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

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Message-ID: <503C6B5D.9030100@linux.vnet.ibm.com>
Date:	Tue, 28 Aug 2012 12:25:25 +0530
From:	"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:	Borislav Petkov <bp@...64.org>
CC:	Andi Kleen <andi@...stfloor.org>, tony.luck@...el.com,
	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 08/27/2012 07:48 PM, Borislav Petkov wrote:
> 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.
>

I'm seeing the same here. GCC v4.7 with -O2 generates testb and cmpb, 
which are both 7 bytes long.

Thanks,
Naveen

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