[llvm-dev] RFC: A new llvm-dlltool driver and llvm-lib driver improvements

[Martell Malone via llvm-dev]

Hey Rui,

> I wonder how llvm-dlltool would fit in the entire picture of mingw
> support. I don't think dlltool is the last missing piece. What do you need
> to do other than that to fully support mingw using LLVM toolchain?

Other then changing `lib/MC/WinCOFFStreamer.cpp` to not use -aligncomm
within the EmitCommonSymbol function and a single patch for mingw-w64
itself to pre-populate it's .ctors and .dtors list, so llvm-dlltool is
infact the only missing part really.
This gives us a fully working clang based mingw-w64 C compiler.

C++ and exception handling is a different story.

libc++ is somewhat working with the following test results

Expected Passes    : 2188
Expected Failures  : 44
Unsupported Tests  : 588
Unexpected Failures: 2816

I was able to build it with exceptions disabled and I actually managed to
bootstrap llvm and clang itself.
It didn't run very well though as you can imagine based on the above tests.

It's not a performance issue but a code maintenance issue. The initial
> patches to support mingw was trying to add a new linker driver and a
> different linkin semantics to the COFF linker which seemed too complicated
> to me. IIRC, I suggested adding a shim which translates GNU command line
> arguments to MSVC linker arguments. Didn't it work?

There were just so many differences between link and ld I never followed
down that path. I pushed forward with the short import library support
based on your later suggestions. My patch to hack lld into accepting some
very basic gnu front end arguments was enough to get all the above working
which was enough to develop further.

Best,
Martell

On Mon, Feb 13, 2017 at 8:08 PM, Rui Ueyama <ruiu at google.com> wrote:

> Hi Martell,
>
> I wonder how llvm-dlltool would fit in the entire picture of mingw
> support. I don't think dlltool is the last missing piece. What do you need
> to do other than that to fully support mingw using LLVM toolchain?
>
> On Mon, Feb 13, 2017 at 8:56 AM, Martell Malone via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hey llvm'ers,
>>
>> I have been working on a dlltool replacement for llvm.
>> Here is my initial differential https://reviews.llvm.org/D29892
>> It is based on some functionality that already exists in lld.
>> I added functionality to support, PE COFF Weak Externals and of course a
>> front end to actually use it.
>> I believe the work here can also be used for llvm-lib and lessen the load
>> on lld.
>> I would like some comments about how this could be be structured to live
>> in llvm with a shared code base across lib ar and dlltool.
>> I also have a section below called "Difference from lib" which is
>> somewhat of a rationale for the tool.
>>
>> Many Thanks,
>> Martell
>>
>>
>> Context
>> ==========
>>
>> Awhile back I talked to various llvm'ers about getting mingw-w64 support
>> for lld.
>> There were a few issues raised but the main issue was that mingw-w64
>> should try best to comply with the PECOFF spec, adding support for custom
>> sections and various binutils/mingw hacks would impact the performance of
>> the COFF linker and in general is not something that lld should support.
>>
>
> It's not a performance issue but a code maintenance issue. The initial
> patches to support mingw was trying to add a new linker driver and a
> different linkin semantics to the COFF linker which seemed too complicated
> to me. IIRC, I suggested adding a shim which translates GNU command line
> arguments to MSVC linker arguments. Didn't it work?
>
>
>> Motivation
>> ==========
>>
>> The main motivation was because dlltool and ld did not comply with PECOFF
>> Spec.
>> It has some custom formatting and uses the assembler for some reason to
>> generate import libraries, it did not use the short import library format
>> at all.
>>
>> There has been many work arounds for the problem this creates such as the
>> creation of the `reimp` tool. Which imports MSVC built libs creates a def
>> and uses dlltool so that the binutils linker can use it.
>>
>> We should just be using the short import format and the linker should
>> support that.
>> Thus llvm-dlltool was born.
>>
>>
>> Difference from lib
>> ===================
>>
>> Using PE COFF spec (section 8, Import Library Format) should be self
>> explanatory.
>> lib.exe is able to accept def files and create libraries using this
>> format.
>>
>> example
>>
>>   `LIBRARY "user32.dll"
>>    EXPORTS
>>    MessageBoxA`
>>
>> LIB.exe can create a user32.lib with the function MessageBoxA from the
>> above definition.
>>
>> Mingw-w64 is different MSVC in that we need to compile the runtime.
>> MS provide us with their crt prebuilt so lib.exe doesn't have support for
>> external function aliasing.
>> We often use aliases for posix naming reasons as well as avoid using the
>> MS version of a function.
>>
>> example
>>
>>   `LIBRARY "user32.dll"
>>    EXPORTS
>>    MessageBoxA
>>    MessageBoxW==MessageBoxA`
>>
>> LIB.exe doesn't not have support for this but dlltool does.
>> The best fit for this within the spec is PE COFF spec (Aux Format 3:
>> Weak Externals)
>>
>> Mingw-w64 also uses lib prefix and .a file extensions for the library
>> name so the driver should be different. The above example would be
>> libuser32.a, for when shared and static versions of the same library exist
>> there is the .dll.a variant.
>>
>>
>> Issues
>> =======
>>
>> Like more of the gnu suite dlltool was not designed in one build multi
>> target manner.
>> We would have to introduce custom arguments to specify the target,
>> "i686", "x86_64", "thumb2pe" etc.
>> This will most likely break some level of backwards compatibility with
>> the binutils version.
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170213/94383874/attachment.html>