Ffmpeg asm

Skill DROOdotFOO/agent-skills/skills/ffmpeg-asm

Agent skills, autonomous agents, and MCP-companions for programming

Install

npx -y skills add DROOdotFOO/agent-skills --skill ffmpeg-asm

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One thing to look at

1 stars1 stars. Stars are a popularity signal and not a quality one, but at this level it is likely that nobody has read this closely except its author, and you would be relying on your own review.

What its author says it does

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ffmpeg upstream contributions (hand-written assembly in libavcodec/libavfilter/libswscale) and custom ffmpeg integration (building, embedding, calling libav* APIs). Covers x86_64 (SSE/AVX/AVX2/AVX-512) and AArch64 (NEON/SVE/SVE2) idioms specific to ffmpeg's x86inc.asm and aarch64/asm.S macro frameworks, the checkasm and FATE test harnesses, and the ffmpeg upstream patch workflow. TRIGGER when: editing .asm files under libav*/x86/, .S files under libav*/aarch64/ or libav*/arm/, working with x86inc.asm, x86util.asm, or aarch64/asm.S macros, writing *_init.c SIMD dispatch tables, modifying tests under checkasm/, running FATE, configuring an ffmpeg build (./configure flags, --enable-*), linking libavcodec, libavformat, libavutil, libavfilter, libswscale, or libswresample from C, Rust, Elixir, or Go, preparing a patch for ffmpeg-devel or Patchwork, or comparing libav vs ffmpeg fork divergence. DO NOT TRIGGER when: general SIMD or intrinsics questions outside ffmpeg's macro framework (use droo-stack for C/Rust syntax, native-code for BEAM NIF SIMD), non-ffmpeg codec libraries like libvpx, dav1d, x264, x265, SVT-AV1 unless explicitly bridging through ffmpeg, GStreamer pipelines, application-level video editing UX, or container/protocol parser bugs without an asm or SIMD angle.

SKILL.md

10.4 KB, as published. Nobody here has run it

ffmpeg-asm

You are an ffmpeg DSP contributor. The C reference is the oracle. The assembler is hostile. Every cycle is contested. The upstream reviewer has been doing this since 2007.

Domain knowledge for hand-written SIMD assembly inside libav*, the build/integration surface around it, and the upstream patch workflow. For BEAM NIF wrappers around libavcodec see native-code. For general C or Rust language patterns outside ffmpeg's macros see droo-stack. For higher-level profiling workflow before reaching for asm see performance-profiler.

What You Get

x86inc.asm macro framework reference (cglobal, register naming, AVX/SSE auto-switching)
CPU dispatch and DSP-context init pattern (ff_get_cpu_flags_*, EXTERNAL_* gates)
x86_64 SIMD idioms (SSE through AVX-512) tailored to ffmpeg conventions
AArch64 NEON plus SVE/SVE2 idioms, with Apple M4 SME caveats and 32-bit ARM legacy notes
High-bit-depth (10/12-bit) templating, symbol suffixes, and clipping idioms
Codec hot-path catalog (DCT, motion compensation, deblock, entropy, scaler, colorspace)
Build and integration recipes for embedding ffmpeg in C, Rust, Elixir, Go
checkasm and FATE testing discipline, plus ffmpeg upstream patch workflow

Philosophy

The C reference is truth. Asm exists only to make the same output faster. Every kernel must be bit-exact against the C path under checkasm, every new function must register in the dispatch table in the same patch, and the patch goes to ffmpeg-devel, not GitHub.

Key principles

C reference first, asm second, byte-exact always -- no "close enough" on pixel output
Same-patch checkasm -- a kernel without a checkasm test will be rejected on review
INIT_XMM/INIT_YMM/INIT_ZMM is your ISA gate -- never hand-duplicate SSE/AVX/AVX-512 kernels
Dispatch registration is part of the patch -- a perfect kernel that nobody calls runs zero times
VZEROUPPER on AVX exit, mask-register hygiene on AVX-512 -- Skylake-class transition penalty is real
Contribute via ffmpeg's canonical paths, not the GitHub mirror -- code.ffmpeg.org or git send-email to ffmpeg-devel; see references/upstream-workflow.md
Asm comments describe the computation, not the patch -- drop AI-narration / authoring-meta (// Args are bare reg names, // Chroma-preserving variant of X for...); keep dataflow + register annotations even through param-rename refactors; cover letters report speedup, not "fewer cycles"; see references/upstream-workflow.md "Comment style"

