[Bug tree-optimization/52571] vectorizer changes alignment of common symbols

["rguenther at suse dot de" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52571

--- Comment #4 from rguenther at suse dot de <rguenther at suse dot de> 2012-03-14 09:35:12 UTC ---
On Tue, 13 Mar 2012, ian at airs dot com wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52571
> 
> --- Comment #3 from Ian Lance Taylor <ian at airs dot com> 2012-03-13 15:48:36 UTC ---
> I agree: if the symbol is always common, the linker should use the largest
> alignment.  But the symbol need not always be common.  Consider one file with
> "unsigned long int *p;" and another file with "unsigned long int *p = &i;". 
> That is a valid use of common symbols.  But now gcc might mark the common
> symbol as having an alignment of 16, while leaving the definition with an
> alignment of 8.  The rules of common symbol linking mean that the definition
> will override the common symbol.  But the definition might have an alignment of
> 8, and it might not be possible for the linker to fix that--and in any case,
> the linker won't even try.
> 
> In other words, common symbols are special.  You can't assume that anything you
> change about them will stick, because they might become, in effect, an
> undefined symbol.  I suspect that vect_can_force_dr_alignment_p should be
> checking DECL_COMMON for exactly the reasons that it checks DECL_EXTERNAL.

Ok, thanks for the clarification, I'll produce a patch.

Richard.