[LLVMdev] LLD archive library design

[Nick Kledzik]

On Oct 19, 2012, at 3:41 PM, Shankar Easwaran wrote:

> Hi,
> 
> I have started to work on support for Reading archive libraries in lld and thought of using the llvm/lib/ArchiveReader for this.
> 
> The ArchiveReader doesnot fully support GNU archive libraries (thin archives), do you think we should continue using llvm/lib/ArchiveReader ?
> 
> I was chatting with Michael and looks like there have been discussions and small sketches done on reading archive libraries in lld. Can you provide pointers to the design ?
The general idea is to have a new Reader class (i.e. ReaderArchive)  along with a ReaderOptionsArchive class.  One of the options will be if all members are force loaded or not.  

The ReaderArchive ::parseFile() method will check the force-load option, if true it will parse up all the members and return an vector<>  of File* objects, one for each member.  If force-load is not specified,  ReaderArchive ::parseFile() will just return one FileArchive object which is a subclass of ArchiveLibraryFile.  The find() method of that class will search the table of contents and return a File* object for the member defining the requested symbol, or nullptr if nothing in the archive defines that symbol.  

> 
> I think the first set of features that lld needs to support is
> 
> 1) Read the archive libraries and pull in the right object file which defines the symbol
> 2) Read the archive libraries and pull all the object files (if force load)
> 
> There are os/architecture specific portions of resolving symbols with the archive libraries when it comes to common symbols, do you have any suggestions on this ?

There may need to be some ReaderOptionsArchive flags for how commons should be handled.  There should also be some ResolverOptions for how commons are handled.  

I don't know how much the archive file format varies across platforms.  In particular the table-of-contents file may be named differently and have a different format.  Again we may need ReaderOptionsArchive flags to drive this, or maybe the ReaderOptionsArchive can implicitly figure it out by looking at the content??  

Another issue is how the ReaderArchive knows which Reader to call to instantiate each member.  We have the same problem at the top level of the linker.   You could punt this to the client for now by having the ReaderOptionsArchive contain a Reader* which ReaderArchive and FileArchive use to instantiate member files.

-Nick