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 15:58:59 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
Cc:	tony.luck@...el.com, andi@...stfloor.org, 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 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
        bt    $0,foo(%rip)            8 bytes

Even worse for writes. The best you can usually do is to use chars on
x86.

-Andi
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