[FFmpeg-cvslog] r12165 - trunk/libavcodec/dsputil.c

[Rich Felker]

On Fri, Feb 22, 2008 at 02:42:02PM +0100, Michael Niedermayer wrote:
> On Fri, Feb 22, 2008 at 12:44:13AM -0500, Rich Felker wrote:
> > On Thu, Feb 21, 2008 at 10:35:53PM -0700, Loren Merritt wrote:
> > > On Thu, 21 Feb 2008, Rich Felker wrote:
> > > >
> > > > The const keyword in C is not as strong as in C++. Historically it was
> > > > just a hint for warning generation and binary sectioning. C99
> > > > (possibly also C90?) add some teeth but I think it's still iffy to
> > > > consider const-qualified variables as real constants. They're not
> > > > usable in certain contexts where a constant is required.
> > > 
> > > It doesn't even have to be declared const.
> > > "static unsigned long pb_7f = 0x7f7f7f7f7f7f7f7fUL" works too: gcc sees 
> > > that it is never assigned to and thus is in fact constant. (assuming unit 
> > > at a time, of course)
> > 
> > If I someday switch to gcc 4, -fno-unit-at-a-time is probably the only
> > hope of building with <= 128 megs of ram. I'd rather not have
> > gratuitous dependencies on unit-at-a-time for optimal code when it's
> > trivial to write the optimal version directly.
> 
> I dont see why a properly written compiler would need 128mb ram to figure
> out which static variables arent written to. grep is well capable of doing
> that ...

The issue is that gcc insists on building a syntax tree and all its
optimization structures for the ENTIRE file in order to be able to do
optimizations like this. While your claim about grep is morally true,
it leaves out the fact that in order to perform this optimization for
all variables, you'd need to parse and identify all static variables,
then perform this grep-like checking for each one. And don't forget
about the preprocessor =) which can make grepping hard.

The core issue is that it's stupid to rely on the compiler to gather
information that you could have made explicit in the declarations.
Qualifying variables with const helps both the human reader and the
compiler, and using literal constants (possibly by #define) helps even
more because it's clear at the point they're used that they're
constant and will be inlined. This whole topic is much like the
question of using the inline keyword versus macros. Sure there are
places for both, but with a macro it's a lot more clear what will
happen when you compile...

Rich