[alsa-devel] [RFC][PATCH] ELD routines and proc interface

[Takashi Iwai]

At Fri, 14 Nov 2008 15:47:37 +0800,
Wu Fengguang wrote:
> 
> On Fri, Nov 14, 2008 at 08:43:59AM +0100, Takashi Iwai wrote:
> > At Fri, 14 Nov 2008 15:38:56 +0800,
> > Wu Fengguang wrote:
> > > 
> > > > > > > --- /dev/null
> > > > > > > +++ sound-2.6/sound/pci/hda/hda_eld.c
> > > > > > > +static inline unsigned char grab_bits(const unsigned char *buf,
> > > > > > > +						int byte, int lowbit, int bits)
> > > > > > > +{
> > > > > > > +	BUG_ON(lowbit > 7);
> > > > > > > +	BUG_ON(bits > 8);
> > > > > > > +	BUG_ON(bits <= 0);
> > > > > > 
> > > > > > Can it be rather BUILD_BUG_ON(), BTW?
> > > > > > Or, hmm, doesn't work if it's an inline function?
> > > > > 
> > > > > Yes, converted to BUILD_BUG_ON() and it compiles OK.
> > > > 
> > > > The question is whether this really triggers the build error
> > > > properly.  Could you check it, simply by changing the caller of
> > > > grab_bits() with some invalid values?  Then you should get a compile
> > > > error.
> > > 
> > > BUILD_BUG_ON() won't emit errors! So use BUG_ON()?
> > 
> > Try to make grab_bits() a macro and check whether BUILD_BUG_ON()
> > works.  I think it won't be too bad to use a macro for such a pretty
> > simple case.  If the resultant code looks too ugly, we should switch
> > back to BUG_ON().
> 
> OK, I'm fine with a macro.
> 
> > The difference is that BUILD_BUG_ON() would add no real code while
> > BUG_ON() is a pure run-time check.
> 
> But the code should be optimize away by gcc when the constant
> expression is false? 

Well, I think BUG_ON() code remains in this case because it's no
constant check as it's in a function (although inlined).  Otherwise
BUILD_BUG_ON() should have worked.


Takashi