Fast Base64 stream encoder/decoder

This is an implementation of a base64 stream encoding/decoding library in C99 with SIMD (AVX2, AVX512, NEON, AArch64/NEON, SSSE3, SSE4.1, SSE4.2, AVX) and OpenMP acceleration. It also contains wrapper functions to encode/decode simple length-delimited strings. This library aims to be:

• FAST;
• easy to use;
• elegant.

On x86, the library does runtime feature detection. The first time it's called, the library will determine the appropriate encoding/decoding routines for the machine. It then remembers them for the lifetime of the program. If your processor supports AVX2, SSSE3, SSE4.1, SSE4.2 or AVX instructions, the library will pick an optimized codec that lets it encode/decode 12 or 24 bytes at a time, which gives a speedup of four or more times compared to the "plain" bytewise codec.

AVX512 support is only for encoding at present, utilizing the AVX512 VL and VBMI instructions. Decoding part reused AVX2 implementations. For CPUs later than Cannonlake (manufactured in 2018) supports these instructions.

NEON support is hardcoded to on or off at compile time, because portable runtime feature detection is unavailable on ARM.

Even if your processor does not support SIMD instructions, this is a very fast library. The fallback routine can process 32 or 64 bits of input in one round, depending on your processor's word width, which still makes it significantly faster than naive bytewise implementations. On some 64-bit machines, the 64-bit routines even outperform the SSSE3 ones.

To the author's knowledge, at the time of original release, this was the only Base64 library to offer SIMD acceleration. The author wrote an article explaining one possible SIMD approach to encoding/decoding Base64. The article can help figure out what the code is doing, and why.

Notable features:

• Really fast on x86 and ARM systems by using SIMD vector processing;
• Can use OpenMP for even more parallel speedups;
• Really fast on other 32 or 64-bit platforms through optimized routines;
• Reads/writes blocks of streaming data;
• Does not dynamically allocate memory;
• Valid C99 that compiles with pedantic options on;
• Re-entrant and threadsafe;
• Unit tested;
• Uses Duff's Device.

Acknowledgements

The original AVX2, NEON and Aarch64/NEON codecs were generously contributed by Inkymail, who, in their fork, also implemented some additional features. Their work is slowly being backported into this project.

The SSSE3 and AVX2 codecs were substantially improved by using some very clever optimizations described by Wojciech Muła in a series of articles. His own code is here.

The AVX512 encoder is based on code from Wojciech Muła's base64simd library.

The OpenMP implementation was added by Ferry Toth (@htot) from Exalon Delft.

Building

The lib directory contains the code for the actual library. Typing make in the toplevel directory will build lib/libbase64.o and bin/base64. The first is a single, self-contained object file that you can link into your own project. The second is a standalone test binary that works similarly to the base64 system utility.

The matching header file needed to use this library is in include/libbase64.h.

To compile just the "plain" library without SIMD codecs, type:

make lib/libbase64.o

Optional SIMD codecs can be included by specifying the AVX2_CFLAGS, AVX512_CFLAGS, NEON32_CFLAGS, NEON64_CFLAGS, SSSE3_CFLAGS, SSE41_CFLAGS, SSE42_CFLAGS and/or AVX_CFLAGS environment variables. A typical build invocation on x86 looks like this:

AVX2_CFLAGS=-mavx2 SSSE3_CFLAGS=-mssse3 SSE41_CFLAGS=-msse4.1 SSE42_CFLAGS=-msse4.2 AVX_CFLAGS=-mavx make lib/libbase64.o

AVX2

To build and include the AVX2 codec, set the AVX2_CFLAGS environment variable to a value that will turn on AVX2 support in your compiler, typically -mavx2. Example:

AVX2_CFLAGS=-mavx2 make

AVX512

To build and include the AVX512 codec, set the AVX512_CFLAGS environment variable to a value that will turn on AVX512 support in your compiler, typically -mavx512vl -mavx512vbmi. Example:

AVX512_CFLAGS="-mavx512vl -mavx512vbmi" make

The codec will only be used if runtime feature detection shows that the target machine supports AVX2.

SSSE3

To build and include the SSSE3 codec, set the SSSE3_CFLAGS environment variable to a value that will turn on SSSE3 support in your compiler, typically -mssse3. Example:

SSSE3_CFLAGS=-mssse3 make

The codec will only be used if runtime feature detection shows that the target machine supports SSSE3.

