Compiler: do not prevent deadcode to preserve tailcall

Author: hhugo

compiler/lib/global_deadcode.ml

@@ -434,7 +434,7 @@ let solver vars uses defs live_vars scoped_live_vars =
     + They are returned; or
     + They are applied to a function.
  *)
-let zero prog sentinal live_table =
+let zero prog pure_funs sentinal live_table =
   let compact_vars vars =
     let i = ref (Array.length vars - 1) in
     while !i >= 0 && Var.equal vars.(!i) sentinal do
@@ -486,7 +486,10 @@ let zero prog sentinal live_table =
           let tc =
             (* We don't want to break tailcalls. *)
             match List.last body with
-            | Some (Let (x', Apply _)) when Code.Var.equal x' x -> true
+            | Some (Let (x', (Apply _ as e))) when Code.Var.equal x' x ->
+                if (not (is_live x')) && Pure_fun.pure_expr pure_funs e
+                then false (* we don't want to stop deadcode *)
+                else true
             | Some _ | None -> false
           in
           if tc then Return x else Return (zero_var x)
@@ -581,7 +584,7 @@ let f p ~deadcode_sentinal global_info =
     Print.print_uses uses;
     Print.print_live_tbl live_table);
   (* Zero out dead fields *)
-  let p = zero p deadcode_sentinal live_table in
+  let p = zero p pure_funs deadcode_sentinal live_table in
   if debug ()
   then (
     Format.eprintf "After Zeroing:@.";

compiler/tests-compiler/tailcall.ml

@@ -126,38 +126,16 @@ let%expect_test "global-deadcode-bug" =
     |}
   in
   Util.compile_and_run ~flags:[ "--disable"; "inline" ] prog;
-  [%expect
-    {|
-    /tmp/build_b5b39e_dune/jsoo-test3c37fc/test.js:1787
-         throw err;
-         ^
-
-    TypeError: Cannot read properties of undefined (reading '2')
-        at g (/tmp/build_b5b39e_dune/jsoo-test3c37fc/test.js:1878:23)
-        at f (/tmp/build_b5b39e_dune/jsoo-test3c37fc/test.js:1879:46)
-        at fun1 (/tmp/build_b5b39e_dune/jsoo-test3c37fc/test.js:1880:5)
-        at /tmp/build_b5b39e_dune/jsoo-test3c37fc/test.js:1883:4
-        at Object.<anonymous> (/tmp/build_b5b39e_dune/jsoo-test3c37fc/test.js:1887:3)
-        at Module._compile (node:internal/modules/cjs/loader:1734:14)
-        at Object..js (node:internal/modules/cjs/loader:1899:10)
-        at Module.load (node:internal/modules/cjs/loader:1469:32)
-        at Function._load (node:internal/modules/cjs/loader:1286:12)
-        at TracingChannel.traceSync (node:diagnostics_channel:322:14)
-
-    Node.js v23.11.0
-
+  [%expect {|
     here
-
-    process exited with error code 7
-     node test.js
+    Success!
     |}];
   let program = Util.compile_and_parse ~flags:[ "--disable"; "inline" ] prog in
   Util.print_fun_decl program (Some "fun1");
   [%expect
     {|
     function fun1(param){
-     function g(f){if(f[2]) return;}
-     function f(x){caml_call1(Stdlib[46], cst_here); return g();}
+     function f(x){caml_call1(Stdlib[46], cst_here);}
      f(5000);
      return log_success(0);
     }