Insert random noops to increase security against ROP attacks (llvm)

A pass that adds random noops to X86 binaries to introduce diversity with the goal of increasing security against most return-oriented programming attacks.

Command line options:
  -noop-insertion // Enable noop insertion.
  -noop-insertion-percentage=X // X% of assembly instructions will have a noop prepended (default: 50%, requires -noop-insertion)
  -max-noops-per-instruction=X // Randomly generate X noops per instruction. ie. roll the dice X times with probability set above (default: 1). This doesn't guarantee X noop instructions.

In addition, the following 'quick switch' in clang enables basic diversity using default settings (currently: noop insertion and schedule randomization; it is intended to be extended in the future).
  -fdiversify

This is the llvm part of the patch.
clang part: D3393

http://reviews.llvm.org/D3392
Patch by Stephen Crane (@rinon)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225908 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/Passes.cpp

@@ -583,6 +583,9 @@ void TargetPassConfig::addMachinePasses() {
       addPass(createGCInfoPrinter(dbgs()), false, false);
   }
 
+  if (TM->Options.NoopInsertion)
+    addPass(&NoopInsertionID);
+
   // Basic block placement.
   if (getOptLevel() != CodeGenOpt::None)
     addBlockPlacement();