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Subject: Re: [PATCH] Unswitching outer loops.
To: Yuri Rumyantsev <ysrumyan@gmail.com>,        Richard Biener <richard.guenther@gmail.com>
References: <CAEoMCqRi+=wqTGyJaYa+vQiYcxTz8SQGCk1KDjHTX9Ws2+sp6w@mail.gmail.com> <CAFiYyc3P5Pzy+dbJp1_j6wo8NKJFQGJJP_tntt5vX74xwXo0cw@mail.gmail.com> <CAEoMCqTX7Jw+uLy9MzrsygBKNjLKk2hfZKrCV7exD9kCD1SWAg@mail.gmail.com> <CAFiYyc30yxVbLVYCLX2bEszOP7J00+FZWL8DVvw+Eb7-T63epw@mail.gmail.com> <CAEoMCqTBc5ZJ7OeOGKzt_VZiRU_aJbF39QmYNxF4F8eYJ415PA@mail.gmail.com>
Cc: gcc-patches <gcc-patches@gcc.gnu.org>, Igor Zamyatin <izamyatin@gmail.com>
From: Jeff Law <law@redhat.com>
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Date: Fri, 31 Jul 2015 15:54:00 -0000
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On 07/31/2015 05:17 AM, Yuri Rumyantsev wrote:
> Hi Richard,
>
> I learned your updated patch for 23825 and it is more general in
> comparison with my.
> I'd like to propose you a compromise - let's consider my patch only
> for force-vectorize outer loop only to allow outer-loop
> vecctorization. Note that your approach will not hoist invariant
> guards if loops contains something else except for inner-loop, i.e. it
> won't be empty for taken branch.
> I also would like to answer on your last question - CFG cleanup is
> invoked to perform deletion of single-argument phi nodes from tail
> block through substitution - such phi's prevent outer-loop
> vectorization. But it is clear that such transformation can be done
> other pass.
A single-argument PHI is a degenerate.  phi-only-cprop might be your 
friend :-)

jeff