svn commit: r216161 - in head/sys: amd64/amd64 i386/i386

[Kostik Belousov]

On Sat, Dec 04, 2010 at 11:36:25AM +0100, Roman Divacky wrote:
> On Fri, Dec 03, 2010 at 06:17:16PM -0500, Jung-uk Kim wrote:
> > On Friday 03 December 2010 06:02 pm, Jung-uk Kim wrote:
> > > On Friday 03 December 2010 05:43 pm, Jung-uk Kim wrote:
> > > > On Friday 03 December 2010 05:08 pm, John Baldwin wrote:
> > > > > On Friday, December 03, 2010 4:54:10 pm Jung-uk Kim wrote:
> > > > > > Author: jkim
> > > > > > Date: Fri Dec  3 21:54:10 2010
> > > > > > New Revision: 216161
> > > > > > URL: http://svn.freebsd.org/changeset/base/216161
> > > > > >
> > > > > > Log:
> > > > > >   Explicitly initialize TSC frequency.  To calibrate TSC
> > > > > > frequency, we use DELAY(9) and it may use TSC in turn if TSC
> > > > > > frequency is non-zero.
> > > > >
> > > > > We zero the BSS, so these were already zero.  This just makes
> > > > > the actual kernel file on disk larger by wasting space in .data
> > > > > instead of .bss.
> > > >
> > > > Please note that I didn't touch other variables, e.g.,
> > > > tsc_is_broken, because I knew that.  However, I just wanted to do
> > > > that *explicitly*. Anyway, it is reverted now and SVN will
> > > > remember what I wanted to do. ;-)
> > > >
> > > > BTW, if my memory serves, GCC (and all modern C compilers) put(s)
> > > > zero-initialized variables back in .bss.
> > >
> > > I just tried it.  GCC generates identical binaries as I thought.
> > > However, Clang doesn't do the optimization. :-/
> > 
> > Strangely, Clang increases .bss when a global variable is explicitly 
> > initialized to zero.
> > 
> > -  2 .bss 00000004 0000000000000000 0000000000000000 00000540 2**2
> > +  2 .bss 00000014 0000000000000000 0000000000000000 00000540 2**3
> 
> in my naive test gcc produces:
> 
> .globl foo
> 	.section	.bss
> 	.align 4
> 	.type	foo, @object
> 	.size	foo, 4
> foo:
> 	.zero	4
> 
> 
> and clang produces:
> 
> 	.type	foo, at object             # @foo
> 	.bss
> 	.globl	foo
> 	.align	4
> foo:
> 	.long	0                       # 0x0
> 	.size	foo, 4
> 
> ie. both put them into BSS
I have no idea how and where your gcc is configured, in particular, I
find the non-documenting directive .zero somewhat puzzling.

Behaviour of clang looks like a plain bug, since initialized objects must
be put into the .data section, not into .bss.

Both gcc 4.2.1/FreeBSD and plain 4.5.1 produce
   .comm   foo,4,4
when compiling the file consisting of
long foo;
line. For amd64, substitute 4 by 8.
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