linux-kernel - Re: [PATCH] x86: Use asm-goto to implement mutex fast path on x86-64

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Message-ID: <20130701143646.GG23515@pd.tnic>
Date:	Mon, 1 Jul 2013 16:36:47 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] x86: Use asm-goto to implement mutex fast path on x86-64

On Mon, Jul 01, 2013 at 07:30:14AM -0700, H. Peter Anvin wrote:
> Unconditional branches don't need prediction. The branch predictor
> is used for conditional branches and in some hardware designs for
> indirect branches. Unconditional direct branches never go through
> the branch predictor simply because the front end can know with 100%
> certainty where the flow of control will be.

I meant the conditional Jcc branches:

ffffffff81774920 <mutex_lock>:

...

ffffffff8177492d:       f0 ff 0b                lock decl (%rbx)
ffffffff81774930:       79 08                   jns    ffffffff8177493a <mutex_lock+0x1a>



ffffffff81774730 <mutex_unlock>:

...

ffffffff81774738:       f0 ff 07                lock incl (%rdi)
ffffffff8177473b:       7f 0a                   jg     ffffffff81774747 <mutex_unlock+0x17>

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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