NEON

This library includes two NEON codecs: one for regular 32-bit ARM and one for the 64-bit AArch64 with NEON, which has double the amount of SIMD registers and can do full 64-byte table lookups. These codecs encode in 48-byte chunks and decode in massive 64-byte chunks, so they had to be augmented with an uint32/64 codec to stay fast on smaller inputs!

Use LLVM/Clang for compiling the NEON codecs. The code generation of at least GCC 4.6 (the version shipped with Raspbian and used for testing) contains a bug when compiling vstq4_u8(), and the generated assembly code is of low quality. NEON intrinsics are a known weak area of GCC. Clang does a better job.

NEON support can unfortunately not be portably detected at runtime from userland (the mrc instruction is privileged), so the default value for using the NEON codec is determined at compile-time. But you can do your own runtime detection. You can include the NEON codec and make it the default, then do a runtime check if the CPU has NEON support, and if not, force a downgrade to non-NEON with BASE64_FORCE_PLAIN.

These are your options:

1. Don't include NEON support;
2. build NEON support and make it the default, but build all other code without NEON flags so that you can override the default at runtime with BASE64_FORCE_PLAIN;
3. build everything with NEON support and make it the default;
4. build everything with NEON support, but don't make it the default (which makes no sense).

For option 1, simply don't specify any NEON-specific compiler flags at all, like so:

CC=clang CFLAGS="-march=armv6" make

For option 2, keep your CFLAGS plain, but set the NEON32_CFLAGS environment variable to a value that will build NEON support. The line below, for instance, will build all the code at ARMv6 level, except for the NEON codec, which is built at ARMv7. It will also make the NEON codec the default. For ARMv6 platforms, override that default at runtime with the BASE64_FORCE_PLAIN flag. No ARMv7/NEON code will then be touched.

CC=clang CFLAGS="-march=armv6" NEON32_CFLAGS="-march=armv7 -mfpu=neon" make

For option 3, put everything in your CFLAGS and use a stub, but non-empty, NEON32_CFLAGS. This example works for the Raspberry Pi 2B V1.1, which has NEON support:

CC=clang CFLAGS="-march=armv7 -mtune=cortex-a7" NEON32_CFLAGS="-mfpu=neon" make

To build and include the NEON64 codec, use CFLAGS as usual to define the platform and set NEON64_CFLAGS to a nonempty stub. (The AArch64 target has mandatory NEON64 support.) Example:

CC=clang CFLAGS="--target=aarch64-linux-gnu -march=armv8-a" NEON64_CFLAGS=" " make

OpenMP

To enable OpenMP on GCC you need to build with -fopenmp. This can be by setting the the OPENMP environment variable to 1.

Example:

OPENMP=1 make

This will let the compiler define _OPENMP, which in turn will include the OpenMP optimized lib_openmp.c into lib.c.

By default the number of parallel threads will be equal to the number of cores of the processor. On a quad core with hyperthreading eight cores will be detected, but hyperthreading will not increase the performance.

To get verbose information about OpenMP start the program with OMP_DISPLAY_ENV=VERBOSE, for instance

OMP_DISPLAY_ENV=VERBOSE test/benchmark

To put a limit on the number of threads, start the program with OMP_THREAD_LIMIT=n, for instance

OMP_THREAD_LIMIT=2 test/benchmark

An example of running a benchmark with OpenMP, SSSE3 and AVX2 enabled:

make clean && OPENMP=1 SSSE3_CFLAGS=-mssse3 AVX2_CFLAGS=-mavx2 make && OPENMP=1 make -C test

API reference

Strings are represented as a pointer and a length; they are not zero-terminated. This was a conscious design decision. In the decoding step, relying on zero-termination would make no sense since the output could contain legitimate zero bytes. In the encoding step, returning the length saves the overhead of calling strlen() on the output. If you insist on the trailing zero, you can easily add it yourself at the given offset.

Flags

Some API calls take a flags argument. That argument can be used to force the use of a specific codec, even if that codec is a no-op in the current build. Mainly there for testing purposes, this is also useful on ARM where the only way to do runtime NEON detection is to ask the OS if it's available. The following constants can be used:

• BASE64_FORCE_AVX2
• BASE64_FORCE_AVX512
• BASE64_FORCE_NEON32
• BASE64_FORCE_NEON64
• BASE64_FORCE_PLAIN
• BASE64_FORCE_SSSE3
• BASE64_FORCE_SSE41
• BASE64_FORCE_SSE42
• BASE64_FORCE_AVX

Set flags to 0 for the default behavior, which is runtime feature detection on x86, a compile-time fixed codec on ARM, and the plain codec on other platforms.

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