Changeset 192855 in webkit for trunk/Source/JavaScriptCore/runtime/JSGlobalObject.cpp

Timestamp:

Nov 30, 2015, 7:39:59 PM (10 years ago)

Author:

[email protected]

Message:

Use a better RNG for Math.random()
​https://bugs.webkit.org/show_bug.cgi?id=151641

Reviewed by Anders Carlsson.

Source/JavaScriptCore:

Updated for interface change.

• runtime/JSGlobalObject.cpp:

(JSC::JSGlobalObject::setInputCursor):

Source/WTF:

Use 64 bits in the random number generator instead of 32 bit. (In
the end, JavaScript, which uses doubles, will only see 52 bits.) This
prevents programs that mulitply a random number by a large constant from
seeing non-random "banding" caused by zeroes in the low 20 bits.

I also took the opportunity to upgrade from GameRandom to Xorshift+,
since Xorshift+ passes more benchmarks for randomness, and is not any
slower or more complicated.

Now let us all remember the fateful words of Steve Weibe, who would be
King of Kong: "The randomness went the opposite way that it usually goes."

• wtf/WeakRandom.h:

(WTF::WeakRandom::WeakRandom):
(WTF::WeakRandom::setSeed): Use standard naming.

(WTF::WeakRandom::seed): This function is safe now. "Unsafe" in function
names makes me itch.

(WTF::WeakRandom::get):
(WTF::WeakRandom::getUint32): Update to 64bit.

(WTF::WeakRandom::advance): The Xorshift+ algorithm.

(WTF::WeakRandom::initializeSeed): Deleted.
(WTF::WeakRandom::seedUnsafe): Deleted.

File:

• trunk/Source/JavaScriptCore/runtime/JSGlobalObject.cpp (modified) (1 diff)

trunk/Source/JavaScriptCore/runtime/JSGlobalObject.cpp

@@ -1015 +1015 @@
     // to avoid threading the input cursor through all the abstraction layers.
     if (cursor.isCapturing())
-        cursor.appendInput<SetRandomSeed>(m_weakRandom.seedUnsafe());
+        cursor.appendInput<SetRandomSeed>(m_weakRandom.seed());
     else if (cursor.isReplaying()) {
         if (SetRandomSeed* input = cursor.fetchInput<SetRandomSeed>())
-            m_weakRandom.initializeSeed(static_cast<unsigned>(input->randomSeed()));
+            m_weakRandom.setSeed(static_cast<unsigned>(input->randomSeed()));
     }
 }