UPDATE: Ilya Albrecht landed the memset implementation from this repo into Folly.

This repository contains high-performance implementations of memset and memcpy. These implementations outperform the folly and glibc implementations. This repository contains several reference implementations in C and assembly. The high-performance implementations are found in the files called "impl.S".

Before reading the source code in this repository you probably want to read an excellent blog post by Joe Bialek about his work to optimize memset for windows.

The charts below compare the code in this repo with other implementations: folly, musl, and glibc. The glibc implementations are measured with and without the elf indirection, as suggested by Dave Zarzycki.

The chart below compares the performance of different memset implementations on buffers of varying sizes and offsets. Unlike the hot loop that hammers a single value, this benchmark is more realistic and takes into account mispredicted branches and the performance of the cpu decoder. The buffers are in the size range 0 to 256. The random function is made of pre-computed random values, to lower the overhead of the random function. This was suggested by Yann Collet. The 'nop' function is used to compute the benchmark setup and call overhead. The numbers below represent the implementation execution time minus the nop function time.

The size of the buffer that memset and memcpy mutates is typically small. The picture below presents the buffer length distribution in google-chrome. Vim, Python, and even server workloads have a similar distribution. The values in the chart represent the power of two buffer size (10 represents the values between 512 and 1024).

The chart below presents a histogram of pointer alignment (from the game minecraft). Most of the pointers that are called by memset and memcpy are aligned to 8-byte values. Some programs have histograms that are not as sharp, meaning that there are more values that are not aligned to 4 or 8-byte boundary.

Memcpy and Memset and frequently called by low-level high-performance libraries. Here is one example of one stack trace from the Firefox codebase:

  (gdb) bt
  #0  __memmove_avx_unaligned_erms () at ../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:225
  #1  in memcpy (__dest=, __src=, __len=40) at /usr/include/x86_64-linux-gnu/bits/string_fortified.h:34
  #2  mozilla::BufferList<InfallibleAllocPolicy>::ReadBytes(mozilla::BufferList<InfallibleAllocPolicy>::IterImpl&, char*, unsigned long) const
  #3  Pickle::ReadBytesInto(PickleIterator*, void*, unsigned int) const (this=, iter=, data=, length=<optimized out>)
  #4  in IPC::Message::ReadFooter(void*, unsigned int, bool) (this=, buffer=, buffer_len=40, truncate=true)
  #5  in mozilla::ipc::NodeController::DeserializeEventMessage(mozilla::UniquePtr<IPC::Message, mozilla::DefaultDelete<IPC::Message> >) (this=, aMessage=...)
  #6  in mozilla::ipc::NodeController::OnEventMessage(mojo::core::ports::NodeName const&, mozilla::UniquePtr<IPC::Message, mozilla::DefaultDelete<IPC::Message> >)
  #7  in mozilla::ipc::NodeChannel::OnMessageReceived(IPC::Message&&) (this=<optimized out>, aMessage=...)
  #8  in IPC::Channel::ChannelImpl::ProcessIncomingMessages() (this=<optimized out>)
  #9  in IPC::Channel::ChannelImpl::OnFileCanReadWithoutBlocking(int) (this=, fd=)
  #10 in base::MessagePumpLibevent::OnLibeventNotification(int, short, void*) (fd=, flags=, context=)
  #11 in event_persist_closure (base=, ev=) at /build/firefox-HSiFn6/firefox-94.0+build3/ipc/chromium/src/third_party/libevent/event.c:1580
  #12 event_process_active_single_queue (base=, activeq=, max_to_process=, endtime=)

The repository contains a few utilities for testing and measuring the performance and correctness of memset and memcpy.

This is a small test harness that verifies the correctness of the implementations. It's really easy to make mistakes with off-by-one errors and run into alignment issues. The exhaustive tester catches these issues.

This is a sample output:

OOOOOOOOOOOXX
           ^
Filling a buffer of length 13. Expected "O" at index 11

The benchmark tool measures the performance of the system libc and the local implementation. The benchmarking tool runs each of the implementations in a loop millions of times. It runs the benchmark several times and picks the least noisy results. It's a good idea to run the benchmark tool and compare some implementation to itself to assess the noise level in the system. The benchmarking tool uses a trampoline to prevent the compiler from inlining and expanding the memset.

The histogram tool is a shared object that collects records calls to memset and memcpy and creates a histogram of the length parameter. It prints the histogram when the program exits cleanly. The shared object can be loaded using LD_PRELOAD (on Linux) or DYLD_INSERT_LIBRARIES (on Mac). Each bucket in the output represents the log2 size of the buffer, and each value represents the number of hits for the bucket.

This is a small utility that swaps the builtin call to memset and memcpy with the local implementation from this project. The shared object can be loaded using LD_PRELOAD (on Linux) or DYLD_INSERT_LIBRARIES (on Mac).