The ocaml_intrinsics library provides an OCaml interface to operations
that have dedicated hardware instructions on some micro-architectures.
Currently, it provides the following operations:
count leading zeros of an integer
count trailing zeros of an integer
count set bits of an integer
cyclic redundancy check
Some operations require more than one instruction to implement.
On x86_64 architecture, the integer operations are implemented
popcnt instructions. Additionally, the
library supports performance monitoring instructions
rdpmc, as well as
crc32 for the x86_64 architecture only.
The library provides default implementations using C stubs for all
targets. Ocaml compiler with support for intrinsics intercepts calls
to these C stubs and emits inline assembly instead.
The library and the compiler support was only tested on x86_64 Linux
The goals of the library are to:
* Expose a portable interface with uniform semantics on all OCaml targets. Under the hood, take care of target-specific details, including C compiler, operating system, and machine architecture support. * Reduce the overhead of OCaml-to-C calls in the implemention, whenever possible, using attributes such as "noalloc", "unboxed", and "untagged". Provides variants of operations, such as tagged and untagged integers, and handle corner cases. The users can choose the most suitable variant in each context. * Make it easier to use dedicated hardware instructions, and evaluate their effects on performance. * Make it easier to experiment with compiler intrinsics. The main difference in generated code between this library's C stubs and compiler intrinsics is that C stubs cannot be inlined. It is not clear whether "inlining" them would noticibly improve performance compared to a direct call. In particular, inlining might also increase register pressure in the caller, especially when new instruction require designated registers as operands (such as rdtsc). Compiler support for intrinsics detects and replaces calls to C stubs annoted with [@@builtin] with the corresponding instructions, on targets that support them. * Make it easier for the users to transition their code from using this library to using compiler intrinsics, whenever the corresponding operation become available in the upstream OCaml compiler.