Date: Fri, 25 Nov 2016 16:31:17 -0500 From: Ed Maste <emaste@freebsd.org> To: "freebsd-toolchain@freebsd.org" <freebsd-toolchain@freebsd.org> Subject: Update on using LLVM's lld linker in the FreeBSD base system Message-ID: <CAPyFy2AqrL4zkjZWXv=56yUUBODAaAV9ATbLv-qAHV5VMjo7vA@mail.gmail.com>
next in thread | raw e-mail | index | archive | help
LLD developers have made much progress since my last update in August. Two options used by the FreeBSD build, -dc and -r, are now implemented. The issues with linker script expression support and symbol version maps have been addressed. At this point an LLD built from subversion can link a working FreeBSD-HEAD kernel and world except for the boot loaders. Here's an update on the plan I posted previously: > 1. Update lld along with the Clang/LLVM 3.9 update that dim@ is working on. > 2. Add the bmake build infrastructure, installing as /usr/bin/ld.lld > on the same architectures that use Clang (amd64, arm, arm64, i386). I > don't think there's a need for a WITH_LLD src.conf knob, but will add > one if desired. Now complete, with Dimitry's import of Clang 3.9.0 in r309124. There is a WITH_/WITHOUT_LLD knob, which defaults to on for amd64 and arm64, and off for all other architectures for now. > 3. Update lld again (most likely to a snapshot from upstream SVN) once > it is able to link an unmodified FreeBSD kernel. This is now possible, but I'm going to wait for 3.9.0 to settle and for the 3.9.1 update to happen first. > 4. Modify the boot loader and kernel builds to avoid using features > not implemented by lld. There are a few outstanding issues in LLD that prevent linking the boot loaders, but I'm hopeful that they will be addressed in the near future. > 5. Introduce a WITH_LLD_AS_LD knob to have /usr/bin/ld be a ld.lld > hardlink instead of /usr/bin/ld.bfd. I've added this already, to allow for testing and experimentation, and to provide some linker on arm64 image builds that can be used to bootstrap a later GNU ld or LLVM lld. It defaults to on for arm64 and off everywhere else. Note that the knob affects the installed linker (/usr/bin/ld), but does not change the linker actually used to build world and kernel, which remains GNU ld. > 6. Request ports exp-runs and issue a call for testing with 3rd party > software. Fix issues found during this process. This can start happening any time now for LLD 3.9.0, either by setting WITH_LLD_AS_LD with poudriere, or by using -fuse-ld=lld in LDFLAGS. For example, % cd /usr/src/bin/ls % LDFLAGS=-fuse-ld=lld make > 7. Switch /usr/bin/ld to ld.lld by default in head for the Clang-using > architectures. Add a WITHOUT_LLD_AS_LD knob to switch back to GNU ld. It's still too early to plan a switch by default.
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?CAPyFy2AqrL4zkjZWXv=56yUUBODAaAV9ATbLv-qAHV5VMjo7vA>