libatomic: Enable LSE128 128-bit atomics for Armv9.4-a

The armv9.4-a architectural revision adds three new atomic operations
associated with the LSE128 feature:

  * LDCLRP - Atomic AND NOT (bitclear) of a location with 128-bit
  value held in a pair of registers, with original data loaded into
  the same 2 registers.
  * LDSETP - Atomic OR (bitset) of a location with 128-bit value held
  in a pair of registers, with original data loaded into the same 2
  registers.
  * SWPP - Atomic swap of one 128-bit value with 128-bit value held
  in a pair of registers.

It is worth noting that in keeping with existing 128-bit atomic
operations in `atomic_16.S', we have chosen to merge certain
less-restrictive orderings into more restrictive ones.  This is done
to minimize the number of branches in the atomic functions, minimizing
both the likelihood of branch mispredictions and, in keeping code
small, limit the need for extra fetch cycles.

Past benchmarking has revealed that acquire is typically slightly
faster than release (5-10%), such that for the most frequently used
atomics (CAS and SWP) it makes sense to add support for acquire, as
well as release.

Likewise, it was identified that combining acquire and release typically
results in little to no penalty, such that it is of negligible benefit
to distinguish between release and acquire-release, making the
combining release/acq_rel/seq_cst a worthwhile design choice.

This patch adds the logic required to make use of these when the
architectural feature is present and a suitable assembler available.

In order to do this, the following changes are made:

  1. Add a configure-time check to check for LSE128 support in the
  assembler.
  2. Edit host-config.h so that when N == 16, nifunc = 2.
  3. Where available due to LSE128, implement the second ifunc, making
  use of the novel instructions.
  4. For atomic functions unable to make use of these new
  instructions, define a new alias which causes the _i1 function
  variant to point ahead to the corresponding _i2 implementation.

libatomic/ChangeLog:

	* Makefile.am (AM_CPPFLAGS): add conditional setting of
	-DHAVE_FEAT_LSE128.
	* acinclude.m4 (LIBAT_TEST_FEAT_AARCH64_LSE128): New.
	* config/linux/aarch64/atomic_16.S (LSE128): New macro
	definition.
	(libat_exchange_16): New LSE128 variant.
	(libat_fetch_or_16): Likewise.
	(libat_or_fetch_16): Likewise.
	(libat_fetch_and_16): Likewise.
	(libat_and_fetch_16): Likewise.
	* config/linux/aarch64/host-config.h (IFUNC_COND_2): New.
	(IFUNC_NCOND): Add operand size checking.
	(has_lse2): Renamed from `ifunc1`.
	(has_lse128): New.
	(HWCAP2_LSE128): Likewise.
	* configure.ac: Add call to
	LIBAT_TEST_FEAT_AARCH64_LSE128.
	* configure (ac_subst_vars): Regenerated via autoreconf.
	* Makefile.in: Likewise.
	* auto-config.h.in: Likewise.

1 files changed, 3 insertions, 0 deletions

diff --git a/libatomic/auto-config.h.in b/libatomic/auto-config.h.in
index ab3424a..7c78933 100644
--- a/libatomic/auto-config.h.in
+++ b/libatomic/auto-config.h.in
@@ -105,6 +105,9 @@
 /* Define to 1 if you have the <dlfcn.h> header file. */
 #undef HAVE_DLFCN_H
 
+/* Have LSE128 support for 16 byte integers. */
+#undef HAVE_FEAT_LSE128
+
 /* Define to 1 if you have the <fenv.h> header file. */
 #undef HAVE_FENV_H