ContextQMD
Back to Fastled

ARM Assembly Optimization for libhelix-mp3

src/third_party/libhelix_mp3/README_ARM_ASSEMBLY.md

3.10.32.6 KB
Original Source

ARM Assembly Optimization for libhelix-mp3

The libhelix-mp3 decoder includes highly optimized ARM assembly implementations of the polyphase filter in real/arm/asmpoly_gcc.S.

Automatic Selective Compilation

The assembly file is always included in the build, but uses preprocessor guards to compile only on supported platforms:

c
#if defined(__GNUC__) && defined(__arm__) && !defined(__thumb__) && !defined(__thumb2__)
// ARM32 assembly code here
#endif

✅ Platforms Where Assembly IS Used

  • ARM Cortex-A processors (Raspberry Pi with Linux, BeagleBone, etc.)
  • Legacy ARM7/ARM9 processors
  • Any ARM platform supporting full ARM32 instructions (not just Thumb-2)

❌ Platforms Where Assembly is SKIPPED

  • ARM Cortex-M processors (Teensy, Arduino ARM boards, RP2040)
    • Uses optimized C++ with Thumb-2 inline assembly instead
  • ESP32 (Xtensa architecture)
    • Uses Xtensa-optimized inline assembly instead
  • x86/AVR/other platforms
    • Uses portable C++ implementations

How It Works

  1. Build System: asmpoly_gcc.S is always included via library.json srcFilter
  2. Preprocessor: Guards detect platform capabilities at compile time
  3. ARM32 platforms: Assembly code compiles, C++ polyphase is excluded
  4. Thumb-2 platforms: Assembly code is empty, C++ polyphase compiles
  5. No user action needed: Everything is automatic!

Performance

Platform TypeImplementationPerformance
ARM Cortex-A (Pi Linux)ARM32 assemblyFastest (~1-2ms/frame better)
ARM Cortex-M (Teensy)C++ + Thumb-2 inline asmAlready optimized
ESP32 (Xtensa)C++ + Xtensa inline asmAlready optimized
AVR/x86/OthersPortable C++Good, compiler-optimized

Technical Details

Assembly File

  • asmpoly_gcc.S - GCC-compatible ARM32 assembly with preprocessor guards
  • Compiles to empty object file on non-ARM32 platforms (no overhead)

C++ Fallback (polyphase.cpp)

cpp
#if !(defined(__GNUC__) && defined(__arm__) && !defined(__thumb__) && !defined(__thumb2__))
#define COMPILE_CPP_POLYPHASE  // Compile C++ version
#endif

Function Naming

Assembly functions use xmp3_ prefix (via STATNAME macro):

  • xmp3_PolyphaseMono
  • xmp3_PolyphaseStereo

These automatically match the C function declarations in coder.h.

Removed Files

  • asmpoly.s - ARM ADS assembler format (deleted, incompatible with GCC)
  • asmmisc.s - ARM ADS assembler format (deleted, incompatible with GCC)

Only asmpoly_gcc.S (GCC format with preprocessor support) is kept.