platform(name = "vm>any",
parents = ["@local_config_platform//:host"],
constraint_values = [
"@rules_ocaml//platform/arch:sys",
"@rules_ocaml//platform/executor:vm",
])
platform(name = "sys>any",
parents = ["@local_config_platform//:host"],
constraint_values = [
"@rules_ocaml//platform/arch:sys",
"@rules_ocaml//platform/executor:sys",
])
Transitions Last updated Feb 11, 2025
What does not work: target platforms with unconstrained emitter.
Scenario: sys>vm ⇒ vm>any. Since the buildhost emitter is vm, the
target host executor must be vm. Since we’re building ordinary code,
which will not itself emit any code when run on the target platform,
the target emitter is irrelevant, so we call it "any", and omit any
constraint on @rules_ocaml//platform/emitter:
The problem with this is that the emitter constraint defaults to
sys. Which means that vm>any will effectivley resolve to vm>sys.
This won’t work, because any targets that select on
@rules_ocaml//platform/emitter will get the value for the target
platform, which for vm>any will be the default, sys, when what
they really need is the emitter of the build platform.
But, the easy fix for this is to change ocaml_imports to select on executor. Since the target platform executor is always the same as the build platform emitter, this is what we want. The import targets need to select the archive needed in the build env, but they are evaluated in the target platform context.
What about cross-compiling compilers or other tools that have an emitter that emits code? Even then, the tool transition ensures that build tools will be built with target executor == build emitter.
Question: ppx executables emit binary ASTs. Are they the same for both executors? I.e. if the ppx runs on the vm does it emit the same binary AST as when it is built to run natively? IOW a ppx.exe is like a compiler, it translates from one language to another. So it has an executor and an emitter.