[llvm-dev] RFC: APIs for bitcode files containing multiple modules

[Mehdi Amini via llvm-dev]

> On Oct 25, 2016, at 6:28 PM, Peter Collingbourne <peter at pcc.me.uk> wrote:
> 
> Hi all,
> 
> As mentioned in my recent RFC entitled "RFC: a more detailed design for ThinLTO + vcall CFI" I would like to introduce the ability for bitcode files to contain multiple modules. In https://reviews.llvm.org/D24786 <https://reviews.llvm.org/D24786> I took a step towards that by proposing a change to the module format so that the block info block is stored at the top level. The next step is to think about what the API would look like for reading and writing multiple modules.
> 
> Here's what I have in mind. To create a multi-module bitcode file, you would create a BitcodeWriter object and add modules to it:
> 
> BitcodeWriter W(OS);
> W.addModule(M1);
> W.addModule(M2);
> W.write();

That requires the two modules to lives longer than the bitcode write, the API could be:

BitcodeWriter W(OS);
W.writeModule(M1);
// delete M1
// ...
// create M2
W.writeModule(M2);

(Maybe you had this in mind, but the API naming didn’t reflect it so I’m not sure).


> 
> Reading a multi-module bitcode file would be supported with a BitcodeReader class. Each of the functional reader APIs in ReaderWriter.h would have a member function on BitcodeReader. We would also have a next() member function which would move to the next module in the file. For example:
> 
> BitcodeReader R(MBRef);
> Expected<bool> B = R.hasGlobalValueSummary();
> std::unique_ptr<Module> M1 = R.getLazyModule(Ctx); // lazily load the first module
> R.next();
> std::unique_ptr<Module> M2 = R.parseBitcodeFile(Ctx); // eagerly load the second module


That makes the API quite stateful, you may have good implementation reason for this, but they’re not clear to me.
I rather see the bitcode reader as a random access container, iterating over modules.

— 
Mehdi


> 
> We'd continue to support the existing functional APIs in ReaderWriter.h for convenience in the common case where the bitcode file has a single module.
> 
> Thanks,
> -- 
> -- 
> Peter

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