The last Step-4 slice: real POSIX threads for a static FreeBSD build,
plus the TLS setup Step 3 deferred (threads need per-thread %fs TLS, and
the main thread needs it too for threadvar access).
runtime.syscall.freebsd gains four FreeBSD-specific leaves (all numbers
verified against release/14.0.0 sys/sys/syscall.h): sysarch=165 (set the
%fs base for TLS via AMD64_SET_FSBASE=129, passing a POINTER to the base
value), _umtx_op=454 (the futex analogue), thr_new=455 (the clone
analogue), thr_exit=431 (terminate ONE thread — NOT exit(2)/SYS_exit,
which kills the process).
runtime.start.static.freebsd gains the full TLS machinery from its Linux
sibling: the auxv PT_TLS walk (AT_PHDR/PHENT/PHNUM and PT_TLS are the same
values on FreeBSD as Linux), the variant-II block build, BuildThreadTLS/
FreeThreadTLS, and _BlaiseStartC now calls SetupTLS(auxv) before main.
The one difference from Linux: the %fs base is set via
sysarch(AMD64_SET_FSBASE, @base) rather than arch_prctl(ARCH_SET_FS, base).
runtime.thread.static.freebsd (new) is the FreeBSD sibling of
runtime.thread.static.linux — it DEFINES the bare pthread_* names the RTL
imports. The 3-state Drepper mutex (CAS/Xchg) is target-invariant and
copied verbatim; only the wait/wake primitive is swapped
futex -> _umtx_op(UMTX_OP_WAIT_UINT_PRIVATE=15 / WAKE_PRIVATE=16). Thread
create fills a 104-byte struct thr_param at the amd64 offsets
(start_func@0 arg@8 stack_base@16 stack_size@24 tls_base@32 tls_size@40
child_tid@48 parent_tid@56 flags@64 rtp@72), memset-zeroed first so flags/
rtp/spare are 0, and calls thr_new. child_tid/parent_tid point at an
Int64 (FreeBSD long*, not Linux's 4-byte). FreeBSD has no
CLONE_CHILD_CLEARTID, so join uses a dedicated JoinWord: the child
trampoline runs StartRoutine, sets JoinWord:=1, _umtx_op-wakes it, then
thr_exit(nil); pthread_join _umtx_op-waits while JoinWord=0.
Both new/edited units are standalone (nothing in the Linux build graph
uses them), so the Linux self-host is untouched. Test
TestLink_FreeBSDThreadLeaf_SyscallNumbers asserts thr_new(455)/_umtx_op
(454)/thr_exit(431) bytes are present and Linux's clone(56)/futex(202)
are absent. Step 4 (the FreeBSD syscall leaf) is now complete.
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