Bug 774364 - Part 4: Add setRNGState testing function. r=sstangl

js/src/builtin/TestingFunctions.cpp

@@ -2761,6 +2761,23 @@ GetLcovInfo(JSContext* cx, unsigned argc, Value* vp)
     return true;
 }
 
+#ifdef DEBUG
+static bool
+SetRNGState(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    if (!args.requireAtLeast(cx, "SetRNGState", 1))
+        return false;
+
+    double seed;
+    if (!ToNumber(cx, args[0], &seed))
+        return false;
+
+    cx->compartment()->rngState = static_cast<uint64_t>(seed) & RNG_MASK;
+    return true;
+}
+#endif
+
 static const JSFunctionSpecWithHelp TestingFunctions[] = {
     JS_FN_HELP("gc", ::GC, 0, 0,
 "gc([obj] | 'compartment' [, 'shrinking'])",
@@ -3208,6 +3225,12 @@ gc::ZealModeHelpText),
 "  Generate LCOV tracefile for the given compartment.  If no global are provided then\n"
 "  the current global is used as the default one.\n"),
 
+#ifdef DEBUG
+    JS_FN_HELP("setRNGState", SetRNGState, 1, 0,
+"setRNGState(seed)",
+"  Set this compartment's RNG state.\n"),
+#endif
+
     JS_FS_HELP_END
 };
 

js/src/jit-test/tests/basic/math-random.js

@@ -0,0 +1,44 @@
+function test() {
+  // With this seed, state won't be reset in 10000 iteration.  The result is
+  // deterministic and it can be used to check the correctness of
+  // implementation.
+  setRNGState(0x12341234);
+
+  function f() {
+    let x = [];
+    for (let i = 0; i < 10000; i++)  {
+      x.push(Math.random());
+    }
+    return x;
+  }
+  let x = f();
+  assertEq(x[0], 0.3562073961260165);
+  assertEq(x[10], 0.9777930699941514);
+  assertEq(x[100], 0.9146259915430884);
+  assertEq(x[1000], 0.315983055288946);
+  assertEq(x[2000], 0.7132284805929497);
+  assertEq(x[3000], 0.9621073641614717);
+  assertEq(x[4000], 0.3928228025111996);
+  assertEq(x[5000], 0.555710685962832);
+  assertEq(x[6000], 0.5207553912782503);
+  assertEq(x[7000], 0.08268413491723015);
+  assertEq(x[8000], 0.031796243723989925);
+  assertEq(x[9000], 0.900683320457098);
+  assertEq(x[9999], 0.7750389203054577);
+
+  // With this seed, state will be reset before calculating high bits.
+  // The result is nondeterministic, but it should be in [0, 1) range.
+  setRNGState(0);
+  x = f();
+  assertEq(x[0] >= 0 && x[0] < 1, true);
+
+  // With this seed, high bits will be 0 and state will be reset before
+  // calculating low bits.  The result is also nondeterministic, but it should
+  // be in [0, 1 / (1 << 26)) range.
+  setRNGState(0x615c0e462aa9);
+  x = f();
+  assertEq(x[0] >= 0 && x[0] < 1 / (1 << 26), true);
+}
+
+if (typeof setRNGState == "function")
+  test();