Add support for `-funified-lto`

[Temtaime]

Hello !
I built my C library with -flto=full flag, and i pass -flto=full to ldc too.

But on the link stage i get this error:
ld.lld: error: unified LTO compilation must use compatible bitcode modules (use -funified-lto)

Any way ldc supports unified lto ?
I checked that i do not pass -funified-lto to clang and there's no option in ld.lld to disable unified lto.

[kinke]

The official LDC builds themselves are LTO'd across D and C++, so yeah, definitely possible. You probably just need to make sure all 3 components (ldc, clang, lld) are based on the same LLVM version, for compatible bitcode files.

[JohanEngelen]

Should already work indeed, but it requires all compilation to either use thin LTO or full LTO. Supporting -funified-lto would enable mixing them. Changed this issue into a feature request.

For implementation, see: https://reviews.llvm.org/D123804

[Temtaime]

Thanks for a reply !
Is the info on release page is correct ?
The prebuilt packages use v20.1.5 but ldc depends on llvm 21

[kinke]

but ldc depends on llvm 21

Surely none of the official packages on GitHub. See ldc2 --version for what LLVM version your LDC is using.