Compiler: Global DCE does not break TC

Author: hhugo

compiler/lib/global_deadcode.ml

@@ -352,7 +352,14 @@ let zero prog sentinal live_table =
     let branch =
       (* Zero out return values in last instruction, otherwise do nothing. *)
       match block.branch with
-      | Return x, loc -> Return (zero_var x), loc
+      | Return x, loc ->
+          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 _ | None -> false
+          in
+          if tc then Return x, loc else Return (zero_var x), loc
       | Raise (_, _), _
       | Stop, _
       | Branch _, _

compiler/tests-compiler/direct_calls.ml

@@ -57,7 +57,7 @@ let%expect_test "direct calls without --enable effects" =
   [%expect
     {|
     function test1(param){
-     function f(g, x){caml_call1(g, x); return;}
+     function f(g, x){return caml_call1(g, x);}
      var _d_ = 7;
      f(function(x){return x + 1 | 0;}, _d_);
      var _e_ = 4.;
@@ -66,7 +66,7 @@ let%expect_test "direct calls without --enable effects" =
     }
     //end
     function test2(param){
-     function f(g, x){caml_call1(g, x); return;}
+     function f(g, x){return caml_call1(g, x);}
      var _c_ = 7;
      f(function(x){return x + 1 | 0;}, _c_);
      f(function(x){return caml_call2(Stdlib[28], x, cst_a$0);}, cst_a);
@@ -130,16 +130,14 @@ let%expect_test "direct calls with --enable effects" =
   [%expect
     {|
     function test1(param, cont){
-     function f(g, x){g(undef); return;}
+     function f(g, x){return g(undef);}
      f(function(x){return;}, undef);
      f(function(x){return;}, undef);
      return cont(0);
     }
     //end
     function test2(param, cont){
-     function f(g, x, cont){
-      return caml_cps_exact_call2(g, x, function(_m_){return cont(undef);});
-     }
+     function f(g, x, cont){return caml_cps_exact_call2(g, x, cont);}
      var _f_ = 7;
      function _g_(x, cont){return cont(undef);}
      return caml_cps_exact_call3
@@ -148,9 +146,7 @@ let%expect_test "direct calls with --enable effects" =
               _f_,
               function(_h_){
                function _i_(x, cont){
-                var _k_ = Stdlib[28];
-                return caml_cps_call3
-                        (_k_, x, cst_a$0, function(_l_){return cont(undef);});
+                return caml_cps_call3(Stdlib[28], x, cst_a$0, cont);
                }
                return caml_cps_exact_call3
                        (f, _i_, cst_a, function(_j_){return cont(0);});

compiler/tests-compiler/effects_toplevel.ml

@@ -73,9 +73,7 @@ let%expect_test "test-compiler/lib-effects/test1.ml" =
                      [11, caml_string_of_jsbytes("abc"), 0],
                      caml_string_of_jsbytes("abc")];
                  function g(param, cont){
-                  var _f_ = Stdlib_Printf[2];
-                  return caml_cps_call2
-                          (_f_, _a_, function(_g_){return cont(undef);});
+                  return caml_cps_call2(Stdlib_Printf[2], _a_, cont);
                  }
                  caml_callback(g, [undef]);
                  var _b_ = 1;