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From: Nick Desaulniers <ndesaulniers@google.com>
To: Sasha Levin <sashal@kernel.org>
Cc: Nathan Chancellor <natechancellor@gmail.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"# 3.4.x" <stable@vger.kernel.org>,
	clang-built-linux@googlegroups.com
Subject: Re: Backport of commit a75bb4eb9e565b9f5115e2e8c07377ce32cbe69a
Date: Tue, 23 Apr 2019 12:42:36 -0700	[thread overview]
Message-ID: <CAKwvOdki6rNSj_vDKkCKDpm5SMLvaYP5uM-tB58ehVLvJyRjog@mail.gmail.com> (raw)
In-Reply-To: <20190423193512.GP17719@sasha-vm>

On Tue, Apr 23, 2019 at 12:35 PM Sasha Levin <sashal@kernel.org> wrote:
>
> On Tue, Apr 23, 2019 at 12:04:21PM -0700, Nathan Chancellor wrote:
> >Hi Greg and Sasha,
> >
> >Please apply this commit to 4.4 through 5.0 (patches are threaded in
> >reply to this one), which will prevent Clang from emitting references
> >to compiler runtime functions and doing some performance-killing
> >optimization when using CONFIG_CC_OPTIMIZE_FOR_SIZE.
> >
> >Please let me know if I did something wrong or if there are any
> >objections.
>
> This looks like a fix for a performance regression, which don't usually
> end up in stable unless they are severe enough.
>
> This patch looks simple enough, and seeing peterz on it suggests to me
> that this was significant enough for someone to notice. Is this really
> the case, or is this just one of those %0.01 performance regression
> things?

I don't recall specific measurements, but when I tried Peter's
suggestion and measured it, I wondered to myself how the initial patch
was ever correct.  The revert (what Nathan sent) should really be
applied for folks building w/ Clang+CONFIG_CC_OPTIMIZE_FOR_SIZE=y,
which is what some CrOS devices (and some Qualcomm BSPs) are doing.
Rather than cherry pick this patch around to a bunch of different
trees, it would be much better for this to land in -stable and for
those trees not to diverge over this.

Nathan, thank you for sending.
-- 
Thanks,
~Nick Desaulniers

  reply	other threads:[~2019-04-23 19:42 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-23 19:04 Nathan Chancellor
2019-04-23 19:04 ` [PATCH 4.14] Revert "kbuild: use -Oz instead of -Os when using clang" Nathan Chancellor
2019-04-23 19:04 ` [PATCH 4.19] " Nathan Chancellor
2019-04-23 19:04 ` [PATCH 4.4] " Nathan Chancellor
2019-04-23 19:04 ` [PATCH 4.9] " Nathan Chancellor
2019-04-23 19:04 ` [PATCH 5.0] " Nathan Chancellor
2019-04-23 19:35 ` Backport of commit a75bb4eb9e565b9f5115e2e8c07377ce32cbe69a Sasha Levin
2019-04-23 19:42   ` Nick Desaulniers [this message]
2019-04-23 19:45   ` Nathan Chancellor
2019-04-23 19:56     ` Matthias Kaehlcke
2019-04-23 20:03       ` Nathan Chancellor
2019-04-23 23:46         ` Sasha Levin
2019-04-24  5:58           ` Sedat Dilek

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