Reland of Allow Math.random to be called when creating a custom startup snapshot.

src/js/math.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 (function(global, utils) {
 "use strict";
 
 %CheckIsBootstrapping();
 
 // -------------------------------------------------------------------
 // Imports
 
-define kRandomBatchSize = 64;
 // The first two slots are reserved to persist PRNG state.
 define kRandomNumberStart = 2;
 
 var GlobalFloat64Array = global.Float64Array;
 var GlobalMath = global.Math;
 var GlobalObject = global.Object;
 var InternalArray = utils.InternalArray;
 var NaN = %GetRootNaN();
-var nextRandomIndex = kRandomBatchSize;
+var nextRandomIndex = 0;
 var randomNumbers = UNDEFINED;
 var toStringTagSymbol = utils.ImportNow("to_string_tag_symbol");
 
 //-------------------------------------------------------------------
 
 // ECMA 262 - 15.8.2.1
 function MathAbs(x) {
   x = +x;
   return (x > 0) ? x : 0 - x;
 }
@@ skipping 27 matching lines @@
   return %_MathLogRT(TO_NUMBER(x));
 }
 
 // ECMA 262 - 15.8.2.13
 function MathPowJS(x, y) {
   return %_MathPow(TO_NUMBER(x), TO_NUMBER(y));
 }
 
 // ECMA 262 - 15.8.2.14
 function MathRandom() {
-  if (nextRandomIndex >= kRandomBatchSize) {
+  // While creating a startup snapshot, %GenerateRandomNumbers returns a
+  // normal array containing a single random number, and has to be called for
+  // every new random number.
+  // Otherwise, it returns a pre-populated typed array of random numbers. The
+  // first two elements are reserved for the PRNG state.
+  if (nextRandomIndex <= kRandomNumberStart) {
     randomNumbers = %GenerateRandomNumbers(randomNumbers);
-    nextRandomIndex = kRandomNumberStart;
+    nextRandomIndex = randomNumbers.length;
   }
-  return randomNumbers[nextRandomIndex++];
+  return randomNumbers[--nextRandomIndex];
 }
 
 function MathRandomRaw() {
-  if (nextRandomIndex >= kRandomBatchSize) {
+  if (nextRandomIndex <= kRandomNumberStart) {
     randomNumbers = %GenerateRandomNumbers(randomNumbers);
-    nextRandomIndex = kRandomNumberStart;
+    nextRandomIndex = randomNumbers.length;
   }
-  return %_DoubleLo(randomNumbers[nextRandomIndex++]) & 0x3FFFFFFF;
+  return %_DoubleLo(randomNumbers[--nextRandomIndex]) & 0x3FFFFFFF;
 }
 
 // ECMA 262 - 15.8.2.15
 function MathRound(x) {
   return %RoundNumber(TO_NUMBER(x));
 }
 
 // ECMA 262 - 15.8.2.17
 function MathSqrtJS(x) {
   return %_MathSqrt(+x);
@@ skipping 160 matching lines @@
 // Exports
 
 utils.Export(function(to) {
   to.MathAbs = MathAbs;
   to.MathExp = MathExp;
   to.MathFloor = MathFloorJS;
   to.IntRandom = MathRandomRaw;
 });
 
 })