Date: Thu, 11 Sep 2003 18:32:45 +0100
From: Jamie Lokier <jamie@shareable.org>
To: Andi Kleen <ak@suse.de>
Cc: richard.brunner@amd.com, linux-kernel@vger.kernel.org, akpm@osdl.org,
       torvalds@osdl.org
Subject: Re: [PATCH] 2.6 workaround for Athlon/Opteron prefetch errata
Message-ID: <20030911173245.GJ29532@mail.jlokier.co.uk>
References: <99F2150714F93F448942F9A9F112634C0638B196@txexmtae.amd.com> <20030911012708.GD3134@wotan.suse.de> <20030911165845.GE29532@mail.jlokier.co.uk> <20030911190516.64128fe9.ak@suse.de>
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Andi Kleen wrote:
> > 	if ((addr & 3) == 0)
> > 		return 0;
> 
> Maybe. But gcc generates quite good code (binary decision tree) code for
> the switch() statement already so I don't think it is worth it to go 
> through complications just to avoid that.
> 
> The decoder looks big in C, but when you take a look at its hot path in 
> assembly it is quite fast.

I wonder.  No part of the signal path looks especially slow, and yet
as a whole it isn't that fast.  The odd percentage slowdown here and
there is quite bothersome.

GCC generates a decision tree.  You realise that about half of the
branches in that tree will be mispredicted?

-- Jamie
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