linux-kernel - Re: [PATCH 1/2] x86/mce: Pack boolean MCE boot flags into a structure

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Date:	Mon, 27 Aug 2012 16:18:45 +0200
From:	Borislav Petkov <bp@...64.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
	tony.luck@...el.com, bp@...64.org, gong.chen@...ux.intel.com,
	ananth@...ibm.com, masbock@...ux.vnet.ibm.com, x86@...nel.org,
	linux-kernel@...r.kernel.org, lcm@...ibm.com, mingo@...hat.com,
	tglx@...utronix.de, linux-edac@...r.kernel.org
Subject: Re: [PATCH 1/2] x86/mce: Pack boolean MCE boot flags into a structure

On Mon, Aug 27, 2012 at 03:58:59PM +0200, Andi Kleen wrote:
> On Mon, Aug 27, 2012 at 04:55:03PM +0530, Naveen N. Rao wrote:
> > Many MCE boot flags are boolean in nature, but are declared as integers
> > currently. We can pack these into a bitfield to save some space.
> 
> Note that this doesn't necessarily save anything because it needs more
> code to access, and accesses are more common than the flag
> 
> 	cmpl  $0,foo(%rip)            7 bytes
>         testl $1,foo(%rip)           10 bytes

I got 7 bytes:

12e9:       f6 05 00 00 00 00 02    testb  $0x2,0x0(%rip)        # 12f0 <mce_start_timer.isra.18+0x30>

vs the old:

1654:       83 3d 00 00 00 00 00    cmpl   $0x0,0x0(%rip)        # 165b <mce_start_timer.clone.15+0x37>

also 7 bytes.

-- 
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Boris.

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