Design of LibSWIFFT Code

At a high level, LibSWIFFT includes a layer of C code that provides the C API and a layer of C++ code on top of it that provides the C++ API. New features are normally added to the C layer and then wrapped by the C++ layer.

The C layer is composed of the main API and the microarchitecture-specific APIs, discussed in more detail below. In regular use cases, one would normally use the main API only, which selects the best microarchitecture-specific API for the platform the library was built for. In more sophisticated use cases, one could use a microarchitecture-specific APIs directly, e.g., to provide an optimized feature. The main C and C++ APIs are respectively provided by the swifft.h and swifft.hpp headers.

LibSWIFFT has the following file and directory structure:

File or Directory	Description
- include	root directory of headers
- libswifft	directory of LibSWIFFT headers
- common.h	LibSWIFFT public C common definitions
- swifft.h	LibSWIFFT public C API
- swifft.hpp	LibSWIFFT public C++ API
- swifft_avx.h	LibSWIFFT public C API for AVX
- swifft_avx2.h	LibSWIFFT public C API for AVX2
- swifft_avx512.h	LibSWIFFT public C API for AVX512
- swifft_common.h	LibSWIFFT public C definitions
- swifft_iset.inl	LibSWIFFT public C API expansion for instruction-sets
- swifft_ver.h	LibSWIFFT public C API
- src	directory of LibSWIFFT sources
- swifft.c	LibSWIFFT public C implementation
- swifft.inl	LibSWIFFT internal C code expansion
- swifft_avx.c	LibSWIFFT public C implementation for AVX
- swifft_avx2.c	LibSWIFFT public C implementation for AVX2
- swifft_avx512.c	LibSWIFFT public C implementation for AVX512
- swifft_impl.inl	LibSWIFFT internal C definitions
- swifft_keygen.cpp	LibSWIFFT internal C code generation
- swifft_ops.inl	LibSWIFFT internal C code expansion
- transpose_8x8_16_sse2.inl	LibSWIFFT internal C code for matrix transposing

The main C API has the following organization:

• FFT functions: SWIFFT_fft{,sum}. These are the two low-level stages in a SWIFFT hash computation and are normally not used directly.
• Transformation functions: SWIFFT_{Compute,Compact}. These transform from input to output and from output to compact forms.
• Arithmetic functions: SWIFFT_{,Const}{Set,Add,Sub,Mul}. These set, add, subtract, or multiply given two output forms or one output form and a constant value.
• Functions for multiple blocks: These are functions with Multiple as part of their name. They operate on a number of blocks given as a parameter, rather than one block like the corresponding (i.e., without Multiple) single-block functions.

The main C++ API has the following organization:

• Wrapper classes: Swifft{Input,Output,Compact}. These wrap input, output, and compact buffers with automatic memory-alignment.
• Wrapper logical operators: The wrapper classes provide equality and inequality operators.
• Wrapper arithmetic operators: The wrapper classes provides operators for setting, adding, subtracting, and multiplying the current SwifftOutput instance with another or with a constant value.

The microarchitecture-specific APIs have the following organization:

• Similarity to the main C API: Each microarchitecure-specific function has the same name as a corresponding main C API but with an added suffix, the same parameter signature, and the same semantics.
• Name-suffix depending on microarchitecture feature: There are 3 sets of microarchitecture-specific functions corresponding to the 3 suffixes _AVX, _AVX2, and _AVX512 that respectively provide implementations optimized for a microarchitecture supporting AVX, AVX2, and AVX512F instruction-sets.

The major code conventions used in LibSWIFFT are:

• SWIFFT-related symbols of the C API start with SWIFFT\_.
• Non-SWIFFT-related symbols of the C API start with LIBSWIFFT\_
• Symbols in the C++ API are in namespace LibSwifft.
• SWIFFT-related functions are thread-safe and constant-time.
• Arguments are assumed non-overlapping in memory.

LibSWIFFT code uses conditional compilation directives with the following goals:

• Prevent multiple compilations of the same header.
• Compile microarchitecture-specific code only on a supporting platform.
• Compile C++ or OpenMP code only on a supporting compiler.