[PATCH v4 0/4] Libatomic: Add LSE128 atomics support for AArch64

[Victor Do Nascimento <victor.donascimento@arm.com>]

v4 updates

  1. Make use of HWCAP2_LSE128, as defined in the  Linux kernel v6.7
  for feature check.  This has required adding a new patch to the
  series, enabling ifunc resolvers to read a second arg of type
  `__ifunc_arg_t *', from which the `_hwcap2' member can be queried
  for LSE128 support.  HWCAP2_LSE128, HWCAP_ATOMICS and __ifunc_arg_t
  are conditionally defined in the `host-config.h' file to allow
  backwards compatibility with older versions of glibc which lack
  definitions for these.

  2. Run configure test LIBAT_TEST_FEAT_LSE128 unconditionally,
  renaming it to LIBAT_TEST_FEAT_AARCH64_LSE128.  While it may seem
  counter-intuitive to run an aarch64 test on non-aarch64 targets, the
  Automake manual makes it clear:

    "Note that you must arrange for every AM_CONDITIONAL to be
     invoked every time configure is run. If AM_CONDITIONAL is
     run conditionally (e.g., in a shell if statement), then
     the result will confuse automake."

  Failure to do so has been found to result in Libatomic build
  failures on arm and x86_64 targets.

  3. Minor changes in the implementations of {ENTRY|END}_FEAT and
  ALIAS macros used in `config/linux/aarch64/atomic_16.S'

  4. Improve commit message in PATCH 2/3 documenting design choice
  around merging REL and ACQ_REL memory orderings in LSE128 atomic
  functions.

Regression-tested on aarch64-none-linux-gnu on Cortex-A72 and
LSE128-enabled Armv-A Base RevC AEM FVP.

---

Building upon Wilco Dijkstra's work on AArch64 128-bit atomics for
Libatomic, namely the patches from [1] and [2],  this patch series
extends the library's  capabilities to dynamically select and emit
Armv9.4-a LSE128 implementations of atomic operations via ifuncs at
run-time whenever architectural support is present.

Regression tested on aarch64-linux-gnu target with LSE128-support.

[1] https://gcc.gnu.org/pipermail/gcc-patches/2023-June/620529.html
[2] https://gcc.gnu.org/pipermail/gcc-patches/2023-August/626358.html

Victor Do Nascimento (4):
  libatomic: atomic_16.S: Improve ENTRY, END and ALIAS macro interface
  libatomic: Add support for __ifunc_arg_t arg in ifunc resolver
  libatomic: Enable LSE128 128-bit atomics for armv9.4-a
  aarch64: Add explicit checks for implicit LSE/LSE2 requirements.

 libatomic/Makefile.am                        |   3 +
 libatomic/Makefile.in                        |   1 +
 libatomic/acinclude.m4                       |  19 ++
 libatomic/auto-config.h.in                   |   3 +
 libatomic/config/linux/aarch64/atomic_16.S   | 247 ++++++++++++++++---
 libatomic/config/linux/aarch64/host-config.h |  60 ++++-
 libatomic/configure                          |  61 ++++-
 libatomic/configure.ac                       |   3 +
 libatomic/configure.tgt                      |   2 +-
 9 files changed, 358 insertions(+), 41 deletions(-)

-- 
2.42.0