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diff --git a/lib/Transforms/Instrumentation/DebugIR.cpp b/lib/Transforms/Instrumentation/DebugIR.cpp
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+++ b/lib/Transforms/Instrumentation/DebugIR.cpp
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+//===--- DebugIR.cpp - Transform debug metadata to allow debugging IR -----===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// A Module transform pass that emits a succinct version of the IR and replaces
+// the source file metadata to allow debuggers to step through the IR.
+//
+// The location where the IR file is emitted is the same as the directory
+// operand of the !llvm.dbg.cu metadata node present in the input module. The
+// file name is constructed from the original file name by stripping the
+// extension and replacing it with "-debug-ll" or the Postfix string specified
+// at construction.
+//
+// FIXME: instead of replacing debug metadata, additional metadata should be
+// used to point capable debuggers to the IR file without destroying the
+// mapping to the original source file.
+//
+// FIXME: this pass should not depend on the existance of debug metadata in
+// the module as it does now. Instead, it should use DIBuilder to create the
+// required metadata.
+//
+//===----------------------------------------------------------------------===//
+
+#include <string>
+
+#include "llvm/ADT/OwningPtr.h"
+#include "llvm/ADT/ValueMap.h"
+#include "llvm/Assembly/AssemblyAnnotationWriter.h"
+#include "llvm/DebugInfo.h"
+#include "llvm/DIBuilder.h"
+#include "llvm/InstVisitor.h"
+#include "llvm/IR/Instruction.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Pass.h"
+#include "llvm/Transforms/Instrumentation.h"
+#include "llvm/Transforms/Utils/Cloning.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/ToolOutputFile.h"
+#include "llvm/Support/FormattedStream.h"
+using namespace llvm;
+
+namespace {
+
+/// Builds a map of Value* to line numbers on which the Value appears in a
+/// textual representation of the IR by plugging into the AssemblyWriter by
+/// masquerading as an AssemblyAnnotationWriter.
+class ValueToLineMap : public AssemblyAnnotationWriter {
+  ValueMap<const Value *, unsigned int> Lines;
+  typedef ValueMap<const Value *, unsigned int>::const_iterator LineIter;
+
+public:
+
+  /// Prints Module to a null buffer in order to build the map of Value pointers
+  /// to line numbers.
+  ValueToLineMap(Module *M) {
+    raw_null_ostream ThrowAway;
+    M->print(ThrowAway, this);
+  }
+
+  // This function is called after an Instruction, GlobalValue, or GlobalAlias
+  // is printed.
+  void printInfoComment(const Value &V, formatted_raw_ostream &Out) {
+    Out.flush();
+    Lines.insert(std::make_pair(&V, Out.getLine() + 1));
+  }
+
+  /// If V appears on a line in the textual IR representation, sets Line to the
+  /// line number and returns true, otherwise returns false.
+  bool getLine(const Value *V, unsigned int &Line) const {
+    LineIter i = Lines.find(V);
+    if (i != Lines.end()) {
+      Line = i->second;
+      return true;
+    }
+    return false;
+  }
+};
+
+/// Removes debug intrisncs like llvm.dbg.declare and llvm.dbg.value.
+class DebugIntrinsicsRemover : public InstVisitor<DebugIntrinsicsRemover> {
+  void remove(Instruction &I) { I.eraseFromParent(); }
+
+public:
+  void visitDbgDeclareInst(DbgDeclareInst &I) { remove(I); }
+  void visitDbgValueInst(DbgValueInst &I) { remove(I); }
+  void visitDbgInfoIntrinsic(DbgInfoIntrinsic &I) { remove(I); }
+};
+
+/// Removes debug metadata (!dbg) nodes from all instructions as well as
+/// metadata named "llvm.dbg.cu" in the Module.
