Subzero: Add a random number generator.

This is inital work necessary for diversification support in Subzero.
The random number generator implementation is temporary.  It will
eventually use a cryptographically secure pseudorandom number
generator (perhaps from LLVM, if LLVM gets one).

Add the -rng-seed= option to seed the random number generator from
the command line.

BUG=none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/455593004

src/IceRNG.cpp

diff --git a/src/IceRNG.cpp b/src/IceRNG.cpp
new file mode 100644
index 0000000..6a9f515
--- /dev/null
+++ b/src/IceRNG.cpp
@@ -0,0 +1,46 @@
+//===- 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)));
+
+} // 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;
+  return State % Max;
+}
+
+} // end of namespace Ice