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

[Duncan P. N. Exon Smith]

> On 2014-Jul-30, at 17:19, Reid Kleckner <rnk at google.com> wrote:
> 
>> 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.

I was selfishly hoping (1) was better (deleting code is easier), but I
guess (2) is cleaner so if no one objects I'll move forward with that.

I should be able to throw a patch together tomorrow.