+class DebugMetadataRemover : public InstVisitor<DebugMetadataRemover> {
+public:
+  void visitInstruction(Instruction &I) {
+    if (I.getMetadata(LLVMContext::MD_dbg))
+      I.setMetadata(LLVMContext::MD_dbg, 0);
+  }
+
+  void run(Module *M) {
+    // Remove debug metadata attached to instructions
+    visit(M);
+
+    // Remove CU named metadata (and all children nodes)
+    NamedMDNode *Node = M->getNamedMetadata("llvm.dbg.cu");
+    M->eraseNamedMetadata(Node);
+  }
+};
+
+/// Replaces line number metadata attached to Instruction nodes with new line
+/// numbers provided by the ValueToLineMap.
+class LineNumberReplacer : public InstVisitor<LineNumberReplacer> {
+  /// Table of line numbers
+  const ValueToLineMap &LineTable;
+
+  /// Table of cloned values
+  const ValueToValueMapTy &VMap;
+
+  /// Directory of debug metadata
+  const DebugInfoFinder &Finder;
+
+public:
+  LineNumberReplacer(const ValueToLineMap &VLM, const DebugInfoFinder &Finder,
+                     const ValueToValueMapTy &VMap)
+      : LineTable(VLM), VMap(VMap), Finder(Finder) {}
+
+  void visitInstruction(Instruction &I) {
+    DebugLoc Loc(I.getDebugLoc());
+
+    unsigned Col = 0; // FIXME: support columns
+    unsigned Line;
+    if (!LineTable.getLine(VMap.lookup(&I), Line))
+      // Instruction has no line, it may have been removed (in the module that
+      // will be passed to the debugger) so there is nothing to do here.
+      return;
+
+    DebugLoc NewLoc;
+    if (!Loc.isUnknown())
+      // I had a previous debug location: re-use the DebugLoc
+      NewLoc = DebugLoc::get(Line, Col, Loc.getScope(I.getContext()),
+                             Loc.getInlinedAt(I.getContext()));
+    else if (MDNode *scope = findFunctionMD(I.getParent()->getParent()))
+      // I had no previous debug location, but M has some debug information
+      NewLoc =
+          DebugLoc::get(Line, Col, scope, /*FIXME: inlined instructions*/ 0);
+    else
+      // Neither I nor M has any debug information -- nothing to do here.
+      // FIXME: support debugging of undecorated IR (generated by clang without
+      //        the -g option)
+      return;
+
+    addDebugLocation(const_cast<Instruction &>(I), NewLoc);
+  }
+
+private:
+
+  /// Returns the MDNode that corresponds with F
+  MDNode *findFunctionMD(const Function *F) {
+    for (DebugInfoFinder::iterator i = Finder.subprogram_begin(),
+                                   e = Finder.subprogram_end();
+         i != e; ++i) {
+      DISubprogram S(*i);
+      if (S.getFunction() == F)
+        return *i;
+    }
+    // cannot find F -- likely means there is no debug information
+    return 0;
+  }
+
+  void addDebugLocation(Instruction &I, DebugLoc Loc) {
+    MDNode *MD = Loc.getAsMDNode(I.getContext());
+    I.setMetadata(LLVMContext::MD_dbg, MD);
+  }
+};
+
+class DebugIR : public ModulePass {
+  std::string Postfix;
+  std::string Filename;
+
+  /// Flags to control the verbosity of the generated IR file
+  bool hideDebugIntrinsics;
+  bool hideDebugMetadata;
+
+public:
+  static char ID;
+
+  const char *getPassName() const { return "DebugIR"; }
+
+  // FIXME: figure out if we are compiling something that already exists on disk
+  // in text IR form, in which case we can omit outputting a new IR file, or if
+  // we're building something from memory where we actually need to emit a new
+  // IR file for the debugger.
+
+  /// Output a file with the same base name as the original, but with the
+  /// postfix "-debug-ll" appended.
+  DebugIR()
+      : ModulePass(ID), Postfix("-debug-ll"), hideDebugIntrinsics(true),
+        hideDebugMetadata(true) {}
+
+  /// Customize the postfix string used to replace the extension of the
+  /// original filename that appears in the !llvm.dbg.cu metadata node.
