[cfe-dev] [PATCH] Wrap clang modules inside Mach-O/ELF/COFF containers

[Adrian Prantl]

> On Dec 11, 2014, at 2:56 PM, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
> On Thu, Dec 11, 2014 at 2:40 PM, Argyrios Kyrtzidis <kyrtzidis at apple.com> wrote:
> The .pcm file is currently independent of debug info, meaning the compiler invocation will be able to use the same .pcm file regardless of whether the invocation had enabled debug info or not; with this change if an invocation had built a module file with debug info disabled, it would be inapplicable to the same invocation that had debug info enabled and would have to rebuild it; essentially we are tying module building with debug info. The module file as the “collection of semantic info” is conceptually independent from debug info.
> 
> I imagine the side table of entity hashes to AST constructs could be generated regardless of whether the module is built with debug info - it's probably cheap/small enough?
> 
> If generating the actual debug info in the module is too expensive to be always-on, we could make it conditional and have the compiler just build it on the fly if it wasn't present in the module. (but, ideally, it is cheap enough that we don't mind always putting it in the module and then ignoring it if the compilation referencing the module doesn't have -g)

To add to what David said, another advantage of having everything in one container is that further down the road, we may also embed bitcode for inlining purposes into the module.


-- adrian
>  
> 
> Did you consider having the debug info container being another file (e.g. besides the .pcm) that will reference the .pcm file ? This way, instead of having to update all users of module files, regardless if they care about debug info or not, you’d just make debug info another user of .pcm files, no more special than the others.
> 
> > On Dec 10, 2014, at 2:27 PM, Adrian Prantl <aprantl at apple.com> wrote:
> >
> > Hi everyone,
> >
> > As the first step in preparation for module debugging (see http://lists.cs.uiuc.edu/pipermail/cfe-dev/2014-November/040076.html) this patch turns the *.pcm files that are used to store clang modules and precompiled headers in a platform-dependent Mach-O/ELF/COFF container, so that eventually we will be able to store debug information alongside the module in the same file.
> >
> > This is implemented by using the standard LLVM code generation machinery. Instead of directly writing to the output file, the serialized AST blob is attached to an empty llvm::Module as a ModuleFlag. The module is passed to the backend which emits the AST blob into a special “__clang_pch" section in TargetLoweringObjectFile*.
> > On the ASTReader side, any object file is transparently unwrapped and the BitstreamReader is pointed directly to the AST section.
> >
> > Other than the .pcm files having an extra header inside, this patch is not meant to have any user-visible effects.
> >
> > Known bugs: I still need to figure out how to make c-index-test link against and register the available targets (check-all passes, but the modules created by c-index-test currently are plain old .pcm files).
> > Open questions: I made up the name of the new __clang_pch section and the various flags on the different platforms on the spot. I’m open to better suggestions.
> >
> > Let me know what you think!
> >
> > -- adrian
> > <clang.diff><llvm.diff>_______________________________________________
> > cfe-dev mailing list
> > cfe-dev at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
> 
>