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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 05.04.12 at 14:07, Tim Deegan <tim@xxxxxxx> wrote:
> # HG changeset patch
> # User Tim Deegan <tim@xxxxxxx>
> # Date 1333626954 -3600
> # Node ID 5a2f5ab5128e4b13b3fd2dbcae1f084bc922584e
> # Parent  5101e5ed24732919076d5285e55c7b53032749c2
> 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.

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.

Jan

>      struct page_info *page;
>      unsigned int order = get_order_from_pages(max_pages), free_order;
>  
> 
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