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

[Mehdi Amini via llvm-dev]

> On Nov 9, 2015, at 2:07 PM, Reid Kleckner via llvm-dev <llvm-dev at lists.llvm.org> 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.


During the BoF at the LLVM Dev meeting, if I understood correctly (I hope other can confirm, Eric?), some people asked about this issue and it was said that 3.7.1 will take the fix.

— 
Mehdi



> 
> On Mon, Nov 9, 2015 at 9:29 AM, Tom Stellard <tom at stellard.net <mailto: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 <mailto:sabre at nondot.org>> wrote:
> >
> > >
> > > > On Oct 27, 2015, at 8:10 AM, Tom Stellard <tom at stellard.net <mailto:tom at stellard.net>> wrote:
> > > >
> > > > Hi Chris,
> > > >
> > > > I would like to get your opinion on merging r242372
> > > > (http://reviews.llvm.org/rL242372 <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
> > >
> > >
> 
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