Re: Side-effect free printk?

From: Julia Lawall
Date: Wed Dec 03 2014 - 13:03:11 EST

On Wed, 3 Dec 2014, Joe Perches wrote:

> Most all printks uses do not have any side-effects.
> Some however modify local or global state or perform
> IO on various ports.
> Things like:
> drivers/video/fbdev/sa1100fb.c: dev_dbg(fbi->dev, "DBAR1: 0x%08x\n", readl_relaxed(fbi->base + DBAR1));
> drivers/remoteproc/remoteproc_core.c: dev_err(dev, "handling crash #%u in %s\n", ++rproc->crash_cnt,
> CONFIG_PRINTK can be set to 'n', but all direct printk
> calls still evaluate their arguments.
> These calls can unnecessarily increase code size.
> Some printk using macros are defined like:
> #define foo_dbg(fmt, ...) \
> do { \
> if (0) \
> printk(...); \
> } while (0)
> The compiler can optimize any use away so this can
> eliminate any side-effect.
> For the general case, printk arguments that call
> functions that perform simple calculations should not
> qualify unless there is some global state change or
> additional IO.
> So, with the goal of elimination of side-effects from
> as many of the printks as possible (and the eventual
> removal of all of the side-effects), is it possible to
> use coccinelle to list all printk calls that have
> side-effects in their arguments?
> It seems coccinelle would need the entire source tree
> to do this, so I'm not sure it's possible, but it
> doesn't hurt to ask...

I'm not completely sure to understand the question. The following
is certainly possible:


Of course one would want to have a lot more operators than the ones shown.
(As a side note, we are planning to add a metavariable for arithmetic and
side-effecting operators, which will make this sort of thing much

When you say "have the entire source tree", do you mean things like:

printk(..., foo(x));

where it is not clear whether foo performs a side effect or not? That
could indeed be harder to detect.

Perhaps the most relevant case is when there is a definition in the same
file, typically a macro in a header file. It could still be complicated
to make a pattern that would match all of the possibilities.

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