Eliminate useless genexp for(;;) conditions (442342, r=jorendorff).

js/src/jsemit.cpp

@@ -4627,12 +4627,6 @@ js_EmitTree(JSContext *cx, JSCodeGenerator *cg, JSParseNode *pn)
             }
 
             if (pn2->pn_kid2) {
-                if (pn2->pn_kid2->pn_type == TOK_LP &&
-                    pn2->pn_kid2->pn_head->pn_type == TOK_FUNCTION &&
-                    (pn2->pn_kid2->pn_head->pn_flags & TCF_GENEXP_LAMBDA) &&
-                    js_NewSrcNote(cx, cg, SRC_GENEXP) < 0) {
-                    return JS_FALSE;
-                }
                 beq = EmitJump(cx, cg, JSOP_IFNE, top - CG_OFFSET(cg));
                 if (beq < 0)
                     return JS_FALSE;

js/src/jsopcode.cpp

@@ -3868,9 +3868,6 @@ Decompile(SprintStack *ss, jsbytecode *pc, intN nb, JSOp nextop)
                     } else {
                         op = (JSOp) *pc2;
                         op = ((js_CodeSpec[op].format & JOF_PARENHEAD) ||
-                              ((op == JSOP_IFNE || op == JSOP_IFNEX) &&
-                               (!(sn2 = js_GetSrcNote(outer, pc2)) ||
-                                SN_TYPE(sn2) != SRC_GENEXP)) ||
                               ((js_CodeSpec[op].format & JOF_INVOKE) &&
                                GET_ARGC(pc2) == 1) ||
                               ((op == JSOP_IFEQ || op == JSOP_IFEQX) &&

js/src/jsopcode.tbl

@@ -105,7 +105,7 @@ OPDEF(JSOP_LEAVEWITH, 4,  "leavewith",  NULL,         1,  1,  0,  0,  JOF_BYTE)
 OPDEF(JSOP_RETURN,    5,  "return",     NULL,         1,  1,  0,  2,  JOF_BYTE)
 OPDEF(JSOP_GOTO,      6,  "goto",       NULL,         3,  0,  0,  0,  JOF_JUMP)
 OPDEF(JSOP_IFEQ,      7,  "ifeq",       NULL,         3,  1,  0,  4,  JOF_JUMP|JOF_DETECTING)
-OPDEF(JSOP_IFNE,      8,  "ifne",       NULL,         3,  1,  0,  0,  JOF_JUMP)
+OPDEF(JSOP_IFNE,      8,  "ifne",       NULL,         3,  1,  0,  0,  JOF_JUMP|JOF_PARENHEAD)
 
 /* Get the arguments object for the current, lightweight function activation. */
 OPDEF(JSOP_ARGUMENTS, 9, js_arguments_str, js_arguments_str, 1, 0, 1, 18, JOF_BYTE)
@@ -353,7 +353,7 @@ OPDEF(JSOP_DEFLOCALFUN, 138,"deflocalfun",NULL,       5,  0,  0,  0,  JOF_SLOTOB
 /* Extended jumps. */
 OPDEF(JSOP_GOTOX,         139,"gotox",    NULL,       5,  0,  0,  0,  JOF_JUMPX)
 OPDEF(JSOP_IFEQX,         140,"ifeqx",    NULL,       5,  1,  0,  4,  JOF_JUMPX|JOF_DETECTING)
-OPDEF(JSOP_IFNEX,         141,"ifnex",    NULL,       5,  1,  0,  0,  JOF_JUMPX)
+OPDEF(JSOP_IFNEX,         141,"ifnex",    NULL,       5,  1,  0,  0,  JOF_JUMPX|JOF_PARENHEAD)
 OPDEF(JSOP_ORX,           142,"orx",      NULL,       5,  1,  0,  5,  JOF_JUMPX|JOF_DETECTING)
 OPDEF(JSOP_ANDX,          143,"andx",     NULL,       5,  1,  0,  6,  JOF_JUMPX|JOF_DETECTING)
 OPDEF(JSOP_GOSUBX,        144,"gosubx",   NULL,       5,  0,  0,  0,  JOF_JUMPX)

js/src/jsparse.cpp

@@ -2860,6 +2860,22 @@ Statement(JSContext *cx, JSTokenStream *ts, JSTreeContext *tc)
                 pn2 = Expr(cx, ts, tc);
                 if (!pn2)
                     return NULL;
+
+                if (pn2->pn_type == TOK_LP &&
+                    pn2->pn_head->pn_type == TOK_FUNCTION &&
+                    (pn2->pn_head->pn_flags & TCF_GENEXP_LAMBDA)) {
+                    /*
+                     * A generator expression as loop condition is useless.
+                     * It won't be called, and as an object it evaluates to
+                     * true in boolean contexts without any conversion hook
+                     * being called.
+                     *
+                     * This useless condition elimination is mandatory, to
+                     * help the decompiler. See bug 442342.
+                     */
+                    RecycleTree(pn2, tc);
+                    pn2 = NULL;
+                }
             }
 
             /* Parse the update expression or null into pn3. */

js/tests/js1_8/genexps/regress-380237-04.js

@@ -78,7 +78,7 @@ needParens("switch (x) { case xx: }");
 needParens("return xx;");
 needParens("yield xx;");
 needParens("for (xx;;) { }");
-needParens("for (;xx;) { }");
+needParens("for (;xx;) { }", "function anonymous() {\n    for (;;) {\n    }\n}");
 needParens("for (;;xx) { }");
 needParens("for (i in xx) { }");
 needParens("throw xx");
@@ -211,7 +211,7 @@ function doesNotNeedParens(pat)
   //    print("Skipping the over-parenthesization test, because I don't know how to test for over-parenthesization when the pattern doesn't have parens snugly around it.")
 }
 
-function needParens(pat)
+function needParens(pat, exp)
 {
   print("Testing " + pat);
 
@@ -242,8 +242,8 @@ function needParens(pat)
   }
   reportCompare(expect, actual, summary + ': needParens ' + ft);
 
-  roundTripTest(f);
-  overParenTest(f);
+  roundTripTest(f, exp);
+  overParenTest(f, exp);
 }
 
 function rejectLHS(pat)
@@ -268,9 +268,11 @@ function rejectLHS(pat)
 }
 
 
-function overParenTest(f)
+function overParenTest(f, exp)
 {
   var uf = "" + f;
+  if (uf == exp)
+    return;
 
   reportCompare(false, uf.indexOf(genexpParened) == -1, summary + 
                 ': overParenTest genexp snugly in parentheses: ' + uf);
@@ -301,7 +303,7 @@ function sanityCheck(pat)
   reportCompare(expect, actual, summary + ': sanityCheck ' + pat);
 }
 
-function roundTripTest(f)
+function roundTripTest(f, exp)
 {
   // Decompile
   var uf = "" + f;
@@ -321,7 +323,7 @@ function roundTripTest(f)
   }
 
   // Decompile again and make sure the decompilations match exactly.
-  expect = uf;
+  expect = exp || uf;
   actual = "" + euf;
   reportCompare(expect, actual, summary + ': roundTripTest no round-trip change');
 }