Fix pessimistic DImode handling in combine.c:make_field_assignment

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

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.

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);