linux-kernel - Re: [GIT PULL] x86/vdso changes for v3.16

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Date:	Fri, 06 Jun 2014 14:42:29 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Stefani Seibold <stefani@...bold.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Pavel Emelyanov <xemul@...allels.com>
Subject: Re: [GIT PULL] x86/vdso changes for v3.16

On 06/06/2014 02:39 PM, Andy Lutomirski wrote:
>>
>> For accessing memory members doing it via a pointer is pretty much TRT,
>> but for things that might be in register it is undesirable to force it
>> out to memory.
> 
> Do you also believe in the folklore that GCC can optimize code
> sequences like the things in that header?  Because I'm pretty sure
> that no clang or gcc version I've ever seen can do it.
> 

I have seen gcc do some pretty sophisticated memory elision lately.
Don't know if that includes byte swaps.

> On the other hand, even a factor of ten in the time it takes to run
> vdso2c is completely irrelevant.

Yep, as I noted in the patch I sent (which is broken - updated one
included here.)

	-hpa


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