libfdk-aac

Author: Ludovic HENRY <ludovic.henry@qti.qualcomm.com> Date: 2026-06-17 Scope: RISC-V (riscv64/linux) support status for libfdk-aac Audience: Technical leadership, resource allocation strategy Verification policy: Every claim is cross-referenced to a primary upstream source. Items that could not be verified against a second source are marked [NEEDS VERIFICATION].

1. Project Overview

libfdk-aac is an open-source packaging of the Fraunhofer IIS FDK AAC codec library, maintained by Martin Storsjo (mstorsjo) as a personal project on GitHub. The upstream codec code originates from Fraunhofer-Gesellschaft zur Forderung der angewandten Forschung e.V. (Fraunhofer IIS, Erlangen, Germany), copyright 1995-2018. The mstorsjo packaging also incorporates ongoing upstream contributions from Google/AOSP, merged via automated bots (Treehugger Robot, android-build-merge-worker-robot).

The library provides AAC-LC, HE-AACv1, HE-AACv2 encode and decode. It is consumed by FFmpeg (optional link for AAC encoding), GStreamer (gstreamer-plugins-bad fdk-aac plugin), and Android (AOSP ART/media stack).

Governance: None. There is no foundation membership, no steering committee, no MAINTAINERS or OWNERS file, and no corporate sponsorship listed in the repository. Martin Storsjo is the sole gatekeeper. There is no tier policy, no platform support matrix, and no stated policy on accepting architecture-specific contributions.

Community culture on new ports: Neutral. Architecture-specific contributions have been accepted historically – s390x SIMD was added via PR #159 (merged December 2023, ~500 lines of vector intrinsics), SPARC architecture detection was added via PR #178 (merged February 2025), and AArch64 assembly optimization was attempted via PR #47 (closed, not merged). No one has filed a RISC-V optimization request, and no contributor is currently working on it.

License: Fraunhofer FDK AAC proprietary license. Redistribution without copyright fees is permitted under specific conditions, but the license explicitly states “NO EXPRESS OR IMPLIED LICENSES TO ANY PATENT CLAIMS ARE GRANTED.” This restricts the library from being included in main or free sections of most Linux distributions regardless of architecture.

RISE Project involvement: None. No RISE blog post mentions libfdk-aac or audio codec work. libfdk-aac does not appear in the RISE wheel builder package list (74 packages listed). No RISE-funded work exists for this project.

2. Port History and Upstreaming Timeline

Date Event Source
2022-05-13 Single commit 3aabcb6abd adds RISC-V architecture detection to libFDK/include/FDK_archdef.h – 6 preprocessor #define lines selecting 16-bit lookup tables and 32x16 multiply preference (generic C path, no SIMD) commit 3aabcb6abd
2022-05-13 Commit authored by Zhang Ye (haocheng.zy@alibaba-inc.com), committed by Mao Han (han_mao@linux.alibaba.com), both at Alibaba. The Gerrit-style Change-Id suggests internal review at Alibaba before direct push to master. [NEEDS VERIFICATION: no second source confirms Alibaba internal review process] commit 3aabcb6abd
2023+ Debian and Ubuntu package the 2022 commit as part of fdk-aac 2.0.2/2.0.3 and build riscv64 .deb packages using the generic C fallback path Debian tracker, Ubuntu packages

The port is fully upstream in the sense that the architecture detection commit is in the master branch. It is not fully upstream in any meaningful engineering sense – there is no riscv64 CI, no SIMD optimization, and no RISC-V contributor actively maintaining the port. The 2022 commit is the entirety of RISC-V work ever done on this project.

3. Upstream Support Tier

The project has no documented tier policy.

Evidence-based tier assignment:

Criterion amd64 arm64 riscv64
CI runner in upstream repo Yes (ubuntu-latest x86_64) Yes (macOS universal, MSVC ARM64, llvm-mingw aarch64-w64-mingw32) No
SIMD/intrinsic optimization Yes (x86/ headers: abs, clz, fixmul, fixpoint_math) Yes (arm/ headers: clz, cplx_mul, fixmadd, fixmul, scale, scramble) No (generic C fallback only)
Official GitHub release binaries No (no GitHub releases at all; source only) No No
Architecture characterization in FDK_archdef.h Yes Yes Yes (6 lines, 2022)
Distro binary packages Yes (all distros) Yes (all distros) Debian sid, Ubuntu 24.04 only

riscv64 is a third-class target: it compiles, links, and produces correct output via the generic C fallback, but receives no CI coverage, no SIMD acceleration, and limited distro packaging. It is closer to an ad-hoc port than a supported target.

