linux-kernel - RE: kernel + gcc 4.1 = several problems

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Date:	Thu, 4 Jan 2007 07:34:52 -0800 (PST)
From:	Linus Torvalds <torvalds@...l.org>
To:	"Zou, Nanhai" <nanhai.zou@...el.com>
cc:	Grzegorz Kulewski <kangur@...com.net>,
	Alan <alan@...rguk.ukuu.org.uk>,
	Mikael Pettersson <mikpe@...uu.se>, s0348365@....ed.ac.uk,
	76306.1226@...puserve.com, akpm@...l.org, bunk@...sta.de,
	greg@...ah.com, linux-kernel@...r.kernel.org,
	yanmin_zhang@...ux.intel.com
Subject: RE: kernel + gcc 4.1 = several problems

On Thu, 4 Jan 2007, Zou, Nanhai wrote:
> 
> cmov will stall on eflags in your test program.

And that is EXACTLY my point.

CMOV is a piece of CRAP for most things, exactly because it serializes 
three streams of data: the two inputs, and the conditional.

My test-case was actually _good_ for cmov, because there was just the one 
conditional (which was 100% ALU) thing that was serialized. In real life, 
the two data sources also come from memory, and _any_ of them being 
delayed ends up delaying the cmov, and screwing up your out-of-order 
pipeline because you now introduced a serialization point that was very 
possibly not necessary at all.

In contrast, a conditional branch-around serializes absolutely NOTHING, 
because branches get predicted.

> I think you will see benefit of cmov if you can manage to put some 
> instructions which does NOT modify eflags between testl and cmov.

A lot of the time, the conditional _is_ the critical path.

The whole point of this discussion was that cmov isn't really all that 
great. It has fundamental problems that a conditional branch that gets 
predicted simply does not have.

That's qiute apart from the fact that cmov has rather limited semantics, 
and that in 99% of all cases you have to use a conditional branch anyway.

			Linus
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