linux-kernel - Re: [BUG FIX] Make x86_32 uni-processor Atomic ops, Atomic

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Message-ID: <4A1B3241.3080509@zytor.com>
Date:	Mon, 25 May 2009 17:05:21 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	lkml@...ethan.org
CC:	Harald Welte <HaraldWelte@...tech.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [BUG FIX] Make x86_32 uni-processor Atomic ops, Atomic

Michael S. Zick wrote:
> 
> Load Effective Address does two's complement arithmetic?
> I'll take your word for it.
> 

LEA, and all other address calculations use 2's-complement arithmetic:

	leal -1(%ebx),%eax
	leal 0xffffffff(%ebx),%eax

... is the same instruction.

However, gcc has been known to optimize out range checks when operating
on signed integers; it is allowed to do this by the C standard, but it
can give surprising results if the user expected wraparound.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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