Bug 622348 - JavaScript Math.round incorrect for (2^53)-1. r=Waldo

--HG--
extra : rebase_source : 51a780cf5ce9cbcc84c56d8a8dbe2c5d9867b36b

js/src/jit-test/tests/basic/mathRoundBig.js

@@ -0,0 +1,10 @@
+/*
+ * Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/licenses/publicdomain/
+ */
+
+assertEq(Math.round(9007199254740991), 9007199254740991);
+assertEq(Math.round(-19007199254740990), -19007199254740990);
+
+assertEq(Math.round("9007199254740991"), 9007199254740991);
+assertEq(Math.round("-19007199254740990"), -19007199254740990);

js/src/jsmath.cpp

@@ -577,26 +577,39 @@ js_copysign(double x, double y)
 }
 #endif
 
-jsdouble
-js_math_round_impl(jsdouble x)
-{
-    return js_copysign(floor(x + 0.5), x);
-}
 
-JSBool
+JSBool /* ES5 15.8.2.15. */
 js_math_round(JSContext *cx, uintN argc, Value *vp)
 {
-    jsdouble x, z;
+    CallArgs args = CallArgsFromVp(argc, vp);
 
-    if (argc == 0) {
-        vp->setDouble(js_NaN);
-        return JS_TRUE;
+    if (args.length() == 0) {
+        args.rval().setDouble(js_NaN);
+        return true;
     }
-    if (!ToNumber(cx, vp[2], &x))
-        return JS_FALSE;
-    z = js_copysign(floor(x + 0.5), x);
-    vp->setNumber(z);
-    return JS_TRUE;
+
+    double x;
+    if (!ToNumber(cx, args[0], &x))
+        return false;
+
+    int32_t i;
+    if (JSDOUBLE_IS_INT32(x, &i)) { 
+        args.rval().setInt32(i);
+        return true;
+    }
+
+    jsdpun u;
+    u.d = x;
+
+    /* Some numbers are so big that adding 0.5 would give the wrong number */
+    int exponent = ((u.s.hi & JSDOUBLE_HI32_EXPMASK) >> JSDOUBLE_HI32_EXPSHIFT) - JSDOUBLE_EXPBIAS;
+    if (exponent >= 52) {
+        args.rval().setNumber(x);
+        return true;
+    }
+
+    args.rval().setNumber(js_copysign(floor(x + 0.5), x));
+    return true;
 }
 
 static JSBool

js/src/jsmath.h

@@ -121,7 +121,4 @@ js_math_ceil_impl(jsdouble x);
 extern jsdouble
 js_math_floor_impl(jsdouble x);
 
-extern jsdouble
-js_math_round_impl(jsdouble x);
-
 #endif /* jsmath_h___ */

js/src/jsnum.h

@@ -65,6 +65,16 @@
 #endif
 #endif
 
+/* Low-level floating-point predicates. See bug 640494. */
+#define JSDOUBLE_HI32_SIGNBIT   0x80000000
+#define JSDOUBLE_HI32_EXPMASK   0x7ff00000
+#define JSDOUBLE_HI32_MANTMASK  0x000fffff
+#define JSDOUBLE_HI32_NAN       0x7ff80000
+#define JSDOUBLE_LO32_NAN       0x00000000
+
+#define JSDOUBLE_HI32_EXPSHIFT  20
+#define JSDOUBLE_EXPBIAS        1023
+
 typedef union jsdpun {
     struct {
 #if defined(IS_LITTLE_ENDIAN) && !defined(FPU_IS_ARM_FPA)
@@ -77,13 +87,6 @@ typedef union jsdpun {
     jsdouble d;
 } jsdpun;
 
-/* Low-level floating-point predicates. See bug 640494. */
-#define JSDOUBLE_HI32_SIGNBIT   0x80000000
-#define JSDOUBLE_HI32_EXPMASK   0x7ff00000
-#define JSDOUBLE_HI32_MANTMASK  0x000fffff
-#define JSDOUBLE_HI32_NAN       0x7ff80000
-#define JSDOUBLE_LO32_NAN       0x00000000
-
 static inline int
 JSDOUBLE_IS_NaN(jsdouble d)
 {