[patch] Add support for putting constructors and destructos in explicit comdats

[Rafael Espíndola]

I have rebased this patch on top of my refactoring patch since it
makes it quiet a bit simpler. I have attached both.

Because of some bugs in GCC there is a discussion going on
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62306 about the best way
to handle the destructor comdat. The attached patch implements the
suggestion of using a new comdat name.


On 1 September 2014 23:03, Rafael Espíndola <rafael.espindola at gmail.com> wrote:
> There are situations when clang knows that the C1 and C2 constructors
> or the D2 and D2 destructors are identical. We already optimize some
> of these cases, but cannot optimize it when the GlobalValue is
> weak_odr.
>
> The problem with weak_odr is that an old TU seeing the same code will
> have a C1 and a C2 comdat with the corresponding symbols. We cannot
> suddenly start putting the C2 symbol in the C1 comdat as we cannot
> guarantee that the linker will not pick a .o with only C1 in it.
>
> The solution implemented by GCC is to expand the ABI to have a comdat
> whose name uses a C5/D5 suffix and always has both symbols. That is
> what this patch implements.
>
> We could also use C5/D5 for linkonce_odr, but it is probably better to
> change one at a time.
>
> Cheers,
> Rafael
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