Implement xorshift128+ for Math.random.

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 70c587d7454b401fbfe9f2fc61812b039a6a0531..3d8c8aef7ef2d8f609ebc09c91f1c2b695226a73 100644
--- a/src/runtime/runtime-maths.cc
+++ b/src/runtime/runtime-maths.cc
@@ -7,6 +7,7 @@
 #include "src/arguments.h"
 #include "src/assembler.h"
 #include "src/base/utils/random-number-generator.h"
+#include "src/bootstrapper.h"
 #include "src/codegen.h"
 #include "src/third_party/fdlibm/fdlibm.h"
 
@@ -247,18 +248,50 @@ RUNTIME_FUNCTION(Runtime_IsMinusZero) {
 }
 
 
-RUNTIME_FUNCTION(Runtime_InitializeRNG) {
+RUNTIME_FUNCTION(Runtime_GenerateRandomNumbers) {
   HandleScope scope(isolate);
-  DCHECK(args.length() == 0);
-  static const int kSize = 4;
-  Handle<FixedArray> array = isolate->factory()->NewFixedArray(kSize);
-  uint16_t seeds[kSize];
-  do {
-    isolate->random_number_generator()->NextBytes(seeds,
-                                                  kSize * sizeof(*seeds));
-  } while (!(seeds[0] && seeds[1] && seeds[2] && seeds[3]));
-  for (int i = 0; i < kSize; i++) array->set(i, Smi::FromInt(seeds[i]));
-  return *isolate->factory()->NewJSArrayWithElements(array);
+  DCHECK(args.length() == 1);
+  // Random numbers in the snapshot are not really that random.
+  DCHECK(!isolate->bootstrapper()->IsActive());
+  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);
+  } 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);
+  }
+
+  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));
+  }
+  // Create random numbers.
+  for (int i = kState1Offset + 1; i < kRandomBatchSize; i++) {
+    // Generate random numbers using xorshift128+.
+    base::RandomNumberGenerator::XorShift128(&state0, &state1);
+    array[i] = base::RandomNumberGenerator::ToDouble(state0, state1);
+  }
+  // Persist current state.
+  array[kState0Offset] = uint64_to_double(state0);
+  array[kState1Offset] = uint64_to_double(state1);
+  return *typed_array;
 }
 }  // namespace internal
 }  // namespace v8