ld64.lld doesn't respect -macos_version_min

[CendioOssman]

Over the years, Apple's ld64 has accepted multiple ways of specifying the target OS version. The current flag is -platform_version, but it has previously been -macos_version_min and -macosx_version_min.

Apple's ld64 accepts all three, but lld only accepts the latest one. This is a major issue when using gcc, which prefers -macos_version_min.

The end result is that ld64.lld is not a drop in replacement for ld64, as any usage results in:

ld64.lld: error: must specify -platform_version

[llvmbot]

@llvm/issue-subscribers-lld-macho

Author: Pierre Ossman (ThinLinc team) (CendioOssman)

Over the years, Apple's ld64 has accepted multiple ways of specifying the target OS version. The current flag is `-platform_version`, but it has previously been `-macos_version_min` and `-macosx_version_min`.

Apple's ld64 accepts all three, but lld only accepts the latest one. This is a major issue when using gcc, which prefers -macos_version_min.

The end result is that ld64.lld is not a drop in replacement for ld64, as any usage results in:

ld64.lld: error: must specify -platform_version

[CendioOssman]

gcc also wants to use -r, -keep_private_externs and -no_compact_unwind. So there seems to be more things to fix to make lld a full ld64 replacement for gcc.

[CendioOssman]

-r and -keep_private_externs are apparently from libtool, and not gcc. So it's a broader issue. Very new libtool seems to have mitigated the issue a bit, but it seems most projects are still including an older version.

[int3]

IMO gcc should update to the latest platform_version flags rather than have lld support legacy stuff. -r would be nice to support but that is a lot of work. -keep_private_externs should be straightforward though