[llvm-dev] Linking the FreeBSD base system with lld

[Rui Ueyama via llvm-dev]

On Wed, Nov 18, 2015 at 12:06 PM, Ed Maste via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I've been occasionally trying to build the FreeBSD base system (on
> amd64 / x86_64) using lld in order to see how things are progressing.
> With a few hacks in the FreeBSD build I can now link the base system
> userland.
>
> The missing functionality I had to work around is listed below. Some
> of these should probably be addressed in our FreeBSD build
> infrastructure, while others should be handled in lld. I'm listing
> this here only as a snapshot of where lld stands today for our use.
>
> 1. -r / --relocatable (PR 23024)
> There are four or five uses of -r in the FreeBSD base system, and at
> least some of them cannot be handled by just using ar instead.
>
> 2. -E / --export-dynamic
> This is used by our GDB build.
>
> 3. -Y [path]
> Used by the 32-bit compat libraries. GNU ld documents this as "add
> path to the default library search path." I don't understand exactly
> what this option does, versus -L.
>
> 4. unrecognized reloc 22 (R_X86_64_GOTTPOFF)
> Encountered in one of the libc tests.
>
> 5. --no-undefined vs _GLOBAL_OFFSET_TABLE_
> A number of libraries are linked with --no-undefined. These fail wtih
> an error of the form:
> undefined symbol: _GLOBAL_OFFSET_TABLE_ in /<path>tmp/usr/lib/crtbeginS.o
>
> 6. emulation type "elf_i386_fbsd"
> This is used by the 32-bit library build, which fails with "Unknown
> emulation: elf_i386_fbsd"
>

gold doesn't recognize that emulation name. Maybe you want to use elf_i386
instead?


> 7. static linking fails because _end is undefined (PR 25528)
>
> 8. Library search paths
> The FreeBSD build infrastructure builds the toolchain twice: once for
> the build host to build the target FreeBSD libraries and binaries, and
> once for installation as the toolchain installed in the target. The
> host toolchain is built with a set of baked-in library search paths
> (.../tmp/lib and .../tmp/usr/lib).
>
> I have a WIP patch to always apply --sysroot during the FreeBSD build,
> but with this added it seems we do not get $SYSROOT/lib and
> $SYSROOT/usr/lib as default search paths in lld. We also lack a
> convenient way to debug this today in lld; with GNU ld the --verbose
> option dumps a built-in linker script which includes the paths in use,
> but lld does not have something equivalent.


Feel free to add that functionality. We have --verbose flag already, but it
prints out little information.

Also, the kernel link fails because FreeBSD's kernel linker script is
> too complicated for the current parser.
>
>
> For reference, here are the workarounds I applied to build FreeBSD
> userland with lld:
>
> * Remove -Wl,--no-undefined from those library makefiles where it is set
> * Disconnect lib/libpam/statc_modules from the build
> * Disable the addition of -static in bsd.lib.mk
> * Explicitly add the temporary build paths as -L options in various LDFLAGS
> * Set the following options in src.conf:
> WITHOUT_BOOT=yes
> WITHOUT_RESCUE=yes
> WITHOUT_LIB32=yes
> WITHOUT_GDB=yes
> WITHOUT_TESTS=yes
> MODULES_OVERRIDE=
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