4. Technical Architecture and RISC-V-Specific Subsystems

libfdk-aac is a fixed-point AAC codec. Its performance-critical operations are:

  • Fixed-point multiply (32x16, 32x32, MLA)
  • Fixed-point multiply-accumulate for filter banks
  • Complex multiply for the MDCT/FFT butterfly
  • Arithmetic shift with saturation (scale)
  • Count-leading-zeros (clz) for normalization
  • Absolute value (abs)
  • FFT scramble

Each of these has architecture-specific intrinsic implementations for ARM, MIPS, x86, and (partially) PowerPC. None has a RISC-V implementation.

Architecture-specific code inventory:

Component amd64 implementation arm64 implementation riscv64 implementation
abs x86/abs_x86.h no dedicated file (uses generic) missing – generic C
clz x86/clz_x86.h (uses __builtin_clz) arm/clz_arm.h (uses __builtin_clz + ARM hint) missing – generic C loop
fixmul (32x16) x86/fixmul_x86.h arm/fixmul_arm.h missing – 64-bit C cast fallback
fixmadd no dedicated file arm/fixmadd_arm.h missing – generic C
fixpoint_math x86/fixpoint_math_x86.h no dedicated file missing – generic C
scale (shift+sat) no dedicated file arm/scale_arm.h missing – generic C
cplx_mul (FFT) no dedicated file arm/cplx_mul_arm.h missing – generic C
FFT scramble no dedicated file arm/scramble_arm.h missing – generic C
Architecture tuning flags FDK_archdef.h (full) FDK_archdef.h (full) FDK_archdef.h (6 lines: 16-bit tables, MULT_32x16 preference)
RVV Vector intrinsics n/a n/a not present
Assembly (.S files) none in repo none for aarch64 specifically none
Architecture subdirectory libFDK/include/x86/ (4 files) libFDK/include/arm/ (6 files) none – directory does not exist

ISA extensions used on riscv64: None. The 2022 commit selects ARCH_PREFER_MULT_32x16, SINETABLE_16BIT, POW2COEFF_16BIT, LDCOEFF_16BIT, and WINDOWTABLE_16BIT. These are compile-time flags that cause the generic C code to use 16-bit ROM tables (reducing memory bandwidth) and prefer 32x16 over 32x32 multiplies. No Zba, Zbb, Zbc, Zvl, or RVV (Vector) extensions are used.

Functional correctness: The codec compiles and runs on riscv64 via the generic C path. Debian builds it successfully. There are no known correctness bugs.

Performance: No benchmarks exist. The absence of SIMD means every MDCT butterfly, every filter bank convolution, and every fixed-point multiply is executed as a scalar C operation. On a wide-issue out-of-order riscv64 core this will be slower than arm64 with NEON, though no quantitative data is available. For real-time audio on embedded riscv64 (in-order, low-frequency), the generic C path is likely insufficient for high-bitrate HE-AACv2 without hardware acceleration. Data not available: no published cycles/frame figures for libfdk-aac on any riscv64 silicon.

5. Build System, Cross-Compilation, and Toolchain

libfdk-aac supports two build systems: autoconf/automake (configure.ac, Makefile.am, autogen.sh) and CMake (CMakeLists.txt, minimum CMake 3.5.1). Neither has any riscv64-specific logic.

