[pkubaj]

Issue Number: #2697

Problem Summary: brpc does not build on 64-bit PowerPC. butil/build_config.h
and butil/atomicops.h have no entry for the architecture, butil/time.h
fails with "unsupported arch" in clock_cycles(), and bthread has no context
switch for it.

What is changed and the side effects?

Changed:

• build_config.h: ARCH_CPU_PPC64_FAMILY / ARCH_CPU_PPC64, endianness
  from __BYTE_ORDER__.
• atomicops: use atomicops_internals_gcc.h, which gains the Atomic64
  operations it was missing (the builtins already handle 64-bit values).
• time.h: clock_cycles() reads the time base with mftb.
• processor.h: cpu_relax() lowers the SMT thread priority briefly
  (or 1,1,1 / or 2,2,2), as the Linux kernel's cpu_relax() does.
• debugger_posix.cc: trap as the break instruction.
• bthread/context.{h,cpp}: ELFv2 implementation of
  bthread_jump_fcontext / bthread_make_fcontext, derived from
  boost.context's jump_ppc64_sysv_elf_gas.S / make_ppc64_sysv_elf_gas.S.
  The frame holds r14-r31, CR, LR, f14-f31, FPSCR and, when AltiVec is
  enabled, v20-v31 and VRSAVE; all of these are non-volatile in the ELFv2
  ABI, so a switch that dropped them would corrupt values the compiler keeps
  live across the call (the patch attached to Support brpc for PPC64 by HX #2697 saves only the FPRs and
  does not restore FPSCR). A new context starts with the current CR, FPSCR
  and VRSAVE.
• mongo_head.h: ARCH_CPU_LITTLE_ENDIAN is only defined on little-endian
  targets, so the byte-swap is decided by the preprocessor.

Side effects:

• Only the ELFv2 ABI is supported: ppc64le, and big-endian ppc64 with musl.
  ELFv1 (glibc ppc64 big-endian) keeps failing as before. context.h
  keeps its existing Linux-only platform selection; the FreeBSD builds
  below used the FreeBSD port's patch that adds a __FreeBSD__ block
  (upstream has none for any architecture).
• No change for any other architecture; every edit is behind
  __powerpc64__ / ARCH_CPU_PPC64_FAMILY / the new platform macro.

Tested

FreeBSD 15.1 powerpc64le (POWER9, clang 19, protobuf 5.29, system
googletest 1.16):

• Library, tools and BUILD_UNIT_TESTS=ON build. 18 of the 20
  bthread_*unittest binaries pass completely: butex, butex_multi_tag, cond,
  cond_bug, countdown_event, execution_queue, futex, id, key, list, once,
  ping_pong, rwlock, sched_yield, semaphore, timer_thread,
  work_stealing_queue and bthread_unittest.
• bthread_mutex_unittest: every test passes when run on its own
  (--gtest_filter), but the whole binary hangs in
  MutexTest.mix_thread_types after the performance tests (killed after
  5 min). bthread_setconcurrency_unittest: min_concurrency and
  concurrency_by_tag fail their worker-count assertions (250 expected,
  201 seen; "Invalid concurrency_by_tag=1"). Both look like scheduler
  accounting / test-order issues rather than the context switch, which the
  passing butex/cond/ping_pong/work-stealing tests exercise heavily; I could
  not compare against an amd64 FreeBSD run.
• bthread_dispatcher_unittest and bthread_fd_unittest do not compile on
  non-Linux hosts (undeclared n, wait_rc, epfd), independent of this
  change.
• Note for the test build: test/CMakeLists.txt calls use_cxx11(), which
  pins the debug butil/bvar copies to C++11 while protobuf 5 / abseil need
  C++14+ (the library itself gets C++17 from the top-level file); I
  commented that call out for the test run. That is a separate issue.
• A standalone register-pinned stress test of the new context switch (loads
  distinct patterns into f14-f31 and v20-v31 on each side of the switch,
  sets different FPSCR rounding modes per context, 300 000 round trips at
  -O2 and 100 000 at -O0) shows no register or FPSCR corruption on
  powerpc64le or big-endian powerpc64.
• The library also builds on big-endian FreeBSD powerpc64 (ELFv2).
• bthread_context_unittest gained a ppc64 section (FPR/VR/GPR
  preservation and FPSCR rounding mode across switches); it passes on
  powerpc64le and big-endian powerpc64 at -O2, -O0 and without AltiVec, and
  fails when any single register restore is removed from the switch.

Written with the help of Claude Code.

[Copilot]

🟡 Changes recommended

Approval is blocked by an unresolved critical PPC64 TOC restoration bug, an additional critical FreeBSD guard issue, and missing architecture-specific regression coverage.

3 open issues

• Save and restore r2 when switching PPC64 contexts · New
• Enable PPC64 ELFv2 context switching on FreeBSD · New
• Add PPC64 context-switch register-preservation tests · New

What changed in this PR

Adds initial PPC64 ELFv2 support across bthread and butil, including architecture detection, atomic operations, timing, processor relaxation, debugger traps, endian handling, and context switching.

Changes:

• Adds PPC64 architecture and endianness support plus GCC atomic operations.
• Implements PPC64-specific timing, cpu_relax(), debugger traps, and MongoDB byte-order handling.
• Adds ELFv2 context switching with nonvolatile integer, floating-point, condition-register, FPSCR, and optional AltiVec state.

Review findings remain unresolved:

• Critical (3 votes): src/bthread/context.cpp does not preserve and restore the ELFv2 TOC pointer in r2.
• Critical (4 votes): src/bthread/context.h limits PPC context support to Linux, excluding the stated FreeBSD ELFv2 target.
• Moderate (2 votes): No in-tree PPC64-specific register-preservation test covers the new context-switch implementation.

File	Description
src/​butil/​time.h	Reads the PPC64 time base for cycle timing.
src/​butil/​processor.h	Adds the PPC64 cpu_relax() implementation.
src/​butil/​debug/​debugger_posix.cc	Uses the PPC64 trap instruction for debugger breaks.
src/​butil/​build_config.h	Defines PPC64 architecture, bitness, and endianness macros.
src/​butil/​atomicops.h	Selects GCC atomic operations for PPC64.
src/​butil/​atomicops_internals_gcc.h	Adds 64-bit atomic implementations for GCC-based PPC64 builds.
src/​bthread/​context.h	Adds ELFv2 PPC64 context platform selection; its non-Linux guards require correction for FreeBSD support.
src/​bthread/​context.cpp	Implements PPC64 ELFv2 context creation and switching; TOC pointer preservation requires correction.
src/​brpc/​mongo_head.h	Makes MongoDB byte swapping compile-time endian-aware.

_{Review effort: Lite}

[Copilot]

🟡 Changes recommended

Critical unresolved PowerPC ELFv2 TOC-preservation issues can corrupt cross-DSO execution when contexts switch or return.

1 open issue

• Restore caller r2 before returning · New

3 resolved since last review

• Enable PPC64 ELFv2 context switching on FreeBSD
• Save and restore r2 when switching PPC64 contexts
• Add PPC64 context-switch register-preservation tests

_{Review effort: Lite}