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

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Message-ID: <4ACA7577.30407@redhat.com>
Date:	Mon, 05 Oct 2009 18:38:47 -0400
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.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

Frederic Weisbecker wrote:
> 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.

Hmm, but %%reg syntax is only for the special case of gcc-inline
assembly (e.g. assembler template, see
http://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html#s3).
So, I guess it will not be so confusing.

> 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.

Then, can we use @@ for prefix of special variables?? :-)

I'm so anxious about collision between register name and
those vars.

Thank you,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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