Cross-compilation with autoconf:

autoreconf -fi
./configure \
  --host=riscv64-linux-gnu \
  --prefix=/usr \
  CC=riscv64-linux-gnu-gcc \
  CXX=riscv64-linux-gnu-g++ \
  AR=riscv64-linux-gnu-ar
make -j$(nproc)

Cross-compilation with CMake:

cmake -S . -B build \
  -DCMAKE_SYSTEM_NAME=Linux \
  -DCMAKE_SYSTEM_PROCESSOR=riscv64 \
  -DCMAKE_C_COMPILER=riscv64-linux-gnu-gcc \
  -DCMAKE_CXX_COMPILER=riscv64-linux-gnu-g++ \
  -DCMAKE_INSTALL_PREFIX=/usr \
  -DBUILD_PROGRAMS=OFF
cmake --build build -j$(nproc)

No riscv64-specific CMake toolchain file exists in the repository, and none is required because the codebase has no CMAKE_SYSTEM_PROCESSOR branches.

Toolchain requirements: C++98 minimum. Any GCC or Clang from the past 15 years suffices. On Debian/Ubuntu, gcc-riscv64-linux-gnu and g++-riscv64-linux-gnu (GCC 12/13 in bookworm/trixie) are the standard packages. No minimum GCC version for riscv64 is documented in the project; GCC riscv64 support was complete from GCC 7.

QEMU: No QEMU setup exists in upstream CI. To run the test suite cross-compiled, qemu-riscv64-static with binfmt_misc registration enables transparent execution of the cross-compiled binary. No automated test runner script is provided by the project.

Known build failures on riscv64: None reported. Zero issues and zero PRs in the tracker mention any riscv64 build failure.

6. Feature Coverage and Gap Analysis vs arm64 and amd64

Feature matrix:

Feature amd64 arm64 riscv64
AAC-LC decode Yes Yes Yes
HE-AACv1 decode (SBR) Yes Yes Yes
HE-AACv2 decode (PS) Yes Yes Yes
AAC-LC encode Yes Yes Yes
HE-AACv1 encode Yes Yes Yes
SIMD-accelerated fixed-point multiply Yes (x86 intrinsics) Yes (ARM intrinsics) No (generic C)
SIMD-accelerated FFT/MDCT Partial (complex mul) Partial (complex mul, scramble) No
SIMD-accelerated scale/shift No (generic C) Yes (ARM intrinsics) No
clz hardware acceleration Yes (via x86 intrinsics or __builtin_clz) Yes No (generic C loop) [NEEDS VERIFICATION: GCC may generate riscv64 clz from __builtin_clz even without an explicit dispatch; this was not tested]
16-bit ROM tables (memory optimization) Yes Yes Yes (set by 2022 commit)
CI-validated correctness Yes Yes (macOS arm64, MSVC ARM64) No

Functional gaps: None. All codec features work on riscv64.

Performance gaps: Every SIMD-accelerated hot path falls back to generic C on riscv64. The magnitude of the regression cannot be quantified without benchmark data, which does not exist.

Security hardening gaps: No riscv64-specific stack protection, shadow call stack, or CFI configuration exists in the build system. This is not a regression from arm64 – neither target has project-level security hardening configuration. Data not available: no security audit of the riscv64 code path has been published.

Floating-point / NaN semantics: The codec is fixed-point throughout. Floating-point is not used in the audio pipeline. No NaN sensitivity issues apply.

7. CI/CD Infrastructure

The upstream CI is a single file: .github/workflows/ci.yml. It defines 14 jobs triggered on push and pull_request.

CI coverage by target:

Target CI present Runner Test depth
Linux x86_64 (autotools) Yes ubuntu-latest Build + sanitizers (ASan, UBSan)
Linux x86_64 (CMake) Yes ubuntu-latest Build
macOS x86_64+arm64 universal Yes macos-latest Build
Windows x86_64 (MSVC) Yes windows-latest Build
Windows ARM (MSVC cross) Yes windows-latest Build
Windows ARM64 (MSVC cross) Yes windows-latest Build
MinGW x86_64 Yes ubuntu-latest Cross-compile
MinGW AArch64 (llvm-mingw) Yes ubuntu-latest Cross-compile
FFmpeg integration (x86_64) Yes ubuntu-latest Build + functional
riscv64 (any form) No None None

RISE runners: Not used. The RISE Project announced RISC-V Runners in March 2026. libfdk-aac does not use them.

QEMU emulation in CI: Not present.

Adding riscv64 CI would require either a QEMU-based cross-compilation job (functional validation only, no performance data) or a native riscv64 runner via the RISE runners program.

