[PATCH v2 00/10] compat/zlib: allow use of zlib-ng as backend

Hi,

I have recently started to play around with zlib-ng a bit, which is a hard fork of the zlib library. It describes itself as zlib replacement with optimizations for "next generation" systems. As such, it contains several implementations of central algorithms using for example SSE2, AVX2 and other vectorized CPU intrinsics that supposedly speed up in- and deflating data.

And indeed, compiling Git against zlib-ng leads to a significant speedup when reading objects. The following benchmark uses git-cat-file(1) with `--batch --batch-all-objects` in the Git repository:

    Benchmark 1: zlib
      Time (mean ± σ):     52.085 s ±  0.141 s    [User: 51.500 s, System: 0.456 s]
      Range (min … max):   52.004 s … 52.335 s    5 runs

    Benchmark 2: zlib-ng
      Time (mean ± σ):     40.324 s ±  0.134 s    [User: 39.731 s, System: 0.490 s]
      Range (min … max):   40.135 s … 40.484 s    5 runs

    Summary
      zlib-ng ran
        1.29 ± 0.01 times faster than zlib

So we're looking at a ~25% speedup compared to zlib. This is of course an extreme example, as it makes us read through all objects in the repository. But regardless, it should be possible to see some sort of speedup in most commands that end up accessing the object database.

This patch series refactors how we wire up zlib in our project by introducing a new "compat/zlib.h" header function. This header is then later extended to patch over the differences between zlib and zlib-ng, which is mostly just that zlib-ng has a `zng_` prefix for each of its symbols. Like this, we can support both libraries directly, and a new Meson build options allows users to pick whichever backend they like.

In theory, these changes shouldn't be necessary because zlib-ng provides a compatibility layer that make it directly compatible with zlib. But most distros don't allow you to install zlib-ng with that layer is it would mean that zlib would need to be replaced globally. Instead, they typically only provide a version of zlib-ng that only has the `zng_` prefixed symbols.

Given the observed speedup I do think that this is a worthwhile change so that users (or especially hosting providers) can easily switch to zlib-ng without impacting the rest of their system.

Changes in v2:
  - Wire up zlib-ng in our Makefile.
  - Exercise zlib-ng via CI by adapting our "linux-musl" job to use
    Meson and installing zlib-ng.
  - Link to v1: https://lore.kernel.org/r/20250110-b4-pks-compat-drop-uncompress2-v1-0-965d0022a74d@pks.im

The series is built on top of fbe8d3079d (Git 2.48, 2025-01-10) with ps/meson-weak-sha1-build at 6a0ee54f9a (meson: provide a summary of configured backends, 2024-12-30) merged into it.

Thanks!

Patrick

---
Patrick Steinhardt (10):
      compat: drop `uncompress2()` compatibility shim
      git-compat-util: drop `z_const` define
      compat: introduce new "zlib.h" header
      git-compat-util: move include of "compat/zlib.h" into "git-zlib.h"
      compat/zlib: provide `deflateBound()` shim centrally
      compat/zlib: provide stubs for `deflateSetHeader()`
      git-zlib: cast away potential constness of `next_in` pointer
      compat/zlib: allow use of zlib-ng as backend
      ci: switch linux-musl to use Meson
      ci: make "linux-musl" job use zlib-ng

 .github/workflows/main.yml |  2 +-
 .gitlab-ci.yml             |  2 +-
 Makefile                   | 21 +++++++---
 archive-tar.c              |  4 --
 archive.c                  |  1 +
 ci/install-dependencies.sh |  4 +-
 ci/lib.sh                  |  5 +--
 ci/run-build-and-tests.sh  |  3 +-
 compat/zlib-compat.h       | 47 +++++++++++++++++++++++
 compat/zlib-uncompress2.c  | 96 ----------------------------------------------
 config.c                   |  1 +
 csum-file.c                |  3 +-
 environment.c              |  1 +
 git-compat-util.h          | 12 ------
 git-zlib.c                 |  6 +--
 git-zlib.h                 |  2 +
 meson.build                | 24 +++++++++---
 meson_options.txt          |  4 ++
 reftable/block.c           |  1 -
 reftable/system.h          |  1 +
 20 files changed, 100 insertions(+), 140 deletions(-)

Range-diff versus v1:

 1:  5f650c2a6b =  1:  0d442c86cf compat: drop `uncompress2()` compatibility shim
 2:  a94c26ad03 =  2:  9fb474c07a git-compat-util: drop `z_const` define
 3:  4431647ede =  3:  d732ab51ca compat: introduce new "zlib.h" header
 4:  bbca17dfb5 =  4:  b261f9ebcd git-compat-util: move include of "compat/zlib.h" into "git-zlib.h"
 5:  d5315531d8 =  5:  8fa63dc02c compat/zlib: provide `deflateBound()` shim centrally
 6:  27691c395f =  6:  851c90ea6a compat/zlib: provide stubs for `deflateSetHeader()`
 7:  5c47ab5f9b =  7:  54d4a95753 git-zlib: cast away potential constness of `next_in` pointer
 8:  9826f57665 !  8:  e260c57b2e compat/zlib: allow use of zlib-ng as backend
    @@ Commit message
     
         Signed-off-by: Patrick Steinhardt <ps@pks.im>
     
    + ## Makefile ##
    +@@ Makefile: include shared.mak
    + # byte-order mark (BOM) when writing UTF-16 or UTF-32 and always writes in
    + # big-endian format.
    + #
    +-# Define NO_DEFLATE_BOUND if your zlib does not have deflateBound.
    ++# Define NO_DEFLATE_BOUND if your zlib does not have deflateBound. Define
    ++# ZLIB_NG if you want to use zlib-ng instead of zlib.
    + #
    + # Define NO_NORETURN if using buggy versions of gcc 4.6+ and profile feedback,
    + # as the compiler can crash (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49299)
    +@@ Makefile: else
    + endif
    + IMAP_SEND_LDFLAGS += $(OPENSSL_LINK) $(OPENSSL_LIBSSL) $(LIB_4_CRYPTO)
    + 
    +-ifdef ZLIB_PATH
    +-	BASIC_CFLAGS += -I$(ZLIB_PATH)/include
    +-	EXTLIBS += $(call libpath_template,$(ZLIB_PATH)/$(lib))
    ++ifdef ZLIB_NG
    ++	BASIC_CFLAGS += -DHAVE_ZLIB_NG
    ++	ifdef ZLIB_NG_PATH
    ++		BASIC_CFLAGS += -I$(ZLIB_NG_PATH)/include
    ++		EXTLIBS += $(call libpath_template,$(ZLIB_NG_PATH)/$(lib))
    ++	endif
    ++	EXTLIBS += -lz-ng
    ++else
    ++	ifdef ZLIB_PATH
    ++		BASIC_CFLAGS += -I$(ZLIB_PATH)/include
    ++		EXTLIBS += $(call libpath_template,$(ZLIB_PATH)/$(lib))
    ++	endif
    ++	EXTLIBS += -lz
    + endif
    +-EXTLIBS += -lz
    + 
    + ifndef NO_OPENSSL
    + 	OPENSSL_LIBSSL = -lssl
    +
      ## compat/zlib-compat.h ##
     @@
      #ifndef COMPAT_ZLIB_H
 -:  ---------- >  9:  7ae8f413d4 ci: switch linux-musl to use Meson
 -:  ---------- > 10:  2dd1b49e4f ci: make "linux-musl" job use zlib-ng

--- base-commit: b2da7775f8b064ef54920eb0f2e60c7f6df8f995 change-id: 20250110-b4-pks-compat-drop-uncompress2-eb5914459c32