On Mon, 12 May 2014 14:42:04 +0100 James Hogan <james.hogan@xxxxxxxxxx> wrote:
> > --- a/include/linux/compiler.h
> > +++ b/include/linux/compiler.h
> > @@ -324,11 +324,10 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
> >
> > #define __compiletime_assert(condition, msg, prefix, suffix) \
> > do { \
> > - bool __cond = !(condition); \
> > extern void prefix ## suffix(void) __compiletime_error(msg); \
> > - if (__cond) \
> > + if (!(condition)) \
> > prefix ## suffix(); \
> > - __compiletime_error_fallback(__cond); \
> > + __compiletime_error_fallback(!(condition)); \
> > } while (0)
> >
> > #define _compiletime_assert(condition, msg, prefix, suffix) \
> >
>
> Unfortunately this breaks the build of today's linux-next for the Meta
> architecture (arch/metag), which happens to use a fairly old compiler
> (based on gcc 4.2.4) which I presume is the reason why.
>
> A bunch of compile time asserts fail, even in code which should be
> optimised out. E.g. here's one which I analysed:
>
> mm/gup.c: In function ___follow_page_mask___:
> mm/gup.c:208: error: size of array ___type name___ is negative
>
> Line 208 uses HPAGE_PMD_NR which expands to a HPAGE_PMD_SHIFT, which
> expands to a BUILD_BUG(). However that line is inside an if block
> conditioned on pmd_trans_huge(*pmd) which include/asm-generic/pgtable.h
> defines inline to return 0, so the whole block should already be being
> optimised out.
>
> I don't understand why your patch should break things, I suspect it's
> related to the sparse behaviour you're trying to work around, but can we
> please drop this patch until a more portable workaround can be found?
Older gcc's often have this problem.
I suppose that build bustage is more serious than sparse false
positives so yes, let's please try to find an alternative.
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