linux-kernel - Re: [patch 0/2] Immediate Values

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Message-ID: <20080428124833.GE27997@elte.hu>
Date:	Mon, 28 Apr 2008 14:48:33 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [patch 0/2] Immediate Values - jump patching update


* Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:

> Hi Ingo,
> 
> Here is the update to the jump patching optimization taking care of 
> Peter's comments about register liveliness and instruction re-use by 
> gcc optimizations. A good thing : it actually simplifies the code. 
> Unfortunately, it adds 3 bytes to the instructions in i-cache because 
> I now have to use a 5-bytes mov instruction so I can replace it with a 
> 5-bytes jump. Therefore, 9 bytes are added to rather small functions 
> (5-bytes mov + 2-bytes test + 2 bytes conditional branch) and 13 bytes 
> are added to larger functions which needs a 6 bytes conditional branch 
> at the branch site.
> 
> Instead of having to execute a sequence of nop, nop and jump, we now 
> only have to execute the near jump, which jumps either at the address 
> following the conditional branch or at the target address of the 
> conditional branch, depending on the immediate value variable state.
> 
> Thanks to Peter for the review.

thanks Mathieu, i've queued them up for more testing. Your previous 
queue already looked good here so i pushed it out into sched-devel.git 
as you probably noticed.

Sidenote, without trying to bikeshed paint this issue too much: are we 
absolutely sure that (on modern CPU architectures) such a short jump is 
better than just 2-3 NOPs in a sequence? It's a minor (sub-cycle) detail 
in any case.

	Ingo
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