linux-kernel - Re: [PATCH 0/4] jump label patches

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Date:	Tue, 06 Oct 2009 16:24:47 -0700
From:	Richard Henderson <rth@...hat.com>
To:	Roland McGrath <roland@...hat.com>
CC:	Jason Baron <jbaron@...hat.com>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, mathieu.desnoyers@...ymtl.ca, tglx@...utronix.de,
	rostedt@...dmis.org, ak@...e.de, mhiramat@...hat.com
Subject: Re: [PATCH 0/4] jump label patches

On 10/05/2009 10:39 PM, Roland McGrath wrote:
> Of course, a first important point is what the actual compiled code
> sequences look like.  I'm hoping Richard (who implemented the compiler
> feature for us) can help us with making sure our expectations jibe with the
> code we'll really get.  There's no benefit in optimizing our asm not to
> introduce a jump into the hot path if the compiler actually generates the
> tracing path first and gives the hot path a "jmp" around it anyway.

At present, the asm goto extension gives no prediction weights to any 
path.  I had hoped that the -freorder-blocks pass (enabled with -O2) 
would automatically place the relevant fallthrough blocks immediately 
after the asm goto.  It did happen for small test cases, but a message 
from Jason downthread indicates that it doesn't always happen.

> 	if (__builtin_expect(0,0)) do_trace: __attribute__((cold)) { ... }

An attribute cold on a label is something that I've suggested, but have 
not yet implemented.  I think that might be the easiest way to add 
prediction weights to an asm goto.

r~
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