Let's fix instead of reverting flambda_backend_args (#396)

* Revert "Revert "Move flambda-backend specific flags out of ocaml/ subdirectory (#382)" (#395)"

This reverts commit 2a9c5ef.

* Propagate -o2,-o3,-oclassic flambda-backend flags correctly

* Rename Flambda_backend_options module for readability

To distinguish it from the module type with the same name.

* Add documentation comments to Flambda_backend_arg

* Remove underscores in Flambda_backend_options

* Move "how to add a new flag" to HACKING

Author: gretay-js

HACKING.md

@@ -16,6 +16,7 @@ want to modify the Flambda backend.  Jump to:
   - [Pulling changes onto a release branch](#pulling-changes-onto-a-release-branch)
   - [Rebasing to a new major version of the upstream compiler](#rebasing-to-a-new-major-version-of-the-upstream-compiler)
   - [How to add a new intrinsic to the compiler](#how-to-add-a-new-intrinsic-to-the-compiler)
+  - [How to add a new command line option](#how-to-add-a-new-command-line-option)
   - [Installation tree comparison script](#installation-tree-comparison-script)
 
 ## Branches, pull requests, etc.
@@ -293,6 +294,17 @@ library, and then the compiler.
   relies on the library tests to avoid duplication. Library tests use
   `Core`, but the library itself does not.
 
+## How to add a new command line option
+
+1) Add a ref to `flambda_backend_flags.ml{i}`
+2) Add the flag's constructor `mk_<flag>` in `flambda_backend_args.ml`
+3) Add the callback for the new flag to `Flambda_backend_options` module type
+   in `flambda_backend_args.ml{i}`
+4) List the flag in the body of `Make_flambda_backend_options` functor
+5) Implement the flag in `Flambda_backend_options_impl`
+   by setting the corresponding ref in Flambda_backend_flags
+6) Add the flag to `Extra_params` if it can be set via `OCAMLPARAM`
+
 ## Installation tree comparison script
 
 A target `make compare` exists to run a comparison script that finds differences

backend/asmgen.ml

@@ -222,14 +222,14 @@ let compile_fundecl ~ppf_dump fd_cmm =
   ++ Profile.record ~accumulate:true "linearize" (fun (f : Mach.fundecl) ->
       let res = Linearize.fundecl f in
       (* CR xclerc for xclerc: temporary, for testing. *)
-      if !Clflags.use_ocamlcfg then begin
+      if !Flambda_backend_flags.use_ocamlcfg then begin
         test_cfgize f res;
       end;
       res)
   ++ pass_dump_linear_if ppf_dump dump_linear "Linearized code"
   ++ Compiler_hooks.execute_and_pipe Compiler_hooks.Linear
   ++ (fun (fd : Linear.fundecl) ->
-    if !use_ocamlcfg then begin
+    if !Flambda_backend_flags.use_ocamlcfg then begin
       fd
       ++ Profile.record ~accumulate:true "linear_to_cfg"
            (Linear_to_cfg.run ~preserve_orig_labels:true)