Mingw-w64 support

[klutzy]

This patchset enables rustc to cross-build mingw-w64 outputs.
Tested on mingw + mingw-w64 (mingw-builds, win64/seh/win32-threads/gcc-4.8.1).

I also patched llvm to support Win64 stack unwinding.
klutzy/llvm@ebe22bd

I cross-built test/run-pass/smallest-hello-world.rs and confirmed it works.
However, I also found something went wrong if I don't have custom #[start] routine.

[huonw]

What was the error message(s)/type of crash if there wasn't a custom #[start]?

[alexcrichton]

If we need LLVM patches for stack unwinding, I could throw those in with #8328. I don't think I'm going to find the rusti fixes soon, but we don't want to change LLVM too often.

[graydon]

Wow, this is great! Thanks so much.

[klutzy]

@huonw segfault from non-main thread.
Some more investigation: it seems to work well with std::rt::start_on_main_thread. At least this code works:

#[start]
fn start(argc: int, argv: **u8, crate_map: *u8) -> int {
    do std::rt::start_on_main_thread(argc, argv, crate_map) {
        println("start_on_main_thread");
        do spawn {
            println("spawn");
        }
    };
    return 0;
}

But if I change start_on_main_thread to start, it segfaults (at non-main thread) in msvcrt.dll when it calls _write to print.
(Same symptom if I use MessageBoxA instead of println.)

[klutzy]

With one more llvm patch (needed to build llvm on mingw-w64/SEH),
I cross-built rustc.dll and rustc.exe (with custom #[start] as above).

$ /d/usr/rust-64/bin/rustc.exe  --version
d:\usr\rust-64\bin\rustc.exe 0.8-pre
host: x86_64-w64-mingw32

However, touch empty.rs && rustc empty.rs failed with following message:

task <unnamed> failed at 'borrowed', D:\stone\rust\src\librustc\middle\resolve.rs:608

I don't know yet what it means.

[huonw]

I'm probably missing something, but this code looks like it's exactly the same as the x86 code; is it required to be duplicated like this?

[luqmana]

extern "stdcall" { ... } vs extern { ... }

[klutzy]

Yes, omitting "stdcall" (see also #8509), although for subtle reason.
If extern "stdcall" is given, rustc produces llvm's function declaration with callconv=X86StdcallCallConv, even if win64 does not have such convention.
Then llvm makes it worse: it mangles function name e.g. "MessageBoxA" -> "_MessageBox@16" but win64 only has "MessageBoxA", so link fails.
I think llvm is wrong (it should ignore wrong callconv), but don't know rustc is wrong or not.

[huonw]

This needs a rebase (also, #8328 has now landed, if this was blocked on it).

[klutzy]

rebased; fixed the segfault issue mentioned above (std::rt::start issue), which was due to incomplete context switching.

[klutzy]

Forgot to say llvm patches mentioned above: rebased on current master llvm head. klutzy/llvm@072c759 (let llvm emit segmented stacks on win64) and klutzy/llvm@7efe293 (to build llvm itself on mingw-w64+seh)

[alexcrichton]

Those patches still need to be merged, right? It doesn't look like they were included in this.

[klutzy]

yes, llvm patches are not merged to current llvm therefore still needed.