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

[Martell Malone via llvm-dev]

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.


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.
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