nmoinvaz

/* benchmark_dist1.cc -- compare strategies for the dist=1 path in CHUNKMEMSET */

#include <benchmark/benchmark.h>

extern "C" {
#  include "zbuild.h"
#  include "zutil.h"
}

#if defined(__ARM_NEON) || defined(__ARM_NEON__)

nmoinvaz

diff --git a/deflate_quick.c b/deflate_quick.c
index 6b84388e..41086525 100644
--- a/deflate_quick.c
+++ b/deflate_quick.c
@@ -49,13 +49,19 @@ Z_INTERNAL block_state deflate_quick(deflate_state *s, int flush) {
     unsigned char *window;
     unsigned last = (flush == Z_FINISH) ? 1 : 0;
 
+    /* Hold strstart and lookahead in registers across the inner loop.
+     * Sync to s before fill_window (which mutates them) and quick_*_block

nmoinvaz

zlib-ng: eliminate s->ins_h spill in insert_string_roll inner loop

Background

Inspecting AArch64 disassembly of zlib-ng's level-9 deflate path showed that the rolling-hash insert loop emitted a redundant store of the running hash to memory on every byte processed. The cause was visible in the C source and the fix is small and self-contained.

Root cause

nmoinvaz

zlib-ng PR #2281 — apt-cache v2 results

PR: zlib-ng/zlib-ng#2281 — [CI] Cache Ubuntu .deb packages to speed up installing dependencies.

Follow-up to https://gist.github.com/nmoinvaz/978f248ea7d528c954e3d52fb8dc99c0 — measuring the v2 design after the author refactored into composite actions and adopted the matrix.packages gate.

TL;DR

v2 is a clean win across the board. Total apt-install time across Ubuntu jobs is now −71% vs. develop, up from −47% in v1. Default-only jobs no longer pay cache overhead, and warm cache hits skip the write-back step entirely.

nmoinvaz

zlib-ng PR #2281 — apt-cache analysis and recommendation

PR: zlib-ng/zlib-ng#2281 — [CI] Cache Ubuntu .deb packages to speed up installing dependencies.

TL;DR

The cache works: it cuts total apt-install time across Ubuntu jobs roughly in half. But cmake.yml enables the cache on every Ubuntu job, including the 23 jobs that only install the default libgtest-dev libbenchmark-dev package set. For those, cache-action overhead exceeds the savings. configure.yml already gates on matrix.packages; mirroring that gate in cmake.yml is a one-line fix.

Methodology

nmoinvaz

zng_check_lens: SIMD vs SWAR vs scalar

Validity-check benchmark for the zng_check_lens(lens, codes) function proposed in the PR #2267 discussion. All three variants scan lens[0..codes-1] and return -1 if any entry exceeds MAX_BITS (15). Input is all-valid (random values in [0, 15]) so the worst case — a full scan with no early exit — is measured.

Variants:

nmoinvaz

count_lengths: SWAR vs SIMD

Investigation spun out of the PR #2267 discussion on zlib-ng: can the SIMD paths in count_lengths (inftrees.c) be replaced with a SWAR implementation using zng_memread_8?

SWAR design

Mirror the pair-interleaved 8-bit-lane structure of the active SIMD path. Two pairs of uint64_t accumulators (s1_lo/s1_hi,

nmoinvaz

/* SIMD validity check for a Huffman code-length buffer.
 *
 * Returns 0 if every entry in lens[0..codes-1] is <= MAX_BITS,
 * otherwise returns -1. Called from zng_inflate_table before
 * count_lengths to guard against the out-of-bounds read of one[]
 * described in zlib-ng issue #2266.
 *
 * Main loop scans 8 uint16_t per iteration via 128-bit vector
 * compares; a scalar tail handles the remaining 0-7 entries. No
 * assumptions about buffer padding or caller identity.

nmoinvaz

#!/bin/bash
# Toy-test cross-compile: compile a minimal C file (see
# zlib-ng-pr2261-visibility-test.c) with every GCC cross compiler we can
# reasonably get, twice per architecture (visibility("hidden") and
# visibility("internal")), and diff the assembly output — ignoring the
# .hidden/.internal pseudo-op so any real codegen difference surfaces.
#
# Run via Docker:
#   docker run --rm -v /path/to/scripts:/work -w /work \
#     debian:trixie-slim bash /work/zlib-ng-pr2261-visibility-runall.sh

nmoinvaz

/* Minimal reproducer: MSVC v142 _mm_set_epi64x miscompile, 32-bit x86.
 *
 * cl /O2 /arch:SSE2 repro_v142.c && repro_v142.exe
 *
 * v142 Win32: FAIL (wrong result due to register corruption)
 * v143+ Win32: PASS
 *
 * https://developercommunity.visualstudio.com/t/10853479
 */
#include <stdio.h>