linux-kernel - Re: [PATCH tracing/kprobes v2 1/5] tracing/kprobes: Rename special variables syntax

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Date:	Mon, 5 Oct 2009 23:55:50 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Masami Hiramatsu <mhiramat@...hat.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...e.hu>,
	lkml <linux-kernel@...r.kernel.org>,
	systemtap <systemtap@...rces.redhat.com>,
	DLE <dle-develop@...ts.sourceforge.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@...radead.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ibm.com>,
	"Frank Ch. Eigler" <fche@...hat.com>
Subject: Re: [PATCH tracing/kprobes v2 1/5] tracing/kprobes: Rename special
	variables syntax

On Mon, Oct 05, 2009 at 05:34:24PM -0400, Masami Hiramatsu wrote:
> Hmm, one idea hits me, how about this? :)
> - %register
> - %%spvars (%%retval, %%arg0)

The problem is that such % or %% symbols have a specific
mean in some other well known areas.

If we borrow the % from the AT&T assembly syntax style
to use register names, that we can retrieve in gcc inline
assembly, then one may expect %% to have a meaning inspired
from the same area. %% has its sense in gcc inline assembly,
but applied there, it looks confusing.

I mean, I'm trying to think like someone reading a perf probe
command line without any documentation. The more this person
can understand this command line without documentation, the better.
We know that % is used for register names, some people know that %%
is used for register names too but when we are in gcc inline assembly
with var to reg resolution and need true registers name.
Then if I try to mirror this sense from gcc to perf probe use,
I feel confused, especially in the case of %%arg1.

In this case, we should rather have %%register and %arg0 :)

Hm, %register is a clear pattern.

Somehow, %retval looks clear too, retval is verbose enough and
% is still logical as return values are most of the time (always?)
put in a register.

But %%arg0 looks confusing.

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