ARM asm fails with latest Clang, non-private labels cannot appear between .cfi_startproc / .cfi_endproc pairs

[mstorsjo]

The ARM assembly, in particular https://github.com/libffi/libffi/blob/master/src/arm/sysv.S#L139-L213 (the sequence of ffi_call_VFP followed by ffi_call_SYSV) fails to compile for Windows/ARM with the very latest Clang (the latest nightly since today).

The breaking change in Clang is this commit: https://reviews.llvm.org/D155245

[MC][AsmParser] Diagnose improperly nested .cfi frames

Compiling fails with an error like this:

../src/arm/sysv.S:156:37: error: non-private labels cannot appear between .cfi_startproc / .cfi_endproc pairs
.globl ffi_call_SYSV; ; .align 3; ; ffi_call_SYSV:
                                    ^
../src/arm/sysv.S:141:2: error: previous .cfi_startproc was here
 .cfi_startproc
 ^

For unknown reasons, this error doesn't seem to trigger on Linux targets, as mentioned in https://reviews.llvm.org/D155245#4657075. However the overall sentiment seems to be that this construct, having one CFI region cover two global functions, is invalid.

[mstorsjo]

FWIW, the relevant commit in Clang was reverted in llvm/llvm-project@797b68c, and they're considering restricting the scope of when this is reported as an error in https://reviews.llvm.org/D155245#4657083.

[billatarm]

It also seems that cfi_startproc could be moved below the label as well, testing seems to indicate that it's fine and we can just work around it. TBH in all other code ive looked at using CFI directives (which isn't much), its always below the label FWIW.