From: Ingo Molnar <mingo@elte.hu>
To: Andy Lutomirski <luto@MIT.EDU>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
John Stultz <johnstul@us.ibm.com>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 2/6] x86-64: Don't generate cmov in vread_tsc
Date: Tue, 29 Mar 2011 08:15:46 +0200 [thread overview]
Message-ID: <20110329061546.GA27398@elte.hu> (raw)
In-Reply-To: <6921987c30552b781bd96c58451169a6074f2c96.1301324270.git.luto@mit.edu>
* Andy Lutomirski <luto@MIT.EDU> wrote:
> - /* 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 *)&__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. */
> + asm volatile ("");
barrier() would do the same, right?
Also, a nit, please use the customary (multi-line) comment style:
/*
* Comment .....
* ...... goes here.
*/
specified in Documentation/CodingStyle.
Thanks,
Ingo
next prev parent reply other threads:[~2011-03-29 6:16 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-03-28 15:06 [PATCH 0/6] x86-64: Micro-optimize vclock_gettime Andy Lutomirski
2011-03-28 15:06 ` [PATCH 1/6] x86-64: Optimize vread_tsc's barriers Andy Lutomirski
2011-03-29 6:18 ` Ingo Molnar
2011-03-28 15:06 ` [PATCH 2/6] x86-64: Don't generate cmov in vread_tsc Andy Lutomirski
2011-03-29 6:15 ` Ingo Molnar [this message]
2011-03-29 11:52 ` Andrew Lutomirski
2011-03-28 15:06 ` [PATCH 3/6] x86-64: Put vsyscall_gtod_data at a fixed virtual address Andy Lutomirski
2011-03-28 17:49 ` Thomas Gleixner
2011-03-28 18:09 ` Andrew Lutomirski
2011-03-28 21:35 ` Andrew Lutomirski
2011-03-28 23:13 ` Thomas Gleixner
2011-03-28 15:06 ` [PATCH 4/6] x86-64: vclock_gettime(CLOCK_MONOTONIC) can't ever see nsec < 0 Andy Lutomirski
2011-03-29 6:21 ` Ingo Molnar
2011-03-29 11:54 ` Andrew Lutomirski
2011-03-28 15:06 ` [PATCH 5/6] x86-64: Omit frame pointers on vread_tsc Andy Lutomirski
2011-03-29 6:24 ` Ingo Molnar
2011-03-28 15:06 ` [PATCH 6/6] x86-64: Turn off -pg and turn on -foptimize-sibling-calls for vDSO Andy Lutomirski
2011-03-29 6:27 ` [PATCH 0/6] x86-64: Micro-optimize vclock_gettime Ingo Molnar
2011-04-06 18:20 ` Andi Kleen
2011-04-06 20:10 ` Andrew Lutomirski
2011-04-06 20:14 ` Andi Kleen
2011-04-06 20:49 ` Andrew Lutomirski
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