Use separate random number generator for each randomization pass

src/IceRNG.h

 //===- subzero/src/IceRNG.h - Random number generator -----------*- C++ -*-===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
 /// This file declares a random number generator.
 ///
 //===----------------------------------------------------------------------===//
 
 #ifndef SUBZERO_SRC_ICERNG_H
 #define SUBZERO_SRC_ICERNG_H
 
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Compiler.h"
+#include "IceDefs.h"
+
 #include <cstdint>
 
 namespace Ice {
 
 class RandomNumberGenerator {
   RandomNumberGenerator() = delete;
   RandomNumberGenerator(const RandomNumberGenerator &) = delete;
   RandomNumberGenerator &operator=(const RandomNumberGenerator &) = delete;
 
 public:
   explicit RandomNumberGenerator(uint64_t Seed, llvm::StringRef Salt = "");
+  /// Create a random number generator with: global seed, randomization pass ID
+  /// and a salt uint64_t integer.
+  /// @param Seed should be a global seed.
+  /// @param RandomizationPassID should be one of RandomizationPassesEnum.
+  /// @param Salt should be an additional integer input for generating unique
+  /// RNG.
+  /// The global seed is 64 bits; since it is likely to originate from the
+  /// system time, the lower bits are more "valuable" than the upper bits. As
+  /// such, we merge the randomization pass ID and the salt into the global seed
+  /// by xor'ing them into high bit ranges. We expect the pass ID to fit within
+  /// 4 bits, so it gets shifted by 60 to merge into the upper 4 bits. We expect
+  /// the salt (usually the function sequence number) to fit within 12 bits, so
+  /// it gets shifted by 48 before merging.
+  explicit RandomNumberGenerator(uint64_t Seed,
+                                 RandomizationPassesEnum RandomizationPassID,
+                                 uint64_t Salt = 0);
   uint64_t next(uint64_t Max);
 
 private:
   uint64_t State;
 };
 
 /// This class adds additional random number generator utilities. The
 /// reason for the wrapper class is that we want to keep the
 /// RandomNumberGenerator interface identical to LLVM's.
 class RandomNumberGeneratorWrapper {
@@ skipping 17 matching lines @@
 /// sample implementation at cppreference.com.
 template <class RandomIt, class RandomFunc>
 void RandomShuffle(RandomIt First, RandomIt Last, RandomFunc &&RNG) {
   for (auto i = Last - First - 1; i > 0; --i)
     std::swap(First[i], First[RNG(i + 1)]);
 }
 
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICERNG_H