linux-kernel - Re: [PATCH v2] x86: Use asm-goto to implement mutex fast path on x86-64

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Message-ID: <51CD9A62.3020404@zytor.com>
Date:	Fri, 28 Jun 2013 07:14:58 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Wedson Almeida Filho <wedsonaf@...il.com>
CC:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86: Use asm-goto to implement mutex fast path on
 x86-64

On 06/28/2013 04:50 AM, Wedson Almeida Filho wrote:
> The new implementation allows the compiler to better optimize the code; the
> original implementation is still used when the kernel is compiled with older
> versions of gcc that don't support asm-goto.
> 
> Compiling with gcc 4.7.3, the original mutex_lock() is 60 bytes with the fast
> path taking 16 instructions; the new mutex_lock() is 42 bytes, with the fast
> path taking 12 instructions.
> 
> The original mutex_unlock() is 24 bytes with the fast path taking 7
> instructions; the new mutex_unlock() is 25 bytes (because the compiler used
> a 2-byte ret) with the fast path taking 4 instructions.
> 
> The two versions of the functions are included below for reference.
> 

As Ingo said, looks very nice.  However, it is really too late for Linux
3.11, so I'm going to put it on a queue I already have for 3.12.

	-hpa


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