[llvm-dev] Request to merge r242372 into the 3.7 branch: Fix for C API incompatibility between 3.6 and 3.7.

[Tom Stellard via llvm-dev]

On Mon, Nov 09, 2015 at 10:22:01PM +0000, Eric Christopher wrote:
> We covered this particular patch a bit at the C API BoF at the conference,
> and I think the general opinion is that this was just an unintentional bug
> and that fixing it is in 3.7.1 is the best solution forward.
> 

Did anyone object to the ABI breakage this would cause between 3.7.0 and 3.7.1?
My impression is that most users of the stable releases care a lot about ABI
stability, and this change will affect everyone not just users of the C API.

We will also need to update the SONAME for 3.7.1 and fix the symlinks to account
for this change.

If the consensus is that fixing the C API here is worth breaking ABI between
3.7.0 and 3.7.1, that is fine. I just want to make sure people were aware
of the downsides of breaking the ABI here.

-Tom

> I'll send out more on the C API BoF as I get out from under a pile of email.
> 
> -eric
> 
> 
> On Mon, Nov 9, 2015 at 2:07 PM Reid Kleckner <rnk at google.com> wrote:
> 
> > We knew this was an ABI break. Here was the logic for why we want it in
> > the 3.7.1 release:
> >
> > The C API is supposed to be stable. The fact that the 3.7.0 release
> > contained a modified LLVMBuildLandingPad function was a bug. Anyone who
> > relies on this API basically cannot use the 3.7.0 release. The set of users
> > using the C API and dealing with EH is probably small, so we chose not a to
> > rush out a 3.7.1 release with a fix for this. Instead, we let the fix roll
> > into 3.7.1, which will now have a backwards compatible C API and ABI with
> > pre 3.7.0 LLVM.
> >
> > ---
> >
> > Anyway, I don't really care whether this gets merged or not. I am not a
> > stakeholder in the stability of the C API. I would really prefer it if the
> > C API users voiced an opinion on whether they want the C API to be more
> > stable or closer to LLVM's in memory representation.
> >
> > On Mon, Nov 9, 2015 at 9:29 AM, Tom Stellard <tom at stellard.net> wrote:
> >
> >> On Wed, Oct 28, 2015 at 05:32:17AM +0000, Eric Christopher wrote:
> >> > On Tue, Oct 27, 2015 at 8:15 PM Chris Lattner <sabre at nondot.org> wrote:
> >> >
> >> > >
> >> > > > On Oct 27, 2015, at 8:10 AM, Tom Stellard <tom at stellard.net> wrote:
> >> > > >
> >> > > > Hi Chris,
> >> > > >
> >> > > > I would like to get your opinion on merging r242372
> >> > > > (http://reviews.llvm.org/rL242372) into the 3.7 branch.
> >> > > > The signature of the C API function LLVMBuildLandingPad
> >> > > > changed from the 3.6.0 to 3.7.0 release, so the C API
> >> > > > is currently incompatible between these to releases.
> >> > >
> >> > > This is a narrow enough API that it is probably only used by a few
> >> > > clients.  I’d be happy for us to do whatever those clients would like
> >> to
> >> > > see with this.
> >> > >
> >> >
> >> > Normally I'd prefer not to change API in a point release, but I think
> >> it's
> >> > fine to change it back here to match the previous releases and current
> >> > trunk. Just needs something in the release notes.
> >> >
> >>
> >> Adding the correct llvm-dev list this time...
> >>
> >> I've just realized that this patch changes the ABI and would make 3.7.0
> >> and 3.7.1
> >> binary incompatible.  I think breaking the stable ABI is worse than
> >> breaking the C API,
> >> so I would prefer to either drop this patch or come up with a work-around.
> >>
> >> One possible work-around would be to keep the LLVMBuildLandingPad
> >> signature the
> >> same and then add this to Core.h:
> >>
> >> #ifdef DEBUG // Not sure whether we should use this or ifndef NDEBUG
> >>   #define LLVMBuildLandingPad(B, Ty, PersFn, Name) \
> >>     dbgs() << "Warning: PersnFn parameter ignored.  You must explicitly
> >> set the " \
> >>               "personality function on the parent function with " \
> >>               " LLVMSetPersonalityFn().  This behavior changed in LLVM
> >> 3.7"; \
> >>     LLVMBuildLandingPad(B, Ty, Name)
> >> #else
> >>   #define LLVMBuildLandingPad(B, Ty, PersFn, Name)
> >>     LLVMBuildLandingPad(B, Ty, Name);
> >>
> >> I'm open to other suggestions.  What do people think about this?
> >>
> >> -Tom
> >>
> >>
> >> >
> >> >
> >> > >
> >> > > -Chris
> >> > >
> >> > > >
> >> > > > Merging this commit will make the 3.7.1 C API compatible
> >> > > > with 3.6.x and current trunk, but it will make the 3.7.1 C API
> >> > > > incompatible with 3.7.0.
> >> > > >
> >> > > > Not merging this commit will mean the 3.7.x C API will
> >> > > > be incompatible with 3.6.x and current trunk, but
> >> > > > the C API for 3.7.0 and 3.7.1 will be compatible.
> >> > > >
> >> > > > It took me a while to wrap my head around this, let me know if you
> >> have
> >> > > > any questions.
> >> > > >
> >> > > > -Tom
> >> > >
> >> > >
> >>
> >
> >