Optimize if_return for single if/return cases.

Fixes #1089

lib/compress.js

@@ -535,6 +535,7 @@ merge(Compressor.prototype, {
 
         function handle_if_return(statements, compressor) {
             var self = compressor.self();
+            var multiple_if_returns = has_multiple_if_returns(statements);
             var in_lambda = self instanceof AST_Lambda;
             var ret = [];
             loop: for (var i = statements.length; --i >= 0;) {
@@ -572,7 +573,8 @@ merge(Compressor.prototype, {
                         }
                         //---
                         // if (foo()) return x; [ return ; ] ==> return foo() ? x : undefined;
-                        if ((ret.length == 0 || ret[0] instanceof AST_Return) && stat.body.value && !stat.alternative && in_lambda) {
+                        if (multiple_if_returns && (ret.length == 0 || ret[0] instanceof AST_Return)
+                            && stat.body.value && !stat.alternative && in_lambda) {
                             CHANGED = true;
                             stat = stat.clone();
                             stat.alternative = ret[0] || make_node(AST_Return, stat, {
@@ -664,6 +666,17 @@ merge(Compressor.prototype, {
                 }
             }
             return ret;
+
+            function has_multiple_if_returns(statements) {
+                var n = 0;
+                for (var i = statements.length; --i >= 0;) {
+                    var stat = statements[i];
+                    if (stat instanceof AST_If && stat.body instanceof AST_Return) {
+                        if (++n > 1) return true;
+                    }
+                }
+                return false;
+            }
         };
 
         function eliminate_dead_code(statements, compressor) {

test/compress/if_return.js

@@ -0,0 +1,207 @@
+if_return_1: {
+    options = {
+        if_return    : true,
+        sequences    : true,
+        conditionals : true,
+        comparisons  : true,
+        evaluate     : true,
+        booleans     : true,
+        unused       : true,
+        side_effects : true,
+        dead_code    : true,
+    }
+    input: {
+        function f(x) {
+            if (x) {
+                return true;
+            }
+        }
+    }
+    expect: {
+        function f(x){if(x)return!0}
+    }
+}
+
+if_return_2: {
+    options = {
+        if_return    : true,
+        sequences    : true,
+        conditionals : true,
+        comparisons  : true,
+        evaluate     : true,
+        booleans     : true,
+        unused       : true,
+        side_effects : true,
+        dead_code    : true,
+    }
+    input: {
+        function f(x, y) {
+            if (x)
+                return 3;
+            if (y)
+                return c();
+        }
+    }
+    expect: {
+        function f(x,y){return x?3:y?c():void 0}
+    }
+}
+
+if_return_3: {
+    options = {
+        if_return    : true,
+        sequences    : true,
+        conditionals : true,
+        comparisons  : true,
+        evaluate     : true,
+        booleans     : true,
+        unused       : true,
+        side_effects : true,
+        dead_code    : true,
+    }
+    input: {
+        function f(x) {
+            a();
+            if (x) {
+                b();
+                return false;
+            }
+        }
+    }
+    expect: {
+        function f(x){if(a(),x)return b(),!1}
+    }
+}
+
+if_return_4: {
+    options = {
+        if_return    : true,
+        sequences    : true,
+        conditionals : true,
+        comparisons  : true,
+        evaluate     : true,
+        booleans     : true,
+        unused       : true,
+        side_effects : true,
+        dead_code    : true,
+    }
+    input: {
+        function f(x, y) {
+            a();
+            if (x) return 3;
+            b();
+            if (y) return c();
+        }
+    }
+    expect: {
+        function f(x,y){return a(),x?3:(b(),y?c():void 0)}
+    }
+}
+
+if_return_5: {
+    options = {
+        if_return    : true,
+        sequences    : true,
+        conditionals : true,
+        comparisons  : true,
+        evaluate     : true,
+        booleans     : true,
+        unused       : true,
+        side_effects : true,
+        dead_code    : true,
+    }
+    input: {
+        function f() {
+            if (x)
+                return;
+            return 7;
+            if (y)
+                return j;
+        }
+    }
+    expect: {
+        function f(){if(!x)return 7}
+    }
+}
+
+if_return_6: {
+    options = {
+        if_return    : true,
+        sequences    : true,
+        conditionals : true,
+        comparisons  : true,
+        evaluate     : true,
+        booleans     : true,
+        unused       : true,
+        side_effects : true,
+        dead_code    : true,
+    }
+    input: {
+        function f(x) {
+          return x ? true : void 0;
+          return y;
+        }
+    }
+    expect: {
+        // suboptimal
+        function f(x){return!!x||void 0}
+    }
+}
+
+if_return_7: {
+    options = {
+        if_return    : true,
+        sequences    : true,
+        conditionals : true,
+        comparisons  : true,
+        evaluate     : true,
+        booleans     : true,
+        unused       : true,
+        side_effects : true,
+        dead_code    : true,
+    }
+    input: {
+        function f(x) {
+            if (x) {
+                return true;
+            }
+            foo();
+            bar();
+        }
+    }
+    expect: {
+        // suboptimal
+        function f(x){return!!x||(foo(),void bar())}
+    }
+}
+
+issue_1089: {
+    options = {
+        if_return    : true,
+        sequences    : true,
+        conditionals : true,
+        comparisons  : true,
+        evaluate     : true,
+        booleans     : true,
+        unused       : true,
+        side_effects : true,
+        dead_code    : true,
+    }
+    input: {
+        function x() {
+            var f = document.getElementById("fname");
+            if (f.files[0].size > 12345) {
+                alert("alert");
+                f.focus();
+                return false;
+            }
+        }
+    }
+    expect: {
+        function x() {
+            var f = document.getElementById("fname");
+            if (f.files[0].size > 12345)
+                return alert("alert"), f.focus(), !1;
+        }
+    }
+}