+  DebugIR(StringRef postfix, bool hideDebugIntrinsics, bool hideDebugMetadata)
+      : ModulePass(ID), Postfix(postfix),
+        hideDebugIntrinsics(hideDebugIntrinsics),
+        hideDebugMetadata(hideDebugMetadata) {}
+
+private:
+  // Modify the filename embedded in the Compilation-Unit debug information of M
+  bool replaceFilename(Module &M, const DebugInfoFinder &Finder) {
+    bool changed = false;
+
+    // Sanity check -- if llvm.dbg.cu node exists, the DebugInfoFinder
+    // better have found at least one CU!
+    if (M.getNamedMetadata("llvm.dbg.cu"))
+      assert(Finder.compile_unit_count() > 0 &&
+             "Found no compile units but llvm.dbg.cu node exists");
+
+    for (DebugInfoFinder::iterator i = Finder.compile_unit_begin(),
+                                   e = Finder.compile_unit_end();
+         i != e; ++i) {
+      DICompileUnit CU(*i);
+      Filename = CU.getFilename();
+
+      // Replace extension with postfix
+      size_t dot = Filename.find_last_of(".");
+      if (dot != std::string::npos)
+        Filename.erase(dot);
+      Filename += Postfix;
+
+      CU.setFilename(Filename, M.getContext());
+      changed = true;
+    }
+    return changed;
+  }
+
+  /// Replace existing line number metadata with line numbers that correspond
+  /// with the IR file that is seen by the debugger.
+  void addLineNumberMetadata(Module *M, const ValueToLineMap &VLM,
+                             const ValueToValueMapTy &VMap,
+                             const DebugInfoFinder &Finder) {
+    LineNumberReplacer Replacer(VLM, Finder, VMap);
+    Replacer.visit(M);
+  }
+
+  void writeDebugBitcode(Module *M) {
+    std::string error;
+    tool_output_file OutFile(Filename.c_str(), error);
+    OutFile.keep();
+    formatted_raw_ostream OS;
+    OS.setStream(OutFile.os());
+    M->print(OS, 0);
+  }
+
+  void removeDebugIntrinsics(Module *M) {
+    DebugIntrinsicsRemover Remover;
+    Remover.visit(M);
+  }
+
+  void removeDebugMetadata(Module *M) {
+    DebugMetadataRemover Remover;
+    Remover.run(M);
+  }
+
+  void updateAndWriteDebugIRFile(Module *M, const DebugInfoFinder &Finder) {
+    // The module we output in text form for a debugger to open is stripped of
+    // 'extras' like debug intrinsics that end up in DWARF anyways and just
+    // clutter the debug experience.
+
+    ValueToValueMapTy VMap;
+    Module *DebuggerM = CloneModule(M, VMap);
+
+    if (hideDebugIntrinsics)
+      removeDebugIntrinsics(DebuggerM);
+
+    if (hideDebugMetadata)
+      removeDebugMetadata(DebuggerM);
+
+    // FIXME: remove all debug metadata from M once we support generating DWARF
+    // subprogram attributes.
+
+    ValueToLineMap LineTable(DebuggerM);
+    addLineNumberMetadata(M, LineTable, VMap, Finder);
+    writeDebugBitcode(DebuggerM);
+  }
+
+  bool runOnModule(Module &M) {
+    // Stores existing debug info needed when creating new line number entries.
+    DebugInfoFinder Finder;
+    Finder.processModule(M);
+
+    bool changed = replaceFilename(M, Finder);
+    if (changed)
+      updateAndWriteDebugIRFile(&M, Finder);
+    return changed;
+  }
+};
+
+} // anonymous namespace
+
+char DebugIR::ID = 0;
+INITIALIZE_PASS(DebugIR, "debug-ir", "Enable debugging IR", false, false)
+
+ModulePass *llvm::createDebugIRPass(StringRef FilenamePostfix,
+                                    bool hideDebugIntrinsics,
+                                    bool hideDebugMetadata) {
+  return new DebugIR(FilenamePostfix, hideDebugIntrinsics, hideDebugMetadata);
+}