remove spurious self-assignments. Closes #1081

terser · Apr 28, 2022 · adf7e85 · adf7e85
1 parent 56a05f0
commit adf7e85
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Showing 4 changed files with 29 additions and 2 deletions.
diff --git a/lib/compress/index.js b/lib/compress/index.js
@@ -3081,6 +3081,17 @@ def_optimize(AST_Assign, function(self, compressor) {
     }
 
     var def;
+    // x = x ---> x
+    if (
+        self.operator === "="
+        && self.left instanceof AST_SymbolRef
+        && self.left.name !== "arguments"
+        && !(def = self.left.definition()).undeclared
+        && self.right.equivalent_to(self.left)
+    ) {
+        return self.right;
+    }
+
     if (compressor.option("dead_code")
         && self.left instanceof AST_SymbolRef
         && (def = self.left.definition()).scope === compressor.find_parent(AST_Lambda)) {
@@ -3103,6 +3114,7 @@ def_optimize(AST_Assign, function(self, compressor) {
             || parent instanceof AST_Sequence && parent.tail_node() === node);
     }
     self = self.lift_sequences(compressor);
+
     if (self.operator == "=" && self.left instanceof AST_SymbolRef && self.right instanceof AST_Binary) {
         // x = expr1 OP expr2
         if (self.right.left instanceof AST_SymbolRef

diff --git a/lib/compress/tighten-body.js b/lib/compress/tighten-body.js
@@ -203,7 +203,7 @@ export function trim_unreachable_code(compressor, stat, target) {
     });
 }
 
-// Tighten a bunch of statements together, and perform statement-level optimization.
+/** Tighten a bunch of statements together, and perform statement-level optimization. */
 export function tighten_body(statements, compressor) {
     var in_loop, in_try;
     var scope = compressor.find_parent(AST_Scope).get_defun_scope();

diff --git a/test/compress/assignment.js b/test/compress/assignment.js
@@ -236,3 +236,18 @@ op_equals_right_global_var: {
         x = g() &   x;
     }
 }
+
+spurious_assignment: {
+    options = {}
+    input: {
+        var x = "PASS";
+        x = x;
+        console.log(x);
+    }
+    expect: {
+        var x = "PASS";
+        x;
+        console.log(x);
+    }
+    expect_stdout: "PASS"
+}
diff --git a/test/compress/collapse_vars.js b/test/compress/collapse_vars.js
@@ -779,7 +779,7 @@ collapse_vars_assignment: {
         function log(x) { return console.log(x), x; }
         function f0(c) {
             var a = 3 / c;
-            return a = a;
+            return a;
         }
         function f1(c) {
             return 1 - 3 / c;