From Fedora Project Wiki

Transitioning to Zlib-ng as compatible replacement for Zlib


Replace Zlib with Zlib-ng. This change is strongly tied to Changes/MinizipNGTransition.


Current status

Detailed Description

Zlib has long served as a reliable compression library, but its performance on modern CPU architectures does not fully utilize their capabilities. The transition to Zlib-ng aims to enhance the compression efficiency and performance for Fedora while ensuring compatibility with existing packages and libraries.


Benefit to Fedora

  • Zlib-ng supports hardware acceleration when available
    • Adler32 implementation using SSSE3, AVX2, AVX512, AVX512-VNNI, Neon, VMX & VSX
    • CRC32-B implementation using PCLMULQDQ, VPCLMULQDQ, ACLE, & IBM Z
    • Hash table implementation using CRC32-C intrinsics on x86 and ARM
    • Slide hash implementations using SSE2, AVX2, ARMv6, Neon, VMX & VSX
    • Compare256 implementations using SSE2, AVX2, Neon, POWER9 & RVV
    • Inflate chunk copying using SSE2, SSSE3, AVX, Neon & VSX
    • Support for hardware-accelerated deflate using IBM Z DFLTCC
  • Includes Intel and Cloudflare optimizations
  • Uses code sanitizers, fuzzing, and code coverage


  • Proposal owners:
    • Zlib-ng will be rebuilt with zlib-compatible API, obsoleting zlib-1.2
      • Zlib-ng compat mode will be distributed in a new package called zlib-ng-compat, with its respective zlib-ng-compat-devel.
      • Initially, zlib-ng-compat will provide zlib = 1.2.13 and zlib-ng-compat-devel will provide zlib-devel = 1.2.13.
      • These versions will be updated following the support added to zlib-ng upstream.
    • Considering the new packages promise to be ABI-compatible with zlib, the packages depending on zlib won't be rebuilt.
      • This may help to catch ABI-compatibility issues soon.
  • Other developers: Help with build failures may be requested.
  • Release engineering: No action required
  • Policies and guidelines: N/A
  • Trademark approval: N/A
  • Alignment with Objectives: N/A

Upgrade/compatibility impact

Zlib-ng has a zlib-compat mode that aims to preserve both API and ABI compatibility with the original Zlib. However certain application tests may rely on hardcoded checksums for the compressed output. These tests could fail due to various changes, even if the compressed output remains valid and decompresses correctly.

How To Test

Update the system, verify that it does not break other packages.

User Experience

This change will increase compression efficiency and performance


Zlib is a dependency for over 100 packages. Most of them are expected not to fail or break.

Contingency Plan

  • Contingency mechanism: Revert the change to Zlib and rebuild dependent packages.
  • Contingency deadline: Beta freeze
  • Blocks release? No


Release Notes