Allow clang-cl.exe /c -emit-llvm

[Quuxplusone]

Bugzilla Link	PR35190
Status	NEW
Importance	P enhancement
Reported by	Andreas Haferburg (andreas.haferburg@gmx.de)
Reported on	2017-11-03 01:21:42 -0700
Last modified on	2018-06-03 17:44:34 -0700
Version	5.0
Hardware	PC Windows NT
CC	d.maljutin@yandex.ru, llvm-bugs@lists.llvm.org
Fixed by commit(s)
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There is no -emit-llvm flag in gcc, but the GCC driver was extended to support -emit-llvm. The equivalent for cl.exe would be /c -emit-llvm, but it doesn't work.

What I found very surprising is that clang-cl /c -Xclang -emit-llvm hello_world.c doesn't even work. Maybe that is a bug.

All the examples on https://clang.llvm.org/docs/UsersManual.html are written for clang.exe, and they don't work anymore for clang-cl.exe.

The workaround for getting clang-cl.exe to emit IR is to run clang-cl.exe -###, and then replace the -emit-obj with -emit-llvm, which I personally find quite unintuitive.

The goal for clang-cl.exe is to be compatible with cl.exe. But in my opinion, it should also remain compatible with LLVM: clang.exe should be LLVM+gcc, and clang-cl.exe should be LLVM+cl.

Cf. https://bugs.llvm.org/show_bug.cgi?id=35120

[Quuxplusone]

Second this. Seems very strange that this is not working and manually adding
all ms-compatibility flags set up by clang-cl is very cumbersome.

Currently this is the error clang-cl emits when trying to -Xclang -emit-llvm:

fatal error LNK1107: invalid or corrupt file: cannot read at 0xFEA
clang-cl.exe: error: linker command failed with exit code 1107 (use -v to see
invocation)