Hand-in-Hand#

Hand-in-Hand is the name of the mechanism that allows other LLVM projects to use LLVM-libc’s internal C++ APIs instead of calling the public libc interface. This is useful for cases where the C interface doesn’t match the desired interface.

The original use case for the Hand-in-Hand interface was to let libc++ use LLVM-libc’s string to float conversion internals. The libc interface (strtof) takes a null terminated string with no maximum length while the libc++ interface (from_chars<float>) takes a string with a start and an end. If libc++ had used the public interface it would have had to allocate a new null terminated string before calling strtof, but with Hand-in-Hand libc++ handles its own parsing and then passes the parsed information to LLVM-libc’s conversion code. This is better for performance and cuts down on code duplication in the LLVM repository.

Hand-in-Hand works by LLVM-libc exposing a set of headers in the /libc/shared/ directory. These headers make the interface more explicit and easier to maintain. The client library includes the shared headers by depending on the llvm-libc-common-utilities target which sets up the necessary includes and defines. The client library then includes "shared/<header>" to get the necessary components. All of the functions shared via Hand-in-Hand are header only.

The Hand-in-Hand interface is intended to be an internal implementation detail, and it has no guarantees of stability. When the internal LLVM-libc interface is changed the other users inside of the LLVM repository are updated in the same commit. This allows LLVM-libc to update their interface without breaking their users.

Current Hand-in-Hand users:

• Libc++ uses it for from_chars<float/double>.

• OpenMP uses it for printf on GPUs.

• [WIP] clang uses it for APFloat functions.

For more information check out the 2024 talk about the original Project:

• slides

• video