linux-kernel - Re: [PATCH RFC 00/11] lock monitor: Separate features related to lock

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Message-ID: <20100319030611.GE22095@nowhere>
Date:	Fri, 19 Mar 2010 04:06:13 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Hitoshi Mitake <mitake@....info.waseda.ac.jp>,
	Jason Baron <jbaron@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, h.mitake@...il.com,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [PATCH RFC 00/11] lock monitor: Separate features related to
	lock

On Thu, Mar 18, 2010 at 10:40:42PM -0400, Mathieu Desnoyers wrote:
> Well, the use-case that drove the asm goto implementation _is_ the tracepoints.
> ;)
> 
> > 
> > But, looking at __DO_TRACE:
> > 
> > 	if (it_func) {						\
> > 		do {						\
> > 			((void(*)(proto))(*it_func))(args);	\
> > 		} while (*(++it_func));				\
> > 	}
> > 
> > I would expect the compiler not to load the parameters in the stack
> > before first checking the branch.
> 
> Note that you have to put that in its full context. It's a macro expanded within
> a static inline function. The initial parameters are passed to the static
> inline, not directly as "args" here. So parameters with side-effects have to be
> evaluated before their result can be passed to the static inline function, so in
> that sense their evaluation cannot be moved into the conditional branch.


Evaluation yeah, I agree. A function passed as an argument is
going to be evaluated indeed, or whatever thing that has a side effect.
But there is nothing here that need to setup the parameters to the stack
right before the true tracepoint call, not until we passed the branch check
once.

 
> > So, the fact that parameters are not loaded before we know we'll call
> > the tracepoint is something we already have or is it something that the jump
> > label brings in the package somehow?
> 
> It's standard compiler optimization behavior.


Sure. My doubt is: currently with the upstream version, does the
compiler tend to load the parameters to the stack before the branch is
checked? Or is this a magic that jmp labels bring for whatever reason?

Thanks.

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