[Patch][pr19848] Produce explicit comdats in clang

[Richard Smith]

On Fri, Dec 19, 2014 at 12:51 PM, Rafael Espíndola <
rafael.espindola at gmail.com> wrote:
>
> There is quiet a bit of history behind this.
>
> The llvm IR until very recently had no support for comdats. This was a
> problem when targeting C++ on ELF/COFF as just using weak linkage
> would cause quiet a bit of dead bits to remain on the executable
> (unless -ffunction-sections, -fdata-sections and --gc-sections were
> used).
>
> To fix the problem, llvm's codegen will just assume that any weak or
> linkonce that is not in an explicit comdat should be output in one
> with the same name as the global.
>
> This unfortunately breaks cases like pr19848 where a weak symbol is
> not expected to be part of any comdat.
>
> Now that we have explicit comdats in the IR, we can finally get both
> cases right.
>
> This first patch just makes clang give explicit comdats to
> GlobalValues where it is allowed to.
>
> A followup patch to llvm will then stop implicitly producing comdats.
>

I'm not sure this is the right direction, but if it is, this change looks
reasonable to me.

Assuming we want to go in this direction, I think we should also consider
switching from a weak linkage to a strong linkage for entities governed by
the ODR; using weak linkage is a hack, since these symbols are really not
weak in the usual sense. That exposes a hole in our comdat support: we
should have the ability to define a discardable comdat (which need not be
emitted if none of its symbols are used in the current TU); a discardable
comdat with an external function definition seems like the right
representation for an inline function.
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