8. Distribution and Release Status

GitHub releases: None. The project ships source tarballs via git tags only. No binary release assets exist for any architecture.

Distribution packages:

Distribution Package name riscv64 available Version Build status Notes
Debian sid libfdk-aac2t64, libfdk-aac-dev Yes 2.0.3-1 Installed (built on rv-manda-03 buildd, ~482 days ago) non-free/misc section
Ubuntu 24.04 (noble) libfdk-aac2, libfdk-aac-dev Yes 2.0.2-3~ubuntu4 Available (460.4 kB download) cross-compiled on amd64 builder
Arch Linux RISC-V port libfdk-aac No 2.0.3-2 (x86_64 only) Not ported fdk-aac is in Arch extra for x86_64; not carried in archriscv community port
Fedora fdk-aac Unknown Unknown Unknown Data not available: Koji build system was not accessible during research
PyPI n/a n/a n/a Not present (HTTP 404) No Python wrapper exists on PyPI

To get a working riscv64 binary:

  • On Debian sid: apt install libfdk-aac-dev (non-free repo must be enabled)
  • On Ubuntu 24.04: apt install libfdk-aac-dev (universe repo; non-free-firmware may also be needed)
  • On any other distribution: build from source using the cross-compilation steps in Section 5

The Debian and Ubuntu riscv64 binaries are compiled from the generic C fallback path. No native riscv64 hardware validation was performed by Debian/Ubuntu in producing these packages [NEEDS VERIFICATION: Debian buildd rv-manda-03 may run test suites; this was not confirmed from the research data].

9. Dependencies

libfdk-aac is intentionally self-contained. All codec subsystems (libAACdec, libAACenc, libSBR, libMpegTP, libFDK, libSYS) are bundled within the repository. There are no external library dependencies for the core codec.

Dependency table:

Dependency Role riscv64 build riscv64 test riscv64 release Blocking issues
libm (glibc) Optional: sin() for fixed-point table generation; linked as -lm where math is not in default libc Yes – glibc riscv64 ships libm Yes – glibc riscv64 is tested Yes – all major distros None
libstdc++ / libc++ C++ runtime; all sources are C++ Yes – GCC riscv64 support complete since GCC 7 Functional Ships in all riscv64 toolchains None
CMake / autoconf (build-time only) Build system n/a n/a n/a None
ARM SIMD (libFDK/include/arm/, libFDK/src/arm/) Optional performance: NEON intrinsics for fixmul, scale, cplx_mul, scramble, FFT Not applicable Not applicable Not applicable No riscv64 equivalent; see Section 6
MIPS DSP (libFDK/include/mips/, libFDK/src/mips/) Optional performance: MIPS DSP intrinsics Not applicable Not applicable Not applicable No riscv64 equivalent
x86 SIMD (libFDK/include/x86/) Optional performance: SSE/x86 intrinsics Not applicable Not applicable Not applicable No riscv64 equivalent
PowerPC (libFDK/include/ppc/) Optional performance: PPC intrinsics (partial) Not applicable Not applicable Not applicable No riscv64 equivalent

The only dependency deserving deeper attention is glibc/libm on riscv64. glibc has full riscv64 support including optimized math routines. Since libfdk-aac is a fixed-point codec and uses libm only at table-generation time (not in the real-time decode/encode path), glibc riscv64 maturity is not a concern.

11. Known Bugs and Active Issues

RISC-V-specific issues: None. GitHub issue and PR searches for “riscv” in repo:mstorsjo/fdk-aac return zero results (confirmed via GitHub search API). Manual review of all issues and all 26 PRs found no RISC-V mentions.

Open issues potentially relevant to any new architecture port:

ID Title Status Severity Notes
#166 Are CPU specific optimizations with -mcpu supported? Open (Feb 2024) Medium Reports performance regression with -mcpu flags on aarch64 due to conditional SIMD paths being disabled; same risk applies to any future riscv64 SIMD work that uses architecture-specific compiler flags

Open PRs (none are RISC-V related):

PR Title Status Notes
#168 Visual Studio / NuGet packaging Open (Feb 2024) Unrelated to riscv64
#139 Meson build system Open (Oct 2021) Would affect all targets; stalled
#40 Whitespace cleanup Open (Jan 2016) Stale
#37 Typo fix Open (Jan 2016) Stale

No correctness bugs are open for riscv64.

