Bug report

On Windows, a PGO build made with Visual Studio 17.11 (MSVC 14.41, MSC v.1941) decompresses deflate data wrongly on CPUs with AVX-512 (here a Ryzen 9 7950X): test_zlib (test_decompressmaxlen*: "incorrect data check"), test_gzip (test_write_seek_write: "CRC check failed") and test_tarfile fail. The checks are right, the data is wrong: raw inflate of an intact file returns different bytes than Linux gzip, and zlib.crc32() is correct. Builds without PGO and the python.org builds (newer compiler) are fine.

This is zlib-ng/zlib-ng#1883: MSVC from Visual Studio 17.11 miscompiles GET_CHUNK_MAG() (arch/x86/chunkset_avx512.c, used for back-references shorter than 32 bytes). zlib-ng 2.2.5 works around it (zlib-ng/zlib-ng#1884) by turning optimization off for that function for Visual Studio before 17.13 (_MSC_VER < 1943). main, 3.15 and 3.14 bundle 2.2.4.

As discussed on #131438, I would like to request updating the Windows build to zlib-ng 2.2.5. 2.2.5 also checks for ZMM support before using the VPCLMULQDQ CRC code that CPython builds (zlib-ng/zlib-ng#1932) and reverts a deflate_medium change that regressed performance (zlib-ng/zlib-ng#1938). I'm happy to test a build on the affected machine.

• Release: https://github.com/zlib-ng/zlib-ng/releases/tag/2.2.5 (the attached binaries aren't needed)
• Sources: https://github.com/zlib-ng/zlib-ng/archive/refs/tags/2.2.5.tar.gz, SHA-256 5b3b022489f3ced82384f06db1e13ba148cbce38c7941e424d6cb414416acd18 (downloaded 2026-11-09)
• Tag commit: 425439062b114a0f6cf625022c41d929c7e879f9

cc @zooba

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Linked PRs

• gh-161735: Update zlib-ng to 2.2.5 on Windows #161813
• [3.15] gh-161735: Update zlib-ng to 2.2.5 on Windows (GH-161813) #161883
• [3.14] gh-161735: Update zlib-ng to 2.2.5 on Windows (GH-161813) #161884