Add a pass to strip bitcode metadata.

This only works on instruction attachments for now.  Since it is a
ModulePass we can add something to strip NamedMetadata
based on a whitelist, if we want to retain some of that.

It does not touch debug metadata, and leaves -strip-debug to handle that.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3348

Review URL: https://codereview.chromium.org/12844027

1 files changed, 77 insertions, 0 deletions

diff --git a/lib/Transforms/NaCl/StripMetadata.cpp b/lib/Transforms/NaCl/StripMetadata.cpp
new file mode 100644
index 0000000000..a7d2e7eb68
--- /dev/null
+++ b/lib/Transforms/NaCl/StripMetadata.cpp
@@ -0,0 +1,77 @@
+//===- StripMetadata.cpp - Strip non-stable non-debug metadata       ------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// The StripMetadata transformation strips instruction attachment
+// metadata, such as !tbaa and !prof metadata.
+// TODO: Strip NamedMetadata too.
+//
+// It does not strip debug metadata.  Debug metadata is used by debug
+// intrinsic functions and calls to those intrinsic functions.  Use the
+// -strip-debug or -strip pass to strip that instead.
+//
+// The goal of this pass is to reduce bitcode ABI surface area.
+// We don't know yet which kind of metadata is considered stable.
+//===----------------------------------------------------------------------===//
+
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Pass.h"
+#include "llvm/Transforms/NaCl.h"
+
+using namespace llvm;
+
+namespace {
+  class StripMetadata : public ModulePass {
+  public:
+    static char ID;
+    explicit StripMetadata() : ModulePass(ID) {
+      initializeStripMetadataPass(*PassRegistry::getPassRegistry());
+    }
+
+    virtual bool runOnModule(Module &M);
+
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+      AU.setPreservesCFG();
+    }
+  };
+}
+
+char StripMetadata::ID = 0;
+INITIALIZE_PASS(StripMetadata, "strip-metadata",
+                "Strip all non-stable non-debug metadata from a module.",
+                false, false)
+
+ModulePass *llvm::createStripMetadataPass() {
+  return new StripMetadata();
+}
+
+static bool DoStripMetadata(Module &M) {
+  bool Changed = false;
+
+  for (Module::iterator MI = M.begin(), ME = M.end(); MI != ME; ++MI) {
+    for (Function::iterator FI = MI->begin(), FE = MI->end(); FI != FE; ++FI) {
+      for (BasicBlock::iterator BI = FI->begin(), BE = FI->end(); BI != BE;
+           ++BI) {
+        SmallVector<std::pair<unsigned, MDNode *>, 8> InstMeta;
+        // Let the debug metadata be stripped by the -strip-debug pass.
+        BI->getAllMetadataOtherThanDebugLoc(InstMeta);
+        for (size_t i = 0; i < InstMeta.size(); ++i) {
+          BI->setMetadata(InstMeta[i].first, NULL);
+          Changed = true;
+        }
+      }
+    }
+  }
+
+  return Changed;
+}
+
+bool StripMetadata::runOnModule(Module &M) {
+  return DoStripMetadata(M);
+}