linux-kernel - Re: [PATCH 1/1] Speedfreq-SMI call clobbers ECX

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Message-ID: <20080310215145.GB9246@one.firstfloor.org>
Date:	Mon, 10 Mar 2008 22:51:45 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Stephan Diestelhorst <langer_mann@....de>
Cc:	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
	davej@...emonkey.org.uk, cpufreq@...ts.linux.org.uk,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] Speedfreq-SMI call clobbers ECX

> Please also note that these are not clobbers in the strict inline asm
> syntax, but rather dummy output values that correspond to actual

Outputs/Inputs are as bad as clobbers in triggering gcc reload ICEs.

> input parameters. I'd consider this a serious compiler flaw, if not
> bug, if these would not work. But let's not get into GCC vs.

It is, and it's (mostly) fixed in newer versions, but we're stuck with 
supporting the older compiler versions unfortunately.

> > For asms with so many clobbers explicit push/pop is usually safer.
> 
> Yes, but it is unneccessary overhead as the compiler can (should!)
> find more elegant ways to get those registers back if needed.

This code is not performance critical at all. Safer beats faster
any time in such cases.

-Andi

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