OBazl toolchains: modeling the OCaml SDK

Bazel toolchain concepts

Platforms

Toolchains

Platforms and Toolchains Rules

ocaml compilers

standard compiler names:

Standard naming schema: ocaml<emitter>.<executor>, where <emitter> is c or opt, and <executor> is byte or opt. The bazel rules use standard naming for compiler names: ocamlc.byte, ocamlopt.byte, ocamlopt.opt, ocamlc.opt.

The profiling compilers are also use the standard names: ocamlcp.byte, ocamlcp.opt, ocamloptp.byte, ocamloptp.opt.

Names for flambda-enabled compilers use optx:

  • ocamloptx.byte - vm executor, flambda emitter (emits flambda-optimized native code)

  • ocamloptx.opt - native executor (un-optimized), flambda emitter

  • ocamlc.optx - flambda-optimized native executor, bytecode emitter

  • ocamlopt.optx - flambda-optimized native executor, emitting (non-flambda) native code

  • ocamloptx.optx - flambda-optimized native executor, emitting flambda-optimized native code

OCamlCC Bazel naming conventions

Bazel rules and non-compiler build targets (e.g. test targets) use the following abbreviations:

  • vv - vm executor and emitter: ocamlc.byte

  • vs - vm executor, sys emitter: ocamlopt.byte

  • ss - sys executor, sys emitter: ocamlopt.opt

  • sv - sys executor, vm emitter: ocamlc.opt

Flambda uses 'x':

  • vx - vm executor, flambda emitter: ocamloptx.byte

  • xx - flambda executor, flambda emitter: ocamloptx.optx

  • xv - flambda executor, vm emitter: ocamlc.optx

  • xs - flambda executor, sys emitter: ocamlopt.optx

  • sx - sys executor, flambda emitter: ocamloptx.opt

Platforms

A Bazel platform is a named collection of "constraints". For example:

platform(name = "ocamlopt.opt",
         parents = ["@local_config_platform//:host"],
         constraint_values = [
             "@rules_ocaml//platform/arch:sys",
             "@rules_ocaml//platform/executor:sys",
             "@rules_ocaml//platform/emitter:sys"
         ])

What this means, essentially, is that if platform ocamlopt.opt is "activated", then the listed `constraint_value`s will be satistfied - i.e. set to True.

Platforms are activated by passing them as the value of --host_platform or --platforms. For example:

$ bazel build //pkg:tgt --host_platform=@rules_ocaml//platform:ocamlc.opt --platforms=@rules_ocaml//platform:ocamlc.byte

The toolchain selection mechanism chooses a toolchain based on whatever constraints happen to be satisfied.