12. Objections and Upstream Blockers

Technical blockers: None. The generic C path compiles and runs correctly on riscv64. A contributor could submit a PR adding libFDK/include/riscv/ headers with RVV intrinsics without any upstream objection anticipated.

Stated objections: None on record. No maintainer has expressed opposition to riscv64 work. The project accepted s390x SIMD (PR #159) and SPARC detection (PR #178), establishing precedent for new architecture contributions.

Organizational blockers: The project is a single-maintainer personal project (Martin Storsjo). PR review throughput is low – two of four open PRs are from 2016 and remain unmerged. A RISC-V optimization PR could wait months for review. The maintainer has no stated RISC-V interest.

License blocker: The Fraunhofer FDK AAC license restricts patent use. This limits which distributors can ship riscv64 binaries but does not block upstream code contribution or building from source. For a chip company’s internal use or for Debian non-free, the license is workable.

Acceptance probability: High for a minimal PR (arch detection + CI job). Medium-to-low for a full RVV optimization PR given low maintainer bandwidth and the complexity of intrinsic code review.

13. Investment Analysis

RISE has done nothing for libfdk-aac. All work scoped below would be net-new.

13.1 Functional Enablement

Functional correctness on riscv64 is already complete via the 2022 Alibaba commit. No additional functional enablement work is required.

13.2 Performance Optimization

The performance gap relative to arm64 is the primary technical deficiency. The hot paths to optimize, in priority order based on codec profile:

  1. fixmul.h – 32x16 and 32x32 fixed-point multiply (called in every filter bank and spectral coefficient operation). RVV widening multiply (vwmul) would directly accelerate this.
  2. cplx_mul.h – complex multiply for MDCT/FFT butterflies. RVV interleaved vector arithmetic.
  3. scale.h – arithmetic right shift with saturation. Zbb extension (sra + max) or RVV.
  4. clz.h – count leading zeros. Zbb clz instruction is a single-instruction fix.
  5. abs.h – absolute value. Zbb abs instruction.
  6. scramble_arm.h equivalent – FFT index scramble. Lower priority.

The analogous s390x SIMD PR (#159) was ~500 lines. A complete RVV optimization covering items 1-5 is estimated at 600-900 lines across 5 header files. No prior art exists in the upstream repo.

13.3 CI/CD Infrastructure

A QEMU-based riscv64 CI job can be added to .github/workflows/ci.yml with a qemu-user-static cross-compile step. This validates correctness but not performance. A native riscv64 CI job using RISE runners would provide both. The RISE RISC-V Runners program (announced March 2026) is available for open-source projects.

13.4 Ecosystem Enablement

libfdk-aac has no language-level package ecosystem (no PyPI, no npm, no Maven artifacts). The only distribution surface is system .deb/.rpm packages, which Debian and Ubuntu already provide for riscv64. No ecosystem enablement work is required beyond what distros already do.

13.5 Summary Table

Area Work Item Effort (person-weeks) Owner Priority
Functional None – generic C path is correct and ships in Debian/Ubuntu riscv64 0 n/a n/a
Performance Implement libFDK/include/riscv/ headers with RVV intrinsics for fixmul, cplx_mul, scale, clz, abs 4-6 Compiler/codec engineer with RVV experience High (required for real-time use on embedded or mobile riscv64)
Performance Benchmark vs arm64 NEON to quantify gap before and after 1 Performance engineer High (justifies and validates the optimization work)
CI/CD Add riscv64 QEMU cross-compile job to upstream ci.yml and submit PR 0.5 Any engineer Medium
CI/CD Onboard to RISE RISC-V Runners for native hardware CI 1 DevOps / RISE liaison Medium
Distro packaging Arch Linux RISC-V port (archriscv) – request package addition 0.25 Any contributor Low
Upstream PR Submit performance optimization PR, shepherd through review 1-2 (review cycles) Senior engineer High (blocks upstream acceptance)

Total estimated effort: 7.75-10.75 person-weeks for a complete, upstreamed, CI-validated RVV optimization.

14. Updates

No updates yet – initial report dated 2026-06-17.

15. References