[PATCH][cse][3/4] Don't overwrite original rtx when folding source of set

[Kyrill Tkachov <kyrylo dot tkachov at foss dot arm dot com>]

Hi all,

This patch is a consequence of the thread I started at https://gcc.gnu.org/ml/gcc/2016-01/msg00100.html
The problem is that the fold_rtx call in cse_insn may overwrite its argument if the insn argument is non-NULL.
This leads to CSE not considering the original form of the RTX when doing its cost analysis later on.
This led to it picking a normal SImode multiply expression over the original multiply-sign-extend expression
which in my case is cheaper (as reflected in the fixed rtx costs from patch 2)

The simple fix is to pass NULL to fold_rtx so that it will return the candidate folded expression
into src_folded but still retain the original src for analysis.

With this change the gcc.target/arm/wmul-[12].c and the costs fix in patch [2/4]
the tests now generate their expected
sign-extend+multiply (+accumulate) sequences.

Apart from that this patch has no impact codegen on SPEC2006 for arm.
For aarch64 the impact is minimal and inconsequential. I've seen sequences that
select between 1 -1 being turned from a CSINC (of zero) into a CSNEG. Both are valid
and of equal value.

On x86_64 the impact was also minimal. Most benchmarks were not changed at all.
Some showed a negligible reduction in codesize and a slight register-allocation perturbations.
But nothing significant.
Hence, I claim that this patch is low impact.

Bootstrapped and tested on arm, aarch64, x86_64.
Ok for trunk?

Thanks,
Kyrill

2016-01-22  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

    * cse.c (cse_insn): Pass NULL to fold_rtx when initially
    folding the source of a SET.

diff --git a/gcc/cse.c b/gcc/cse.c
index 58b8fc0313dcbfb2036054564746a7832ae52140..2665d9a2733cad7286b41a88753acfcf79be83f1 100644
--- a/gcc/cse.c
+++ b/gcc/cse.c
@@ -4636,7 +4636,7 @@ cse_insn (rtx_insn *insn)
 
       /* Simplify and foldable subexpressions in SRC.  Then get the fully-
 	 simplified result, which may not necessarily be valid.  */
-      src_folded = fold_rtx (src, insn);
+      src_folded = fold_rtx (src, NULL);
 
 #if 0
       /* ??? This caused bad code to be generated for the m68k port with -O2.