Constant merger patch to fix bug 17815 [Constant merging may illegally downgrade global alignment]

[Rafael Espíndola]

On 5 November 2013 06:12, Chris Smowton <chris at smowton.net> wrote:
> Hi all,
>
> I attach a patch to fix the constant merging pass, which can sometimes
> downgrade global alignment when one of those being merged has no specified
> alignment (i.e. getAlignment() == 0), causing apparent null-pointer
> segfaults that are actually x86 vector operation alignment faults.
>
> See http://llvm.org/bugs/show_bug.cgi?id=17815 for a test case and full
> details. Patch is against 3.2 again I'm afraid due to time constraints, but
> it doesn't look as though constmerge has changed significantly since then.

Hi Chris,
Thanks for working on this, it is quiet a bad bug.

It looks like the optimization is simply not valid if only one of the
constants has explicit alignment and we don't have TD, so we should
avoid selecting this pair for replacement in the first place. Can you
check if the attached patch solves the problem for you?

Cheers,
Rafael
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