[PATCH v9 0/5] add pointer compression API

[Paul Szczepanek <paul.szczepanek@arm.com>]

This patchset is proposing adding a new header only library
with utility functions that allow compression of arrays of pointers.

Since this is a header only library a patch needed to be added to amend
the build system to allow adding libraries without source files.

When passing caches full of pointers between threads, memory containing
the pointers is copied multiple times which is especially costly between
cores. A compression method will allow us to shrink the memory size
copied.

The compression takes advantage of the fact that pointers are usually
located in a limited memory region (like a mempool). We can compress them
by converting them to offsets from a base memory address.

Offsets can be stored in fewer bytes (dictated by the memory region size
and alignment of the pointer). For example: an 8 byte aligned pointer
which is part of a 32GB memory pool can be stored in 4 bytes. The API is
very generic and does not assume mempool pointers, any pointer can be
passed in.

Compression is based on few and fast operations and especially with vector
instructions leveraged creates minimal overhead.

The API accepts and returns arrays because the overhead means it only is
worth it when done in bulk.

Test is added that shows potential performance gain from compression. In
this test an array of pointers is passed through a ring between two cores.
It shows the gain which is dependent on the bulk operation size. In this
synthetic test run on ampere altra a substantial (up to 25%) performance
gain is seen if done in bulk size larger than 32. At 32 it breaks even and
lower sizes create a small (less than 5%) slowdown due to overhead.

In a more realistic mock application running the l3 forwarding dpdk
example that works in pipeline mode on two cores this translated into a
~5% throughput increase on an ampere altra.

v2:
* addressed review comments (style, explanations and typos)
* lowered bulk iterations closer to original numbers to keep runtime short
* fixed pointer size warning on 32-bit arch
v3:
* added 16-bit versions of compression functions and tests
* added documentation of these new utility functions in the EAL guide
v4:
* added unit test
* fix bug in NEON implementation of 32-bit decompress
v5:
* disable NEON and SVE implementation on AARCH32 due to wrong pointer size
v6:
* added example usage to commit message of the initial commit
v7:
* rebase to remove clashing mailmap changes
v8:
* put ptr compress into its own library
* add depends-on tag
* remove copyright bumps
* typos
v9
* added MAINTAINERS entries, release notes, doc indexes etc.
* added patch for build system to allow header only library

Paul Szczepanek (5):
  lib: allow libraries with no sources
  ptr_compress: add pointer compression library
  test: add pointer compress tests to ring perf test
  docs: add pointer compression guide
  test: add unit test for ptr compression

 MAINTAINERS                                |   6 +
 app/test/meson.build                       |  21 +-
 app/test/test_ptr_compress.c               | 108 +++++++
 app/test/test_ring.h                       |  92 ++++++
 app/test/test_ring_perf.c                  | 352 ++++++++++++++-------
 doc/api/doxy-api-index.md                  |   1 +
 doc/api/doxy-api.conf.in                   |   1 +
 doc/guides/prog_guide/index.rst            |   1 +
 doc/guides/prog_guide/ptr_compress_lib.rst | 142 +++++++++
 doc/guides/rel_notes/release_24_03.rst     |   6 +
 lib/meson.build                            |  15 +
 lib/ptr_compress/meson.build               |   4 +
 lib/ptr_compress/rte_ptr_compress.h        | 266 ++++++++++++++++
 13 files changed, 883 insertions(+), 132 deletions(-)
 create mode 100644 app/test/test_ptr_compress.c
 create mode 100644 doc/guides/prog_guide/ptr_compress_lib.rst
 create mode 100644 lib/ptr_compress/meson.build
 create mode 100644 lib/ptr_compress/rte_ptr_compress.h

--
2.25.1