Re: [Cocci] Performance issue with quite simple patch?

[Maxime Ripard <maxime@cerno.tech>]

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Hi Julia,

On Wed, Jan 13, 2021 at 11:58:41AM +0100, Julia Lawall wrote:
> On Wed, 13 Jan 2021, Maxime Ripard wrote:
> 
> > Hi!
> >
> > I've been trying to get a patch to rename any variable called "state" in
> > a given set of callbacks.
> >
> > This is the patch that I've come up with:
> >
> > @ plane_atomic_func @
> > identifier helpers;
> > identifier func;
> > @@
> >
> > (
> >  static const struct drm_plane_helper_funcs helpers = {
> >  	...,
> >  	.atomic_check = func,
> > 	...,
> >  };
> > |
> >  static const struct drm_plane_helper_funcs helpers = {
> >  	...,
> >  	.atomic_disable = func,
> > 	...,
> >  };
> > |
> >  static const struct drm_plane_helper_funcs helpers = {
> >  	...,
> >  	.atomic_update = func,
> > 	...,
> >  };
> > )
> 
> You don't need the ...s in the above.  For structure declarations
> Coccinelle is happy as long as what you specify is a subset of what is
> present.  The static and const aren't essential either.  If you remove
> them, the pattern will match whethe thy are present or not.

Oh, that's good to know, thanks!

> >
> > @@
> > identifier plane_atomic_func.func;
> > symbol state;
> > expression e;
> > type T;
> > @@
> >
> >  func(...)
> >  {
> >  	...
> > -	T state = e;
> > +	T plane_state = e;
> >  	<+...
> > -	state
> > +	plane_state
> >  	...+>
> >  }
> >
> > However, it seems like at least on a file (in Linux,
> > drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_plane.c), it takes quite big
> > performance hit with one CPU running at 100% until the timeout is hit.
> >
> > Replacing <+... by ... makes it work instantly, but doesn't really do
> > what I'm expecting, so I guess it's a matter of the patch being
> > subobtimal?
> >
> > Is there a more optimal way of doing it?
> 
> In your rule, I donkt think that there is really any essential connection
> between the declaration and the use?  You just want to change state to
> plane_state when it occurs in one of the functions that you detected.

I'm sorry, I forgot to mention that I only want the change to occur if
it's a variable declared in the function, not an argument to it

> So you could at least try the following and see if it gives any false positives:
>
> @@
> identifier plane_atomic_func.func;
> symbol state;
> expression e;
> type T;
> @@
> 
>  func(...)
>  {
>       <...
> (
> -     T state = e;
> +     T plane_state = e;
> |
> -     state
> +     plane_state
> )
>       ...>
>  }

I would never have thought of that, thanks :)

I tried to adjust it to match only on functions that have such a
variable defined:

@ plane_atomic_func @
identifier helpers;
identifier func;
@@

 struct drm_plane_helper_funcs helpers = {
 	.atomic_check = func,
 };

@ has_variable_state @
identifier plane_atomic_func.func;
symbol state;
expression e;
type T;
@@

 func(...)
 {
 	...
	T state = e;
 	...
 }

@ depends on has_variable_state @
identifier plane_atomic_func.func;
symbol state;
expression e;
type T;
@@

 func(...)
 {
 	<...
(
-	T state = e;
+	T plane_state = e;
|
-	state
+	plane_state
)
 	...>
 }

But it's still choking on that same file
(drivers/gpu/drm//atmel-hlcdc/atmel_hlcdc_plane.c). Every other file
that matches takes at most a few seconds though, so it seems a bit weird

Maxime

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