From: Andi Kleen <andi@firstfloor.org>
To: Jan Hubicka <hubicka@ucw.cz>
Cc: Xinliang David Li <davidxl@google.com>,
GCC Patches <gcc-patches@gcc.gnu.org>,
Teresa Johnson <tejohnson@google.com>
Subject: Re: [PATCH i386]: Enable push/pop in pro/epilogue for modern CPUs
Date: Wed, 12 Dec 2012 18:37:00 -0000 [thread overview]
Message-ID: <m2ehivdslr.fsf@firstfloor.org> (raw)
In-Reply-To: <20121212183036.GB5303@atrey.karlin.mff.cuni.cz> (Jan Hubicka's message of "Wed, 12 Dec 2012 19:30:36 +0100")
Jan Hubicka <hubicka@ucw.cz> writes:
>
> libcall is not faster up to 8KB to rep sequence that is better for regalloc/code
> cache than fully blowin function call.
I noticed btw that some of the generated string instructions are slower
than just calling the C library.
rep scasb etc. is rarely a win over an optimized library function,
it's not very optimized. Perhaps those patterns should just be disabled.
The way to optimize that on modern CPUs is to use PCMP*STR*, but that's
quite a bit more complicated and has some constraints.
>> > /* X86_TUNE_FOUR_JUMP_LIMIT: Some CPU cores are not able to predict more
>> > than 4 branch instructions in the 16 byte window. */
>> > - m_PPRO | m_P4_NOCONA | m_CORE2I7 | m_ATOM | m_AMD_MULTIPLE | m_GENERIC,
>> > + m_PPRO | m_P4_NOCONA | m_ATOM | m_AMD_MULTIPLE | m_GENERIC,
>
> This is special passs to handle limitations of AMD's K7/K8/K10 branch prediction.
> Intel never had similar design, so this flag is pointless.
Actually the Sandy Bridge decoded icache has a limit of 3 jumps per
16 byte window. If you exceed that it falls back to running
the full decoder from the normal icache.
I don't have solid data, but it may be a win for frontend limited
code (otherwise possibly more in power than performance)
I would revisit that for Sandy Bridge
-Andi
--
ak@linux.intel.com -- Speaking for myself only
next prev parent reply other threads:[~2012-12-12 18:37 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-12-08 18:13 Xinliang David Li
2012-12-12 16:37 ` Jan Hubicka
2012-12-12 17:25 ` Xinliang David Li
2012-12-12 17:34 ` Xinliang David Li
2012-12-12 18:30 ` Jan Hubicka
2012-12-12 18:37 ` Andi Kleen [this message]
2012-12-12 18:43 ` Andi Kleen
2012-12-12 18:43 ` Jan Hubicka
2012-12-12 20:56 ` x86-64 medium memory model Leif Ekblad
2012-12-12 20:59 ` H.J. Lu
2012-12-12 21:33 ` Leif Ekblad
2012-12-13 0:16 ` [PATCH i386]: Enable push/pop in pro/epilogue for modern CPUs Xinliang David Li
2012-12-13 0:16 ` Xinliang David Li
2012-12-13 1:19 ` Jan Hubicka
2012-12-13 6:09 ` Xinliang David Li
2012-12-13 6:21 ` Jakub Jelinek
2012-12-13 7:05 ` Xinliang David Li
2012-12-13 19:28 ` Jan Hubicka
2012-12-13 10:22 ` Richard Biener
2012-12-13 19:43 ` H.J. Lu
2012-12-13 20:26 ` Jan Hubicka
2012-12-13 20:28 ` H.J. Lu
2012-12-13 20:40 ` Jan Hubicka
2012-12-13 21:02 ` H.J. Lu
2012-12-13 21:35 ` Jan Hubicka
2012-12-20 12:13 ` Melik-adamyan, Areg
2012-12-20 14:08 ` H.J. Lu
2012-12-20 15:05 ` Jan Hubicka
2012-12-20 15:07 ` Jan Hubicka
2012-12-20 15:22 ` H.J. Lu
2012-12-21 8:28 ` Zamyatin, Igor
2012-12-09 13:50 Uros Bizjak
2012-12-09 17:09 ` Дмитрий Дьяченко
2012-12-10 9:23 ` Richard Biener
2012-12-10 20:42 ` Xinliang David Li
2012-12-10 21:07 ` Mike Stump
2012-12-11 9:49 ` Richard Biener
2012-12-11 17:15 ` Xinliang David Li
2012-12-11 22:53 Xinliang David Li
2012-12-11 23:39 ` Xinliang David Li
2012-12-12 11:00 ` Richard Biener
2012-12-21 7:26 Xinliang David Li
2012-12-21 8:20 ` Zamyatin, Igor
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