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

[Richard Biener <richard dot guenther at gmail dot com>]

On Fri, Jan 22, 2016 at 10:52 AM, Kyrill Tkachov
<kyrylo.tkachov@foss.arm.com> wrote:
> 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?

Ok if the rest of the series is approved (otherwise ok for stage1).

Thanks,
Richard.

> 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.