Fix bug where a ref assignment is moved ouside a conditional

Fixes #7170

Assignment of a ref, or other record with a single mutable field, is compiled to simple variable assignment.
This triggers an incorrect optimisation that moves identical assignments in the true and false branches of a conditional above if they are identical and the guard has no side effects.

Added a check that if the assigments to be moved are to variable, that variable must not occur free in the guard of the conditional.

Author: cristianoc

CHANGELOG.md

@@ -13,6 +13,9 @@
 # 12.0.0-alpha.6 (Unreleased)
 - Fix exponential notation syntax. https://github.com/rescript-lang/rescript/pull/7174
 
+#### :bug: Bug fix
+- Fix bug where a ref assignment is moved ouside a conditional. https://github.com/rescript-lang/rescript/pull/7176
+
 # 12.0.0-alpha.5
 
 #### :rocket: New Feature

compiler/core/js_analyzer.mli

@@ -29,7 +29,7 @@
 
 val free_variables_of_statement : J.statement -> Set_ident.t
 
-val free_variables_of_expression : J.finish_ident_expression -> Set_ident.t
+val free_variables_of_expression : J.expression -> Set_ident.t
 
 (* val no_side_effect_expression_desc :
    J.expression_desc -> bool *)

compiler/core/js_stmt_make.ml

@@ -303,6 +303,16 @@ let if_ ?comment ?declaration ?else_ (e : J.expression) (then_ : J.block) : t =
         aux ?comment (E.or_ e (E.not pred1)) ifso ifso1
       | ifso1 :: ifso_rest, ifnot1 :: ifnot_rest
         when Js_analyzer.eq_statement ifnot1 ifso1
+             && (match ifso1.statement_desc with
+                | Exp
+                    {
+                      expression_desc =
+                        Bin (Eq, {expression_desc = Var (Id v)}, _);
+                      _;
+                    } ->
+                  let guard_vars = Js_analyzer.free_variables_of_expression e in
+                  not (Set_ident.mem guard_vars v)
+                | _ -> true)
              && Js_analyzer.no_side_effect_expression e ->
         (* here we do agressive optimization, because it can help optimization later,
             move code outside of branch is generally helpful later

tests/tests/src/move_ref_assignment.mjs

@@ -0,0 +1,29 @@
+// Generated by ReScript, PLEASE EDIT WITH CARE
+
+
+let j = 0;
+
+let k = 1;
+
+if (j === 0) {
+  j = j + 2 | 0;
+  console.log("j.c");
+} else {
+  j = j + 2 | 0;
+}
+
+j = j + 2 | 0;
+
+if (k === 0) {
+  console.log("k.c");
+}
+
+let j$1 = 0;
+
+let k$1 = 0;
+
+export {
+  j$1 as j,
+  k$1 as k,
+}
+/*  Not a pure module */

tests/tests/src/move_ref_assignment.res

@@ -0,0 +1,21 @@
+type t = {mutable c: int}
+
+let j = {c: 0}
+let k = {c: 1}
+
+if j.c == 0 {
+  j.c = j.c + 2
+  Console.log("j.c")
+} else {
+  j.c = j.c + 2
+}
+
+if k.c == 0 {
+  j.c = j.c + 2
+  Console.log("k.c")
+} else {
+  j.c = j.c + 2
+}
+
+let j = 0
+let k = 0