       ? HOST_WIDE_INT_M1U
>  		       : (HOST_WIDE_INT_1U << len) - 1,
>  		       0);