Allow make debug with dune-based build

.gitignore

@@ -59,3 +59,6 @@ _coverage/
 # Jane Street Merlin support
 /.local-merlin-binaries
 /.local-ocaml-lib
+
+# Debugger support
+ocamlc

HACKING.md

@@ -11,7 +11,9 @@ want to modify the Flambda backend.  Jump to:
 - [Running tests](#running-tests)
 - [Running only part of the upstream testsuite](#running-only-part-of-the-upstream-testsuite)
 - [Running tests with coverage analysis](#running-tests-with-coverage-analysis)
-- [Running the compiler produced by "make hacking" on an example without the stdlib](#running-the-compiler-produced-by-make-hacking-on-an-example-without-the-stdlib)
+- [Running the compiler produced by "make hacking" on an example without the
+  stdlib](#running-the-compiler-produced-by-make-hacking-on-an-example-without-the-stdlib)
+- [Using the ocaml debugger to debug the compiler](#using-the-ocaml-debugger-to-debug-the-compiler)
 - [Getting the compilation command for a stdlib file](#getting-the-compilation-command-for-a-stdlib-file)
 - [Bootstrapping the ocaml subtree](#bootstrapping-the-ocaml-subtree)
 - [Testing the compiler built locally with OPAM](#testing-the-compiler-built-locally-with-opam)
@@ -202,6 +204,34 @@ something like:
 ./_build/_bootinstall/bin/ocamlopt.opt -nostdlib -nopervasives -c test.ml
 ```
 
+## Using the OCaml debugger to debug the compiler
+
+Run `make debug`. This completes three steps:
+
+1. `make install`
+2. Sets up the `ocaml/tools/debug_printers` script so that you can `source
+   ocaml/tools/debug_printers` during a debugging session to see
+   otherwise-abstract variable values.
+3. Symlinks `./ocamlc` to point to the bytecode compiler. This is convenient for
+   emacs integration, because emacs looks for sources starting in the directory
+   containing the executable.
+
+To actually run the debugger from emacs (other workflows are possible; just
+tweak these instructions):
+
+1. Run `M-x camldebug RET`
+2. Choose the `ocamlc` symlink in the root of the repo.
+3. Choose the arguments to pass to `ocamlc`, likely a full path to a test `.ml`
+   file.
+4. Choose the built `ocamldebug`, in your install directory.
+5. Set any breakpoints you want. The easiest way is to navigate to the line
+   where you want the breakpoint and use `C-x C-a C-b` in emacs.
+6. Install the debug printers, with `source ocaml/tools/debug_printers`.
+7. `run` to your breakpoint.
+
+See [the manual section](https://v2.ocaml.org/manual/debugger.html) for more
+information about the debugger.
+
 ## Getting the compilation command for a stdlib file
 
 For example because you need to get the `-dflambda` output because of a bug.
@@ -387,4 +417,4 @@ only runs on Linux, although it shouldn't be hard to port to macOS, especially
 if using GNU binutils.  It is recommended to install the Jane Street `patdiff` executable
 before running `make compare`.  The comparison script has not been maintained since the
 early releases of the Flambda backend; it was written as part of the acceptance process
-for the initial release.
+    for the initial release.

Makefile

@@ -160,3 +160,17 @@ coverage: boot-runtest
 		--source-path=. \
 	  --source-path=_build/default
 	@echo Coverage report generated in _coverage/index.html
+
+.PHONY: debug
+.NOTPARALLEL: debug
+debug: install ocaml/tools/debug_printers ocamlc
+
+ocamlc:
+	ln -s $(prefix)/bin/ocamlc.byte ocamlc
+
+ocaml/tools/debug_printers: ocaml/tools/debug_printers.ml ocaml/tools/debug_printers.cmo
+	echo 'load_printer "ocaml/tools/debug_printers.cmo"' > $@
+	awk '{ print "install_printer Debug_printers." $$2 }' < $< >> $@
+
+ocaml/tools/debug_printers.cmo: ocaml/tools/debug_printers.ml _build/install/main/bin/ocamlc.byte
+	_build/install/main/bin/ocamlc.byte -c -I _build/main/ocaml/.ocamlcommon.objs/byte ocaml/tools/debug_printers.ml

ocaml/tools/dune

@@ -91,16 +91,4 @@
   (libraries ocamlcommon)
   (modules debug_printers))
 
-(rule
-  (target debug_printers)
-  (deps debug_printers.ml %{cmo:debug_printers})
-  (action
-    (with-stdout-to %{target}
-      (progn
-        ; Resorting to Bash instead of the built-in [echo] action because I
-        ; couldn't find a better way to get an absolute path out of Dune
-        (bash "echo load_printer \\\"$(realpath %{cmo:debug_printers})\\\"")
-        (with-stdin-from debug_printers.ml
-          (run awk "{ print \"install_printer Debug_printers.\" $2 }"))))))
-
 ; ocamlcp, ocamloptp and ocamlprof are not currently supported.