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

Reland of Allow Math.random to be called when creating a custom startup snapshot. (patchset #1 id:1 of https://codereview.chromium.org/1798863003/ )

Reason for revert:
This seems not to change performance.

Original issue's description:
> Revert of Allow Math.random to be called when creating a custom startup snapshot. (patchset #2 id:20001 of https://codereview.chromium.org/1780173002/ )
>
> Reason for revert:
> Regresses performance on base64 benchmark.
>
> Original issue's description:
> > Allow Math.random to be called when creating a custom startup snapshot.
> >
> > R=jkummerow@chromium.org
> > BUG=v8:4810
> > LOG=N
> >
> > Committed: https://crrev.com/6a7ec6a3bf779cdd41c66a768fd7a37195ed7b7f
> > Cr-Commit-Position: refs/heads/master@{#34705}
>
> TBR=jkummerow@chromium.org
> # Not skipping CQ checks because original CL landed more than 1 days ago.
> BUG=v8:4810, chromium:594484
> LOG=N
>
> Committed: https://crrev.com/b7be51cd33bc81d768dbf5632ba0c68843448e37
> Cr-Commit-Position: refs/heads/master@{#34739}

TBR=jkummerow@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=v8:4810, chromium:594484
LOG=N

Review URL: https://codereview.chromium.org/1806713003

Cr-Commit-Position: refs/heads/master@{#34820}

src/js/math.js

@@ -10,7 +10,6 @@
 // -------------------------------------------------------------------
 // Imports
 
-define kRandomBatchSize = 64;
 // The first two slots are reserved to persist PRNG state.
 define kRandomNumberStart = 2;
 
@@ -19,7 +18,7 @@ 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");
 
@@ -67,19 +66,24 @@ function MathPowJS(x, 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

src/runtime/runtime-maths.cc

@@ -228,8 +228,18 @@ RUNTIME_FUNCTION(Runtime_MathSqrt) {
 RUNTIME_FUNCTION(Runtime_GenerateRandomNumbers) {
   HandleScope scope(isolate);
   DCHECK(args.length() == 1);
-  // Random numbers in the snapshot are not really that random.
-  CHECK(!isolate->serializer_enabled());
+  if (isolate->serializer_enabled()) {
+    // Random numbers in the snapshot are not really that random. And we cannot
+    // return a typed array as it cannot be serialized. To make calling
+    // Math.random possible when creating a custom startup snapshot, we simply
+    // return a normal array with a single random number.
+    Handle<HeapNumber> random_number = isolate->factory()->NewHeapNumber(
+        isolate->random_number_generator()->NextDouble());
+    Handle<FixedArray> array_backing = isolate->factory()->NewFixedArray(1);
+    array_backing->set(0, *random_number);
+    return *isolate->factory()->NewJSArrayWithElements(array_backing);
+  }
+
   static const int kState0Offset = 0;
   static const int kState1Offset = 1;
   static const int kRandomBatchSize = 64;

test/cctest/test-serialize.cc

@@ -462,7 +462,8 @@ static void PartiallySerializeCustomContext(
           "  e = function(s) { return eval (s); }"
           "})();"
           "var o = this;"
-          "var r = Math.sin(0) + Math.cos(0);"
+          "var r = Math.random();"
+          "var c = Math.sin(0) + Math.cos(0);"
           "var f = (function(a, b) { return a + b; }).bind(1, 2, 3);"
           "var s = parseInt('12345');");
 
@@ -558,7 +559,18 @@ UNINITIALIZED_TEST(PartialSerializerCustomContext) {
                      ->ToNumber(v8_isolate->GetCurrentContext())
                      .ToLocalChecked()
                      ->Value();
-      CHECK_EQ(1, r);
+      CHECK(0.0 <= r && r < 1.0);
+      // Math.random still works.
+      double random = CompileRun("Math.random()")
+                          ->ToNumber(v8_isolate->GetCurrentContext())
+                          .ToLocalChecked()
+                          ->Value();
+      CHECK(0.0 <= random && random < 1.0);
+      double c = CompileRun("c")
+                     ->ToNumber(v8_isolate->GetCurrentContext())
+                     .ToLocalChecked()
+                     ->Value();
+      CHECK_EQ(1, c);
       int f = CompileRun("f()")
                   ->ToNumber(v8_isolate->GetCurrentContext())
                   .ToLocalChecked()