From: Ingo Molnar <mingo@elte.hu>
To: Andy Lutomirski <luto@mit.edu>
Cc: x86@kernel.org, Thomas Gleixner <tglx@linutronix.de>,
Andi Kleen <andi@firstfloor.org>,
linux-kernel@vger.kernel.org
Subject: Re: [RFT/PATCH v2 3/6] x86-64: Don't generate cmov in vread_tsc
Date: Thu, 7 Apr 2011 09:54:56 +0200 [thread overview]
Message-ID: <20110407075456.GC24879@elte.hu> (raw)
In-Reply-To: <49856c9e1325fd1a1f1786f05a7f2befe14666d6.1302137785.git.luto@mit.edu>
* Andy Lutomirski <luto@mit.edu> wrote:
> vread_tsc checks whether rdtsc returns something less than
> cycle_last, which is an extremely predictable branch. GCC likes
> to generate a cmov anyway, which is several cycles slower than
> a predicted branch. This saves a couple of nanoseconds.
>
> Signed-off-by: Andy Lutomirski <luto@mit.edu>
> ---
> arch/x86/kernel/tsc.c | 19 +++++++++++++++----
> 1 files changed, 15 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 858c084..69ff619 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -794,14 +794,25 @@ static cycle_t __vsyscall_fn vread_tsc(void)
> */
>
> /*
> - * This doesn't multiply 'zero' by anything, which *should*
> - * generate nicer code, except that gcc cleverly embeds the
> - * dereference into the cmp and the cmovae. Oh, well.
> + * This doesn't multiply 'zero' by anything, which generates
> + * very slightly nicer code than multiplying it by 8.
> */
> last = *( (cycle_t *)
> ((char *)&VVAR(vsyscall_gtod_data).clock.cycle_last + zero) );
>
> - return ret >= last ? ret : last;
> + if (likely(ret >= last))
> + return ret;
> +
> + /*
> + * GCC likes to generate cmov here, but this branch is extremely
> + * predictable (it's just a funciton of time and the likely is
> + * very likely) and there's a data dependence, so force GCC
> + * to generate a branch instead. I don't barrier() because
> + * we don't actually need a barrier, and if this function
> + * ever gets inlined it will generate worse code.
> + */
> + asm volatile ("");
Hm, you have not addressed the review feedback i gave in:
Message-ID: <20110329061546.GA27398@elte.hu>
Thanks,
Ingo
next prev parent reply other threads:[~2011-04-07 7:55 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-04-07 2:03 [RFT/PATCH v2 0/6] Micro-optimize vclock_gettime Andy Lutomirski
2011-04-07 2:03 ` [RFT/PATCH v2 1/6] x86-64: Clean up vdso/kernel shared variables Andy Lutomirski
2011-04-07 8:08 ` Ingo Molnar
2011-04-07 2:03 ` [RFT/PATCH v2 2/6] x86-64: Optimize vread_tsc's barriers Andy Lutomirski
2011-04-07 8:25 ` Ingo Molnar
2011-04-07 11:44 ` Andrew Lutomirski
2011-04-07 15:23 ` Andi Kleen
2011-04-07 17:28 ` Ingo Molnar
2011-04-07 16:18 ` Linus Torvalds
2011-04-07 16:42 ` Andi Kleen
2011-04-07 17:20 ` Linus Torvalds
2011-04-07 18:15 ` Andi Kleen
2011-04-07 18:30 ` Linus Torvalds
2011-04-07 21:26 ` Andrew Lutomirski
2011-04-08 17:59 ` Andrew Lutomirski
2011-04-09 11:51 ` Ingo Molnar
2011-04-07 21:43 ` Raghavendra D Prabhu
2011-04-07 22:52 ` Andi Kleen
2011-04-07 2:04 ` [RFT/PATCH v2 3/6] x86-64: Don't generate cmov in vread_tsc Andy Lutomirski
2011-04-07 7:54 ` Ingo Molnar [this message]
2011-04-07 11:25 ` Andrew Lutomirski
2011-04-07 2:04 ` [RFT/PATCH v2 4/6] x86-64: vclock_gettime(CLOCK_MONOTONIC) can't ever see nsec < 0 Andy Lutomirski
2011-04-07 7:57 ` Ingo Molnar
2011-04-07 11:27 ` Andrew Lutomirski
2011-04-07 2:04 ` [RFT/PATCH v2 5/6] x86-64: Move vread_tsc into a new file with sensible options Andy Lutomirski
2011-04-07 2:04 ` [RFT/PATCH v2 6/6] x86-64: Turn off -pg and turn on -foptimize-sibling-calls for vDSO Andy Lutomirski
2011-04-07 8:03 ` Ingo Molnar
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