Add test to check PRNG quality.

test/mjsunit/random-bit-correlations.js

Index: test/mjsunit/random-bit-correlations.js
diff --git a/test/mjsunit/random-bit-correlations.js b/test/mjsunit/random-bit-correlations.js
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+// Copyright 2015 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.
+
+// Flags: --random-seed=12
+
+(function() {
+  var kHistory = 2;
+  var kRepeats = 100;
+  var history = new Uint32Array(kHistory);
+
+  function random() {
+    return (Math.random() * Math.pow(2, 32)) >>> 0;
+  }
+
+  function ChiSquared(m, n) {
+    var ys_minus_np1 = (m - n / 2.0);
+    var chi_squared_1 = ys_minus_np1 * ys_minus_np1 * 2.0 / n;
+    var ys_minus_np2 = ((n - m) - n / 2.0);
+    var chi_squared_2 = ys_minus_np2 * ys_minus_np2 * 2.0 / n;
+    return chi_squared_1 + chi_squared_2;
+  }
+  for (var predictor_bit = -2; predictor_bit < 32; predictor_bit++) {
+    // The predicted bit is one of the bits from the PRNG.
+    for (var random_bit = 0; random_bit < 32; random_bit++) {
+      for (var ago = 0; ago < kHistory; ago++) {
+        // We don't want to check whether each bit predicts itself.
+        if (ago == 0 && predictor_bit == random_bit) continue;
+        // Enter the new random value into the history
+        for (var i = ago; i >= 0; i--) {
+          history[i] = random();
+        }
+        // Find out how many of the bits are the same as the prediction bit.
+        var m = 0;
+        for (var i = 0; i < kRepeats; i++) {
+          for (var j = ago - 1; j >= 0; j--) history[j + 1] = history[j];
+          history[0] = random();
+          var predicted;
+          if (predictor_bit >= 0) {
+            predicted = (history[ago] >> predictor_bit) & 1;
+          } else {
+            predicted = predictor_bit == -2 ? 0 : 1;
+          }
+          var bit = (history[0] >> random_bit) & 1;
+          if (bit == predicted) m++;
+        }
+        // Chi squared analysis for k = 2 (2, states: same/not-same) and one
+        // degree of freedom (k - 1).
+        var chi_squared = ChiSquared(m, kRepeats);
+        if (chi_squared > 24) {
+          var percent = Math.floor(m * 100.0 / kRepeats);
+          if (predictor_bit < 0) {
+            var bit_value = predictor_bit == -2 ? 0 : 1;
+            print(`Bit ${random_bit} is ${bit_value} ${percent}% of the time`);
+          } else {
+            print(`Bit ${random_bit} is the same as bit ${predictor_bit} ` +
+              `${ago} ago ${percent}% of the time`);
+          }
+        }
+        // For 1 degree of freedom this corresponds to 1 in a million.  We are
+        // running ~8000 tests, so that would be surprising.
+        assertTrue(chi_squared <= 24);
+        // If the predictor bit is a fixed 0 or 1 then it makes no sense to
+        // repeat the test with a different age.
+        if (predictor_bit < 0) break;
+      }
+    }
+  }
+})();