Enable deterministic random number generation

src/runtime/runtime-maths.cc

 // Copyright 2014 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.
 
 #include "src/runtime/runtime-utils.h"
 
 #include "src/arguments.h"
 #include "src/assembler.h"
 #include "src/base/utils/random-number-generator.h"
 #include "src/bootstrapper.h"
@@ skipping 20 matching lines @@
 
   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 {
+    // If a fixed random seed was requested, reset it whenever a script asks for
+    // random numbers for the first time in this context. This ensures the
+    // script sees a consistent sequence.
+    if (FLAG_random_seed != 0)
+      isolate->random_number_generator()->SetSeed(FLAG_random_seed);

[Yang, 2017/03/23 12:30:13]

I wonder whether it would be easier to simply set state0 and state1 to
FLAG_random_seed, and not touch random_number_generator at all, since its also
used elsewhere.

[Sami, 2017/03/23 17:00:27]

Sorry, missed this comment earlier. Yeah, that would be a simpler change. Done,
PTAL.

     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;
   FixedDoubleArray* raw_cache = *cache;
   // Create random numbers.
   for (int i = 0; i < kInitialIndex; i++) {
     // Generate random numbers using xorshift128+.
     base::RandomNumberGenerator::XorShift128(&state0, &state1);
     raw_cache->set(i, base::RandomNumberGenerator::ToDouble(state0, state1));
   }
 
   // Persist current state.
   raw_cache->set(kState0Offset, uint64_to_double(state0));
   raw_cache->set(kState1Offset, uint64_to_double(state1));
   return Smi::FromInt(kInitialIndex);
 }
 }  // namespace internal
 }  // namespace v8