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From: Denis Vlasenko <vda.linux@googlemail.com>
To: Neil Brown <neilb@suse.de>
Cc: Andi Kleen <ak@suse.de>, Sean Reifschneider <jafo@tummy.com>,
	linux-kernel@vger.kernel.org
Subject: Re: PATCH - x86-64 signed-compare bug, was Re: select() setting ERESTARTNOHAND (514).
Date: Thu, 11 Jan 2007 20:38:03 +0100	[thread overview]
Message-ID: <200701112038.03722.vda.linux@googlemail.com> (raw)
In-Reply-To: <17829.36029.240912.274302@notabene.brown>

On Thursday 11 January 2007 02:02, Neil Brown wrote:
> If regs->rax is unsigned long, then I would think the compiler would
> be allowed to convert
> 
>    switch (regs->rax) {
> 	case -514 : whatever;
>    }
> 
> to a no-op, as regs->rax will never have a negative value.

In C, you never actually compare different types. They always
promoted to some common type first.

both sides of (impicit) == here get promoted to "biggest" integer,
in this case, to unsigned long. "-514" is an int, so it gets
sign extended to the width of "long" and then converted to
unsigned long.
--
vda

  parent reply	other threads:[~2007-01-11 19:39 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-01-10 23:42 Sean Reifschneider
2007-01-11  0:27 ` David Miller
2007-01-11  1:04   ` Sean Reifschneider
2007-01-11  1:15     ` David Miller
2007-01-11  8:25       ` Sean Reifschneider
2007-01-11 22:22         ` bert hubert
2007-01-24  0:50           ` Sean Reifschneider
2007-01-11  0:37 ` PATCH - x86-64 signed-compare bug, was " Neil Brown
2007-01-11  0:40   ` Andi Kleen
2007-01-11  1:02     ` Neil Brown
2007-01-11  1:37       ` Andi Kleen
2007-01-11  4:09       ` Sean Reifschneider
2007-01-11 19:38       ` Denis Vlasenko [this message]
2007-01-11  0:43   ` David Miller

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