[llvm-commits] [lld] r156100 - in /lld/trunk: include/lld/Reader/ include/lld/Reader/Reader.h lib/CMakeLists.txt lib/Reader/ lib/Reader/CMakeLists.txt lib/Reader/COFFReader.cpp tools/lld-core/CMakeLists.txt tools/lld-core/lld-core.cpp

[Michael Spencer]

On Fri, May 4, 2012 at 2:56 PM, Nick Kledzik <kledzik at apple.com> wrote:
> On May 4, 2012, at 1:32 PM, Michael Spencer wrote:
>> On Thu, May 3, 2012 at 2:27 PM, Nick Kledzik <kledzik at apple.com> wrote:
>>>
>>> On May 3, 2012, at 1:52 PM, Michael J. Spencer wrote:
>>>> --- lld/trunk/lib/Reader/COFFReader.cpp (added)
>>>
>>> I would have imagined this file going in:
>>>        lld/trunk/lib/Platforms/Windows/COFFReader.cpp
>>>
>>> In my mind there are three (initial) platforms:  Darwin, Windows, Unix*.  So, we have:
>>>
>>>        lld/trunk/lib/Platforms/Darwin/
>>>        lld/trunk/lib/Platforms/Windows/
>>>        lld/trunk/lib/Platforms/Unix/
>>>
>>> And code only needed by one platform goes under its Platform directory.
>>>
>>> The interface that all Platforms must provide is defined by Platform.h.  So one way to manage this would be to add:
>>>    bool isObjectFile(llvm::MemoryBuffer*);
>>>    File * parseObjectFile(llvm::MemoryBuffer*);
>>> to the Platform class, and wire up your COFF reader behind that in the Windows Platform.
>>>
>>> My assumption is that even though the lld internal model is platform independent, no one is going to want to use other platform object files (e.g. win32 in mach-o).  There are just too many ABI and other conventions implicit in the file format.
>>>
>>> * By Unix I mean ELF based Linux, Solaris, etc. I also image that the Unix platform will be much more customizable to support all the *nix variants.
>>>
>> I believe there are platforms that are sufficiently different to need
>> a different Platform class, but still use ELF. For example, Haiku and
>> PlayStation 3.
> What sort of differences to you see driving the need for a different Platform classes for these ELF based systems?  I just see these as minor variants on the Unix platform.  As I said, I image the Unix Platform would have a lot customization points.  That may mean there are many subclasses of PlatformUnix, or that PlatformUnix has a bunch of knobs which can be set differently to support different OSs.  But in either case, that is all under the lib/Platforms/Unix.

In the case of the PS3 and VITA, they are not Unix like at all, and
have very different ways of representing things like thread local
storage and shared objects.

> As an example of variants under a Platform, on Darwin most of the Platform class methods will operate very differently depending on the architecture (e.g. i386, x86_64, armv7).   I have not decided whether to have different PlatformDarwin subclass for each arch, or have a arch specific delegate object underneath PlatformDarwin.  But that choice does not effect Core linking or any other platform, so it all will be in lib/Platforms/Darwin.
>
>
>> PE/COFF is also used in EFI, which is very very
>> different from Windows.
> On Darwin, we use EFI to boot Macs.  Our tool chain is all mach-o based, and there is a final step to convert the generated mach-o executable to PE/COFF.
>
>
> Perhaps using OS names (Darwin, Windows, Unix) is confusing.  What if the file format names were used instead (MachO, PE-COFF, ELF).   Would it then  make more sense to put the Reader in the Platform (Format) directory?

Depends on how it is laid out.

> A related issue is that in "ld -r" mode, the linker merges object files and creates a new object file.  Where should that Writer live? Given that Object File Readers and Writers need to be kept in sync (because they must remain inverses of each other for every feature), it would make sense to keep that code close together.  My preference is that they all live in the Platform (or file format dir).
>
> -Nick

I agree that the reader/writer/exewriter should all be in the same directory.

One of the major reasons I want to have the formats in a different
directory is layering. The readers and writers shouldn't depend on the
platform at all. Any non-standard extensions (like all the .gnu
weirdness) can be handled by the platforms while reading.

So I propose:
lib/Format/{COFF,ELF,Mach-O}/{Reader/ObjectWriter/ExecutableWriter}.cpp
lib/Platform/{Unix,Darwin,Windows,etc...}

- Michael Spencer