New functionality
* string: New strrchr and stpcpy routines
* string: New Memory Tagging Extension (MTE) variants of strlen and strchr
* math: New vector version of pow(double)
* networking: Optimized ones' complement checksum for 32-bit and 64-bit Arm

Performance improvements
* string: Improved memcpy and memmove (SIMD and non-SIMD) for 64-bit Arm
* string: Improved memset for 64-bit Arm

Add scalar and NEON ones' complement checksumming implementations for
AArch64 and Armv7-A.

Improve comments in math_config.h.

Set WANT_ERRNO to 0 by default.

Use consistent file names.

Further optimize SIMD memcpy. Small cases now include copies up
to 32 bytes. 64-128 byte copies are split into two cases to improve
performance of 64-96 byte copies. Comments have been rewritten.
Performance on the random memcpy benchmark is ~10% faster.

Reading outside the range of the string is only allowed within 16 byte
aligned granules when MTE is enabled.

This implementation is based on string/aarch64/strchr.S

The 64-bit syndrome value is changed to contain only 16 bytes of
data.
The 32 byte loop is unrolled by two 16 byte reads.

Add support for stpcpy on AArch64.

Remove unnecessary code for unused ZVA sizes.
For zero memsets it's faster use DC ZVA for >= 160 bytes.
Add a define which allows skipping the ZVA size test when the
ZVA size is known to be 64 - reading dczid_el0 may be expensive.

Reading outside the range of the string is only allowed within 16 byte
aligned granules when MTE is enabled.

This implementation is based on string/aarch64/strlen.S

Merged the page cross code into the main path and optimized it.
Modified the zeroones mask to ignore the bytes that are loaded but are
not part of the string. Made a special case for when there is 8 bytes
or less to check before the alignment boundary.

Including multiple asm source files into a single top level file
can cause problems, this can be fixed by having one top level
file per target specific source file, but for maintenance and
clarity it's better to use the sub directory structure for selecting
which files to build.

This requires a new ARCH make variable setting in config.mk which
must be consistent with the target of CC.

Note: the __ARM_FEATURE_SVE checks are moved into the SVE asm code.
This is not entirely right: the feature test macro is for ACLE, not
asm support, but this patch is not supposed to change the produced
binaries and some toolchains (e.g. older clang) does not support SVE
instructions.  The intention is to remove these checks eventually
and always build all asm code and only support new toolchains (the
test code will only test the SVE variants if there is target support
for it though).

Further optimize integer memcpy. Small cases now include copies up
to 32 bytes. 64-128 byte copies are split into two cases to improve
performance of 64-96 byte copies. Comments have been rewritten.

Improves glibc's memcpy-random benchmark by ~10% on Neoverse N1.

Some functions were not tested with the statistical ulp error check
tool, this commit adds tests for the current math symbols.

This implementation is a wrapper around the scalar pow with appropriate
call abi. As such it is not expected to be faster than scalar calls,
the new double prec vector pow symbols are provided for completeness.

This was a placeholder for testing the build system before we added
optimized string code and thus no longer needed.

clang does not support c99 fenv_access and may move fp operations out
of conditional blocks causing unconditional fenv side-effects. Here

  if (cond)
    ix = f (x * 0x1p52);

was transformed to

  ix_ = f (x * 0x1p52);
  ix = cond ? ix_ : ix;

where x can be a huge negative value so the mul overflows. The added
barrier should prevent such transformation by significantly increasing
the cost of doing the mul unconditionally.

Found by enh from google on android arm and aarch64 targets.
Fixes github issue #16.

Add strrchr for AArch64. Originally written by Richard Earnshaw, same
code is present in newlib, this copy has minor edits for inclusion into
the optimized-routines repo.

This Assignment Agreement has to be filled in, signed and sent to
optimized-routines-assignment@arm.com by Contributors before their
contributions can be accepted into optimized-routines.

Use L(name) for all assembler labels.

Cleanup string functions to use asmdefs.h, ENTRY and END instead of
defining macros in each file.

