Use separate random number generator for each randomization pass

src/IceRNG.cpp

 //===- subzero/src/IceRNG.cpp - PRNG implementation -----------------------===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 13 matching lines @@
 
 // TODO(wala,stichnot): Switch to RNG implementation from LLVM or C++11.
 //
 // TODO(wala,stichnot): Make it possible to replay the RNG sequence in a
 // subsequent run, for reproducing a bug.  Print the seed in a comment
 // in the asm output.  Embed the seed in the binary via metadata that an
 // attacker can't introspect.
 RandomNumberGenerator::RandomNumberGenerator(uint64_t Seed, llvm::StringRef)
     : State(Seed) {}
 
+RandomNumberGenerator::RandomNumberGenerator(
+    uint64_t Seed, RandomizationPassesEnum RandomizationPassID, uint64_t Salt) {
+  constexpr unsigned NumBitsGlobalSeed = CHAR_BIT * sizeof(State);
+  constexpr unsigned NumBitsPassID = 4;
+  constexpr unsigned NumBitsSalt = 12;
+  static_assert(RPE_num < (1 << NumBitsPassID), "NumBitsPassID too small");
+  State = Seed ^ ((uint64_t)RandomizationPassID
+                  << (NumBitsGlobalSeed - NumBitsPassID)) ^
+          (Salt << (NumBitsGlobalSeed - NumBitsPassID - NumBitsSalt));
+}
 uint64_t RandomNumberGenerator::next(uint64_t Max) {
   // Lewis, Goodman, and Miller (1969)
   State = (16807 * State) % MAX;
   return State % Max;
 }
 
 bool RandomNumberGeneratorWrapper::getTrueWithProbability(float Probability) {
   return RNG.next(MAX) < Probability * MAX;
 }
 
 } // end of namespace Ice