[Math] implement Math.random as TFJ builtin.

src/runtime/runtime-maths.cc

Index: src/runtime/runtime-maths.cc
diff --git a/src/runtime/runtime-maths.cc b/src/runtime/runtime-maths.cc
index 47e560d022a4189275604c710bfd5bc5a87ad2bc..404305a150592ee07a8684e1bb55f290f7cea2b5 100644
--- a/src/runtime/runtime-maths.cc
+++ b/src/runtime/runtime-maths.cc
@@ -15,58 +15,49 @@ namespace internal {
 
 RUNTIME_FUNCTION(Runtime_GenerateRandomNumbers) {
   HandleScope scope(isolate);
-  DCHECK(args.length() == 1);
-  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);
-  }
+  DCHECK(args.length() == 0);
+
+  Handle<Context> native_context = isolate->native_context();
+  DCHECK_EQ(0, native_context->math_random_index()->value());
+
+  static const int kCacheSize = 64;
+  static const int kState0Offset = kCacheSize - 1;
+  static const int kState1Offset = kState0Offset - 1;
+  // The index is decremented before used to access the cache.
+  static const int kInitialIndex = kState1Offset;
 
-  static const int kState0Offset = 0;
-  static const int kState1Offset = 1;
-  static const int kRandomBatchSize = 64;
-  CONVERT_ARG_HANDLE_CHECKED(Object, maybe_typed_array, 0);
-  Handle<JSTypedArray> typed_array;
-  // Allocate typed array if it does not yet exist.
-  if (maybe_typed_array->IsJSTypedArray()) {
-    typed_array = Handle<JSTypedArray>::cast(maybe_typed_array);
+  Handle<FixedDoubleArray> cache;
+  uint64_t state0 = 0;
+  uint64_t state1 = 0;
+  if (native_context->math_random_cache()->IsFixedDoubleArray()) {
+    cache = Handle<FixedDoubleArray>(
+        FixedDoubleArray::cast(native_context->math_random_cache()), isolate);
+    state0 = double_to_uint64(cache->get_scalar(kState0Offset));
+    state1 = double_to_uint64(cache->get_scalar(kState1Offset));
   } else {
-    static const int kByteLength = kRandomBatchSize * kDoubleSize;
-    Handle<JSArrayBuffer> buffer =
-        isolate->factory()->NewJSArrayBuffer(SharedFlag::kNotShared, TENURED);
-    JSArrayBuffer::SetupAllocatingData(buffer, isolate, kByteLength, true,
-                                       SharedFlag::kNotShared);
-    typed_array = isolate->factory()->NewJSTypedArray(
-        kExternalFloat64Array, buffer, 0, kRandomBatchSize);
+    cache = Handle<FixedDoubleArray>::cast(
+        isolate->factory()->NewFixedDoubleArray(kCacheSize, TENURED));
+    native_context->set_math_random_cache(*cache);
+    // Initialize state if not yet initialized.
+    while (state0 == 0 || state1 == 0) {
+      isolate->random_number_generator()->NextBytes(&state0, sizeof(state0));
+      isolate->random_number_generator()->NextBytes(&state1, sizeof(state1));
+    }
   }
 
   DisallowHeapAllocation no_gc;
-  double* array =
-      reinterpret_cast<double*>(typed_array->GetBuffer()->backing_store());
-  // Fetch existing state.
-  uint64_t state0 = double_to_uint64(array[kState0Offset]);
-  uint64_t state1 = double_to_uint64(array[kState1Offset]);
-  // Initialize state if not yet initialized.
-  while (state0 == 0 || state1 == 0) {
-    isolate->random_number_generator()->NextBytes(&state0, sizeof(state0));
-    isolate->random_number_generator()->NextBytes(&state1, sizeof(state1));
-  }
+  FixedDoubleArray* raw_cache = *cache;
   // Create random numbers.
-  for (int i = kState1Offset + 1; i < kRandomBatchSize; i++) {
+  for (int i = 0; i < kInitialIndex; i++) {
     // Generate random numbers using xorshift128+.
     base::RandomNumberGenerator::XorShift128(&state0, &state1);
-    array[i] = base::RandomNumberGenerator::ToDouble(state0, state1);
+    raw_cache->set(i, base::RandomNumberGenerator::ToDouble(state0, state1));
   }
+
   // Persist current state.
-  array[kState0Offset] = uint64_to_double(state0);
-  array[kState1Offset] = uint64_to_double(state1);
-  return *typed_array;
+  raw_cache->set(kState0Offset, uint64_to_double(state0));
+  raw_cache->set(kState1Offset, uint64_to_double(state1));
+  return Smi::FromInt(kInitialIndex);
 }
 }  // namespace internal
 }  // namespace v8