Re: [PATCH] powerpc: Remove the static variable initialisations to 0

[Segher Boessenkool <segher@kernel.crashing.org>]

On Mon, Jul 25, 2022 at 01:27:52PM +1000, Michael Ellerman wrote:
> Segher Boessenkool <segher@kernel.crashing.org> writes:
> > On Sat, Jul 23, 2022 at 03:34:05PM +0200, Michal Suchánek wrote:
> >> Hello,
> >> 
> >> On Sat, Jul 23, 2022 at 05:24:36PM +0800, Jason Wang wrote:
> >> > Initialise global and static variable to 0 is always unnecessary.
> >> > Remove the unnecessary initialisations.
> >> 
> >> Isn't this change also unnecessary?
> >> 
> >> Initializing to 0 does not affect correctness, or even any kind of
> >> semantics in any way.
> >
> > It did make a difference when the kernel was still compiled with
> > -fcommon (which used to be the GCC default on most configurations, it is
> > traditional on Unix).  No explicit initialiser puts an object in .bss if
> > you use -fcommon.  This matters a bit for data layout.
> 
> The kernel has built with -fno-common since ~2002.

2001, yes (255649c18287).  And before that it was important to
initialise everything with static storage duration explicitly in the
source code.  It was part of the collective memory, I wondered if this
patch originated that way?

> I think the belief is that an explicit initialiser of 0 forces the
> variable into .data, but AFAICS that is not true with any compiler we
> support.

Exactly, you get identical code either way, if you use -fno-common.
People will still see this difference if they use a compiler before
GCC 10 for compiling most other things though.


Segher