Re: [Xen-devel] [PATCH 6 of 6] x86: explicitly mark an __initdata variable as used

[Tim Deegan <tim@xxxxxxx>]

At 14:44 +0100 on 05 Apr (1333637053), Jan Beulich wrote:
> >>> On 05.04.12 at 14:07, Tim Deegan <tim@xxxxxxx> wrote:
> > x86: explicitly mark an __initdata variable as used.
> > 
> > This stops LLVM from replacing it with a different, auto-generated
> > variable as part of an optimization.  (The auto-generated variable
> > ends up in the normal data section, failing the check that this
> > file only contains __initdata vars).
> > 
> > Signed-off-by: Tim Deegan <tim@xxxxxxx>
> > 
> > diff -r 5101e5ed2473 -r 5a2f5ab5128e xen/arch/x86/domain_build.c
> > --- a/xen/arch/x86/domain_build.c   Thu Apr 05 12:55:54 2012 +0100
> > +++ b/xen/arch/x86/domain_build.c   Thu Apr 05 12:55:54 2012 +0100
> > @@ -129,7 +129,7 @@ static struct page_info * __init alloc_c
> >      struct domain *d, unsigned long max_pages)
> >  {
> >      static unsigned int __initdata last_order = MAX_ORDER;
> > -    static unsigned int __initdata memflags = MEMF_no_dma;
> > +    static unsigned int __initdata __attribute__((used)) memflags = 
> > MEMF_no_dma;
> 
> Without a code comment, the mere fact that this is (a) totally
> non-obvious, (b) being done differently for two neighboring
> variables of otherwise the exact same kind, and (c) probably
> a compiler bug makes it quite likely that your change will get
> removed again by a future commit.

Sorry, yes this deserves a code comment.  I'll add one. 

I agree that this is a compiler bug, but the LLVM developers disagree,
and I have limited time for arguing with compiler devs.

> Furthermore, it being needed on one but not the other variable
> makes it highly likely that the same issue could surface at any
> time here or elsewhere in the code.

I considered putting the __attribute__((used)) into the definition of
__initdata but thought it was more invasive.  I'm happy to reconsider. 

Tim.

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