dune

 % find . -type f -exec echo "*** {} ***" \; -exec cat {} \; -exec echo "" \;
*** ./src/sub/main.ml ***
let _ =
  Printf.printf "Hey\n"

*** ./src/sub/dune ***
(executable
 (name main))

*** ./dune-project ***
(lang dune 2.9)

*** ./dune ***
(env
  (dev
    (flags (:standard -w +42)))
  (release
    (flags (:standard -O3))))
    
% dune build --verbose --profile release
Workspace root: /usr/home/gahr/lang/ocaml/dune
Running[0]: /usr/bin/getconf _NPROCESSORS_ONLN > /tmp/dune7efe5b.output 2> /dev/null
Auto-detected concurrency: 3
disable binary cache
Running[1]: /usr/home/gahr/.opam/default/bin/ocamlc.opt -config > /tmp/dune589953.output
Dune context:
 { name = "default"
 ; kind = "default"
 ; profile = Release
 ; merlin = true
 ; for_host = None
 ; fdo_target_exe = None
 ; build_dir = "default"
 ; toplevel_path = Some External "/usr/home/gahr/.opam/default/lib/toplevel"
 ; ocaml_bin = External "/usr/home/gahr/.opam/default/bin"
 ; ocaml = Ok External "/usr/home/gahr/.opam/default/bin/ocaml"
 ; ocamlc = External "/usr/home/gahr/.opam/default/bin/ocamlc.opt"
 ; ocamlopt = Ok External "/usr/home/gahr/.opam/default/bin/ocamlopt.opt"
 ; ocamldep = Ok External "/usr/home/gahr/.opam/default/bin/ocamldep.opt"
 ; ocamlmklib = Ok External "/usr/home/gahr/.opam/default/bin/ocamlmklib.opt"
 ; env =
     map
       { "DUNE_OCAML_HARDCODED" : "/usr/home/gahr/.opam/default/lib"
       ; "DUNE_OCAML_STDLIB" : "/usr/home/gahr/.opam/default/lib/ocaml"
       ; "DUNE_SOURCEROOT" : "/usr/home/gahr/lang/ocaml/dune"
       ; "INSIDE_DUNE" : "/usr/home/gahr/lang/ocaml/dune/_build/default"
       ; "MANPATH" :
           "/usr/home/gahr/lang/ocaml/dune/_build/install/default/bin"
       ; "OCAMLFIND_IGNORE_DUPS_IN" :
           "/usr/home/gahr/lang/ocaml/dune/_build/install/default/lib"
       ; "OCAMLPATH" :
           "/usr/home/gahr/lang/ocaml/dune/_build/install/default/lib"
       ; "OCAMLTOP_INCLUDE_PATH" :
           "/usr/home/gahr/lang/ocaml/dune/_build/install/default/lib/toplevel"
       ; "OCAML_COLOR" : "always"
       ; "OPAMCOLOR" : "always"
       }
 ; findlib_path = [ External "/usr/home/gahr/.opam/default/lib" ]
 ; arch_sixtyfour = true
 ; natdynlink_supported = true
 ; supports_shared_libraries = true
 ; ocaml_config =
     { version = "4.13.1"
     ; standard_library_default = "/usr/home/gahr/.opam/default/lib/ocaml"
     ; standard_library = "/usr/home/gahr/.opam/default/lib/ocaml"
     ; standard_runtime = "the_standard_runtime_variable_was_deleted"
     ; ccomp_type = "cc"
     ; c_compiler = "cc"
     ; ocamlc_cflags =
         [ "-O2"; "-fno-strict-aliasing"; "-fwrapv"; "-pthread"; "-fPIC" ]
     ; ocamlc_cppflags = [ "-D_FILE_OFFSET_BITS=64" ]
     ; ocamlopt_cflags =
         [ "-O2"; "-fno-strict-aliasing"; "-fwrapv"; "-pthread"; "-fPIC" ]
     ; ocamlopt_cppflags = [ "-D_FILE_OFFSET_BITS=64" ]
     ; bytecomp_c_compiler =
         [ "cc"
         ; "-O2"
         ; "-fno-strict-aliasing"
         ; "-fwrapv"
         ; "-pthread"
         ; "-fPIC"
         ; "-D_FILE_OFFSET_BITS=64"
         ]
     ; bytecomp_c_libraries = [ "-lm"; "-lpthread" ]
     ; native_c_compiler =
         [ "cc"
         ; "-O2"
         ; "-fno-strict-aliasing"
         ; "-fwrapv"
         ; "-pthread"
         ; "-fPIC"
         ; "-D_FILE_OFFSET_BITS=64"
         ]
     ; native_c_libraries = [ "-lm" ]
     ; cc_profile = []
     ; architecture = "amd64"
     ; model = "default"
     ; int_size = 63
     ; word_size = 64
     ; system = "freebsd"
     ; asm = [ "cc"; "-c" ]
     ; asm_cfi_supported = true
     ; with_frame_pointers = false
     ; ext_exe = ""
     ; ext_obj = ".o"
     ; ext_asm = ".s"
     ; ext_lib = ".a"
     ; ext_dll = ".so"
     ; os_type = "Unix"
     ; default_executable_name = "a.out"
     ; systhread_supported = true
     ; host = "x86_64-unknown-freebsd13.0"
     ; target = "x86_64-unknown-freebsd13.0"
     ; profiling = false
     ; flambda = false
     ; spacetime = false
     ; safe_string = true
     ; exec_magic_number = "Caml1999X030"
     ; cmi_magic_number = "Caml1999I030"
     ; cmo_magic_number = "Caml1999O030"
     ; cma_magic_number = "Caml1999A030"
     ; cmx_magic_number = "Caml1999Y030"
     ; cmxa_magic_number = "Caml1999Z030"
     ; ast_impl_magic_number = "Caml1999M030"
     ; ast_intf_magic_number = "Caml1999N030"
     ; cmxs_magic_number = "Caml1999D030"
     ; cmt_magic_number = "Caml1999T030"
     ; natdynlink_supported = true
     ; supports_shared_libraries = true
     ; windows_unicode = false
     }
 }
Actual targets:
- alias @@default
Running[2]: (cd _build/default && /usr/home/gahr/.opam/default/bin/ocamlc.opt -w -40 -O3 -g -bin-annot -I src/sub/.main.eobjs/byte -no-alias-deps -o src/sub/.main.eobjs/byte/dune__exe__Main.cmo -c -impl src/sub/main.ml)
Command [2] exited with code 2:
$ (cd _build/default && /usr/home/gahr/.opam/default/bin/ocamlc.opt -w -40 -O3 -g -bin-annot -I src/sub/.main.eobjs/byte -no-alias-deps -o src/sub/.main.eobjs/byte/dune__exe__Main.cmo -c -impl src/sub/main.ml)
/usr/home/gahr/.opam/default/bin/ocamlc.opt: unknown option '-O3'.
Usage: ocamlc <options> <files>
Try 'ocamlc --help' for more information.