Modify integer and SIMD versions of memcpy to handle overlaps correctly.

Make __memmove_aarch64 and __memmove_aarch64_simd alias to
__memcpy_aarch64 and __memcpy_aarch64_simd respectively.

Complete sharing of code between memcpy and memmove implementations is
possible without noticeable performance penalty. This is thanks to
moving the source and destination buffer overlap detection after
the code for handling small and medium copies which are overlap-safe
anyway.

Benchmarking shows that keeping two versions of memcpy is necessary
because newer platforms favor aligning src over destination for large
copies. Using NEON registers also gives a small speedup. However,
aligning dst and using general-purpose registers works best for older
platforms. Consequently, memcpy.S and memcpy_simd.S contain memcpy
code which is identical except for the registers used and src vs dst
alignment.

Mention releases.

Create a new memcpy implementation for targets with the NEON extension.

__memcpy_aarch64_simd has been tested on a range of modern
microarchitectures. It turned out to be faster than __memcpy_aarch64 on
all of them, with a performance improvement of 3-11% depending on the
platform.

Include asmdefs.h in memcpy.S to avoid duplicate macro definitions.

Add macro for defining labels in asmdefs.h.

Change the default routine entry point alignment to 64 bytes.

Define a new macro which allows controlling the entry point alignment.

Add include guard to asmdefs.h.

Don't include the makefile fragments of subprojects that aren't built.

With this the build fails more reasonably when SUBS is set incorrectly.

Reorganise the makefiles so subprojects can be more separately used and
maintained.  Still kept the single toplevel Makefile and config.mk.

Subproject Dir.mk is expected to provide all-X, check-X, clean-X and
install-X targets where X is the subproject name and it may use generic
make variables set in config.mk, like CFLAGS_ALL and CC, or subproject
specific variables like X-cflags.

Use .d rather than .2d for element mov instructions in string routines
so the assembly compiles with clang too.

When defined as 0 the vector math code is not built and not tested.

The math_errhandling checks are incorrect in general: it is defined
by the libc math.h which is not appropriate for optimized-routines
provided functions that we are testing.

However even if we want to test a libc implementation, ISO C allows
the setting of errno even if !(math_errhandling&MATH_ERRNO), so
relax the checks.

Vector functions are only used on aarch64, so only define them there.

  math/test/mathbench.c:95:1: warning: '__v_dummyf' defined but not used [-Wunused-function]

gcc-9 started warning if alias symbols have different attributes:

  math/expf.c: At top level:
  math/expf.c:89:21: warning: '__expf_finite' specifies less restrictive attributes than its target 'expf': 'leaf', 'nothrow', 'pure' [-Wmissing-attributes]

so copy the attributes when creating the aliases.

Compilers (incorrectly) warn about unused volatile variables:

  math/math_config.h: In function 'force_eval_float':
  math/math_config.h:188:18: warning: unused variable 'y' [-Wunused-variable]

silence them.

Compiler checks and realated macros need to be done earlier so they are
usable for the static inline functions.

Fix the Makefile so the documented mechanism in the README still works.

Same design as in expf. Worst-case error of __v_exp2f and __v_exp2f_1u
is 1.96 and 0.88 ulp respectively.

It is not clear if round/convert instructions are better or +- Shift.
For expf the latter, for exp2f the former seems more consistently
faster, but both options are kept in the code for now.

Use heredoc instead of pipe when iterating over test cases to avoid
creating a subshell that would break the PASS/FAIL accounting.

Increase the upper bound on medium cases from 96 to 128 bytes.
Now, up to 128 bytes are copied unrolled.

Increase the upper bound on small cases from 16 to 32 bytes so that
copies of 17-32 bytes are not impacted by the larger medium case.

Implicit function declaration is always a bug, but compilers don't
turn it into an error by default for historical reasons, so add it
to the default config.

This is a simple fix to the v_powf code, but in general the vector
code may not work on arbitrary targets even when compiled with
scalar types (s_powf.c), so in the long term may be all s_* should
be disabled for non-aarch64 targets (requires test system and header
changes too).