When to use

Editing .asm under libav*/x86/ or .S under libav*/aarch64/ or libav*/arm/
Working with x86inc.asm, x86util.asm, aarch64/asm.S macro frameworks
Writing or modifying *_init.c SIMD dispatch tables
Adding or fixing tests under tests/checkasm/
Running FATE (make fate-*) and triaging failures
Configuring an ffmpeg build (./configure flags, custom minimal builds)
Linking libav* from C, Rust, Elixir (via NIF), or Go
Preparing a patch for ffmpeg-devel or following up on Patchwork review
Comparing libav vs ffmpeg fork divergence for a shared API surface

When NOT to use

General SIMD or intrinsics work outside ffmpeg's macros -- use droo-stack or native-code
Non-ffmpeg codec libraries' internal asm (dav1d, libvpx, x264, x265, SVT-AV1)
Hardware acceleration APIs (VAAPI, NVENC, VideoToolbox, QSV) -- these wrap vendor drivers
Application-level ffmpeg CLI usage, filter graph composition, muxing recipes
GStreamer, MLT, OBS, and adjacent application ecosystems
Audio DSP theory (psychoacoustic models, perceptual coding math)
Container or protocol parser security bugs without an asm/SIMD angle -- use security-auditor

Reading guide

Writing assembly

Working on	Read
x86inc.asm macros, cglobal, AVX/SSE switching	references/x86inc-macros
CPU feature detection and DSP dispatch tables	references/dispatcher-init
x86_64 SSE/AVX/AVX2/AVX-512 idioms	references/x86-simd-patterns
AArch64 NEON/SVE/SVE2, Apple Silicon, ARMv7 legacy	references/aarch64-neon-sve
10/12-bit pixel asm, BPC templating, HBD dispatch	references/high-bit-depth
DCT/IDCT, motion comp, deblocking, entropy, scaler	references/codec-hot-paths

Integration and process

Working on	Read
`./configure` flags, embedding libav* in C/Rust/etc	references/build-integration
checkasm + FATE byte-exact verification	references/testing-checkasm-fate
ffmpeg upstream patch workflow (both canonical paths)	references/upstream-workflow

Common pitfalls

Mistake	Impact	Fix
Missing `VZEROUPPER` before SSE-style code path	Per-instruction false-dep cost on Skylake+, merge dep on Zen	Add `RET` macro which emits `vzeroupper` for AVX `INIT_YMM` blocks
Submitting kernel without checkasm test	Reviewer rejects, byte-exactness unverified	Add `tests/checkasm/<module>.c` entry in the same patch
New function not registered in `*_init.c`	Kernel ships but nothing calls it	Add function pointer assignment under correct `EXTERNAL_*` flag
AVX-512 without `EXTERNAL_AVX512ICL` gate	Crashes on pre-ICL AVX-512 parts (Skylake-X, Cannon Lake) using VBMI2/GFNI	Gate with the narrowest applicable flag; consider `--disable-avx512`
Hand-duplicated SSE/AVX kernels	Doubles maintenance, divergent bugs	Use `INIT_XMM sse2` / `INIT_YMM avx2` to share source
AVX2 lane-crossing assumed to be SSE-style shuffle	Silent wrong output on upper 128-bit lane	Read AVX2 shuffle docs; use `vperm2i128` for cross-lane
GitHub PR opened against ffmpeg mirror	Ignored -- mirror is read-only	Use `code.ffmpeg.org` or `git send-email` to ffmpeg-devel
AI-narration `//` comments left in asm	Reviewer blocks PR for noise; reads as authoring-meta, not code semantics	Strip comments that explain the refactor or implementation choice; keep dataflow + register annotations only
`//` annotations stripped during param-rename refactor	Block: reviewer flags lost annotations one by one	Carry original comment text verbatim through the rename; change the symbol, not the trailing prose
Cover letter framed as "fewer cycles" / every bench width dumped	Reviewer requests rewrite; speedup is the grep target	Frame as speedup (new/baseline); foreground the widest column; model after `f54841d375`
v(N)->v(N+1) diff edits lines unrelated to review comments	Forgejo `Compare` button useless; review history splinters	Constrain revisions to review comments; split structural changes into a separate prep commit
8-bit-only kernel registered in HBD dispatch path	Integer overflow on first 10-bit frame	Add 10/12-bit variants (`_10`, `_12` suffix); see references/high-bit-depth
`SECTION_RODATA` constants without explicit align	Misaligned `mova` traps	`SECTION_RODATA 32` (or 64 for AVX-512); use `ALIGN`
`--enable-gpl` toggled accidentally on LGPL target	Downstream license obligations change	Audit `./configure` output; keep LGPL builds clean of GPL components