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From: Robin Dapp <rdapp@linux.ibm.com>
To: "H.J. Lu" <hjl.tools@gmail.com>
Cc: Jeff Law <jeffreyalaw@gmail.com>, gcc-patches@gcc.gnu.org
Subject: Re: [RFC] postreload cse'ing vector constants
Date: Wed, 28 Sep 2022 18:48:05 +0200	[thread overview]
Message-ID: <bfa5e551-55b4-a3f3-0835-e92b784dad08@linux.ibm.com> (raw)
In-Reply-To: <CAMe9rOraweH0rw0tEoHWuLqTjNTCr8RmknG-srjTSPC8UpXdQg@mail.gmail.com>

> I opened:
> 
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107061

The online docs for encodekey256 also say

XMM4 through XMM6 are reserved for future usages and software should not
rely upon them being zeroed.

I believe we also zero there.

> This sounds like an issue.

So with your patch for encodekey128 the regression is gone and we zero
(pxor) xmm0 in both versions.  The case I outlined before does not
actually happen since cprop_hardreg propagates the (newly) zeroed
register to the use sites rather than zeroing every time.

I guess this just leaves the situation where we implicitly know that a
reg is zero and by rather zeroing another one we miss the cprop_hardreg
opportunity.  Not sure how common this is and if it's a blocker for this
patch.  No regressions on x86, aarch64, power9 and s390 now.  Most
likely we don't check to that granularity in the test suite and even
here it was more of accidental hit.

  reply	other threads:[~2022-09-28 16:48 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-09-07 14:40 Robin Dapp
2022-09-07 15:06 ` Jeff Law
2022-09-07 15:33   ` Robin Dapp
2022-09-07 15:49     ` Jeff Law
2022-09-08 13:04       ` Robin Dapp
2022-09-27 17:40         ` Robin Dapp
2022-09-27 19:39           ` H.J. Lu
2022-09-28 16:48             ` Robin Dapp [this message]
2022-11-03 12:38               ` Robin Dapp
2022-11-20 16:40                 ` Jeff Law

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