Subzero: Randomly insert nops.

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.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file implements the random number generator.
 //
 //===----------------------------------------------------------------------===//
 
 #include <time.h>
 
 #include "llvm/Support/CommandLine.h"
 
 #include "IceRNG.h"
 
 namespace Ice {
 
 namespace {
 namespace cl = llvm::cl;
 
 cl::opt<unsigned long long>
 RandomSeed("rng-seed", cl::desc("Seed the random number generator"),
            cl::init(time(0)));
 
+const unsigned MAX = 2147483647;
+
 } // end of anonymous namespace
 
 // 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(llvm::StringRef)
     : State(RandomSeed) {}
 
 uint64_t RandomNumberGenerator::next(uint64_t Max) {
   // Lewis, Goodman, and Miller (1969)
-  State = (16807 * State) % 2147483647;
+  State = (16807 * State) % MAX;
   return State % Max;
 }
 
+bool RandomNumberGeneratorWrapper::getTrueWithProbability(float Probability) {
+  return RNG.next(MAX) < Probability * MAX;
+}
+
 } // end of namespace Ice