Re: Automaticly eliminating redundant zero initialisers

[George Anzinger <george at pioneer dot net>]

Tigran Aivazian wrote:
> 
> On Mon, 1 May 2000, Tigran Aivazian wrote:
> > On Mon, 1 May 2000, Graham Stoney wrote:
> >
> > > In a discussion about Linux kernel optimisations, Russell King writes:
> > > > Oh, if any of the mm people are reading this, what about killing the
> > > > redundant zero initialisers so that these variables can be placed in
> > > > the BSS?
> > >
> > > Even better, is there any way to get gcc to treat static and extern variables
> > > with explicit all-bits-zero initializers as though they had no initializers?
> > >
> > > In other words, treat these the same:
> > >     static int initialised=0;       /* wastes space in .data */
> > >     static int uninitialised;       /* implicitly zero, and more efficient */
> > >
> > > This would be useful for all space-conscious environments where the user
> > > knows that .bss is zero-filled. Variables with redundant explicit zero
> > > initializers are effectively wasting .data space, but some programmer like
> > > to use them anyway.
> >
> > # cd /usr/src/linux/arch/i386/kernel
> > # grep -i surprise *S
> > head.S: * Clear BSS first so that there are no surprises...
> >
> > as you see, it has been done explicitly by the kernel, regardless of what
> > gcc may have prepared.
> 
> oops, I just realised you are asking for something slightly different.
> What I meant is that you can now safely ignore zero-initializers because
> BSS is zero-cleared by the kernel explicitly.
> 
> Regards,
> Tigran

Soon as you depend on this some OS dude will fill bss with -1s so he can
tell what was used... or some such.  Seems like a bad idea to depend on
bss being anything at all.

George