Re: [RFC PATCH, i386]: Enable post-reload compare elimination pass

[Uros Bizjak <ubizjak at gmail dot com>]

On Wed, May 10, 2017 at 5:18 PM, Uros Bizjak <ubizjak@gmail.com> wrote:
> On Wed, May 10, 2017 at 4:27 PM, Jakub Jelinek <jakub@redhat.com> wrote:
>> On Tue, May 09, 2017 at 06:06:47PM +0200, Uros Bizjak wrote:
>>> Attached patch enables post-reload compare elimination pass by
>>> providing expected patterns (duplicates of existing patterns with
>>> setters of reg and flags switched in the parallel) for flag setting
>>> arithmetic instructions.
>>>
>>> The merge triggers more than 3000 times during the gcc bootstrap,
>>> mostly in cases where intervening memory load or store prevents
>>> combine from merging the arithmetic insn and the following compare.
>>>
>>> Also, some recent linux x86_64 defconfig build results in ~200 merges,
>>> removing ~200 test/cmp insns. Not much, but I think the results still
>>> warrant the pass to be enabled.
>>
>> Isn't the right fix instead to change the compare-elim.c pass to either
>> accept both reg vs. flags orderings in parallel, or both depending
>> on some target hook, or change it to the order i386.md and most other
>> major targets use and just fix up mn10300/rx (and aarch64?) to use the same
>> order?

Attached patch changes compare-elim.c order to what i386.md expects.

Thoughts?

Uros.

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