From: Alexey Dobriyan <adobriyan@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
Andrew Morton <akpm@linux-foundation.org>,
Andrew Lutomirski <luto@kernel.org>,
Borislav Petkov <bp@alien8.de>,
Josh Poimboeuf <jpoimboe@redhat.com>, Peter Anvin <hpa@zytor.com>,
Denys Vlasenko <dvlasenk@redhat.com>
Subject: Re: x86/asm: __clear_user() micro-optimization (was: "Re: [GIT PULL] x86/asm changes for v4.18")
Date: Wed, 6 Jun 2018 02:20:22 +0300 [thread overview]
Message-ID: <20180605232022.GA4468@avx2> (raw)
In-Reply-To: <CA+55aFxz46_efiQw9Nk_pFbX3AYx+oehPQOk-BDd1Xpbr2Oa3w@mail.gmail.com>
On Tue, Jun 05, 2018 at 04:04:37PM -0700, Linus Torvalds wrote:
> On Tue, Jun 5, 2018 at 4:01 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > On Tue, Jun 5, 2018 at 3:41 PM Alexey Dobriyan <adobriyan@gmail.com> wrote:
> > >
> > > On my potato performance increase is 33%, sheesh.
> > > And CPU starts doing 3 instructions per cycle vs 2.
> >
> > Whee. That's a shockingly big difference. On my CPU (i7-6700K) it
> > makes absolutely no difference whether the values are integers or in
> > registers.
>
> In fact, looking at Agner Fog's instruction lists, I don't see any CPU
> where it would make a difference, except for the P4 (where the
> immediate looks like it's a bad idea because it's an extra uop, but it
> might pack fine and not be noticeable).
>
> But maybe I'm missing something subtle. What CPU, out of morbid interest?
This is Broadwell Xeon E5-2620 v4.
Which is somewhat strange indeed because it should be modern enough.
next prev parent reply other threads:[~2018-06-05 23:20 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-06-04 12:21 [GIT PULL] x86/asm changes for v4.18 Ingo Molnar
2018-06-05 1:58 ` Linus Torvalds
2018-06-05 15:05 ` x86/asm: __clear_user() micro-optimization (was: "Re: [GIT PULL] x86/asm changes for v4.18") Ingo Molnar
2018-06-05 15:47 ` Linus Torvalds
2018-06-05 17:22 ` Alexey Dobriyan
2018-06-05 17:32 ` Linus Torvalds
2018-06-05 22:41 ` Alexey Dobriyan
2018-06-05 23:01 ` Linus Torvalds
2018-06-05 23:04 ` Linus Torvalds
2018-06-05 23:20 ` Alexey Dobriyan [this message]
2018-06-05 23:27 ` Linus Torvalds
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