[LLVMdev] Inconsistent third field in global_ctors (was Re: [llvm] r214321 - UseListOrder: Visit global values)

[Reid Kleckner]

On Wed, Jul 30, 2014 at 3:46 PM, Rafael Espíndola <
rafael.espindola at gmail.com> wrote:

> > Looking a little deeper, it's not "old-style" exactly; rejecting this
> > isn't trivial.
> >
> >   - According to LangRef, the third field is optional [1].
> >
> > [1]:
> http://llvm.org/docs/LangRef.html#the-llvm-global-ctors-global-variable
> >
> >   - It was added in r209015 to support comdat keys.
> >
> >   - The helper functions `llvm::appendToGlobalCtors()` and
> >     `llvm::appendToGlobalDtors()` (in ModuleUtils.cpp) will by default
> >     set up the two-field version.
> >
> >   - Despite the field being optional (and the API defaulting to the
> >     short version), the BitcodeReader auto-upgrades to the 3-element
> >     version.
> >
> > It looks to me like `@llvm.global_ctors` and `@llvm.global_dtors` are in
> > an inconsistent state here.  In particular, with the same version of the
> > tool, if you generate the arrays you get one type, then if you write to
> > bitcode and read it back you get another type.
> >
> > There are a few ways to move forward:
> >
> >  1. Remove the auto-upgrade from the BitcodeReader, making the third
> >     field truly optional.  Why create them one way, and round-trip to
> >     bitcode another way?
> >
> >  2. Make the third field *required*.  This primarily means:
> >
> >     - changing `appendToGlobal?tors()` (etc.?) to add the third field
> >       immediately instead of waiting for a bitcode rount-trip and
> >     - rejecting the two-field version in .ll.
> >
> >     This is another way of removing the inconsistency.
> >
>
> I have a small preference for 2, but just because it makes the current
> IR definition simpler. Reid implemented the auto upgrade, so he
> probably has a better insight on which option is best.
>

Nick was in favor of 2 as well, which is what pushed me to do the
auto-upgrade, even though I initially intended for this to be optional.  I
didn't want to force the complexity of the 3 operand form on non-C++
frontends, basically.  Obviously, having one form is simpler from LLVM's
perspective.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20140730/12f2f73d/attachment.html>