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Re: [PATCH] Improve x86 and + rotate (PR target/82498)
- From: Uros Bizjak <ubizjak at gmail dot com>
- To: Jakub Jelinek <jakub at redhat dot com>
- Cc: "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>
- Date: Thu, 12 Oct 2017 08:39:12 +0200
- Subject: Re: [PATCH] Improve x86 and + rotate (PR target/82498)
- Authentication-results: sourceware.org; auth=none
- References: <20171011205903.GP14653@tucnak> <CAFULd4ZmiFjoYcPbQBuJaUAoDRTXtPcxBp1CEQS1nCB4umRN8g@mail.gmail.com>
On Thu, Oct 12, 2017 at 8:32 AM, Uros Bizjak <ubizjak@gmail.com> wrote:
> On Wed, Oct 11, 2017 at 10:59 PM, Jakub Jelinek <jakub@redhat.com> wrote:
>> Hi!
>>
>> As can be seen on the testcase below, the *<rotate_insn><mode>3_mask
>> insn/splitter is able to optimize only the case when the and is
>> performed in SImode and then the result subreged into QImode,
>> while if the computation is already in QImode, we don't handle it.
>>
>> Fixed by adding another pattern, bootstrapped/regtested on x86_64-linux and
>> i686-linux, ok for trunk?
>
> We probably want to add this variant to *all* *_mask splitters (there
> are a few of them in i386.md, please grep for "Avoid useless
> masking"). Which finally begs a question - should we implement this
> simplification in a generic, target-independent way? OTOH, we already
> have SHIFT_COUNT_TRUNCATED and shift_truncation_mask hooks, but last
> time I try the former, there were some problems in the testsuite on
> x86. I guess there are several targets that would benefit from
> removing useless masking of count operands.
Oh, and there is a strange x86 exception in the comment for
SHIFT_COUNT_TRUNCATED. I'm not sure what "(real or pretended)
bit-field operation" means, but variable-count BT instruction with
non-memory operand (we never generate variable-count BTx with memory
operand) masks its count operand as well.
Uros.