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

[Peter S. Housel]

On 11/04/2014 07:04 AM, Robinson, Paul wrote:
>> From: 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}

and will discard debug info marked with any other version.

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.

-Peter-