[LLVMdev] Using the unused "version" field in the bitcode wrapper (redux)

[Bob Wilson]

> On Nov 4, 2014, at 1:40 PM, Eric Christopher <echristo at gmail.com> wrote:
> 
> 
> 
> On Tue Nov 04 2014 at 12:43:05 PM Peter S. Housel <housel at acm.org <mailto:housel at acm.org>> wrote:
> On 11/04/2014 07:04 AM, Robinson, Paul wrote:
> >> From: llvmdev-bounces at cs.uiuc.edu <mailto:llvmdev-bounces at cs.uiuc.edu> [mailto:llvmdev-bounces at cs.uiuc.edu <mailto:llvmdev-bounces at cs.uiuc.edu>] On Behalf Of Sean Silva
> >>
> >> You haven't established that you really need this. AFAIK Apple's linker
> >> doesn't need this version information and they have shipped LTO for a
> >> while now.
> > Does Apple support library/middleware providers shipping bitcode instead
> > of object code?  That's the most nervous-making scenario for compatibility.
> > LTO by itself needs bitcode only as an ephemeral stage between source and
> > object; it's supporting bitcode as a long-lived on-disk format that keeps
> > us awake at night.
> >
> > I acknowledge the compatibility promise but I've been whacked upside the
> > head too often by QA over the years to take an unverified promise at
> > face value.  I would like worked examples and industry experience reports.
> > --paulr
> 
> I've been bitten by trying to do this sort of thing recently. The
> released LLVM 3.4 and 3.5, and the Xcode 5.x tools, generate debug info
> marked with the module flag:
> 
>      !49 = metadata !{i32 2, metadata !"Debug Info Version", i32 1}
> 
> The tools included with Xcode 6.x generate debug info marked as:
> 
>      !49 = metadata !{i32 2, metadata !"Debug Info Version", i32 600054001}
> 
> 
> This is an apple bug. You should report it.

No, it is intentional. Because our releases are not aligned with any LLVM release, we’re using debug version numbers that are completely different from trunk.

>  
> and will discard debug info marked with any other version.
> 
> 
> This is correct behavior.
>  
> I had originally hoped that our (Open Dylan) compiler could output
> bitcode that people could compile and link using the standard
> /usr/bin/clang installed with Xcode, but it's looking more like we may
> want to provide users with a build of vanilla LLVM/Clang for this
> platform instead.
> 
> Hopefully the debug info improvements currently under way will
> incorporate a compatibility guarantee that Apple-distributed tools will
> stick to as well.
> 
> 
> Unlikely for now though I don't speak for Apple.

We would love to have our debug info be compatible with the open-source version. We would need a stable representation of debug info for that to be possible.
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