diff --git a/llvm/lib/Support/Debug.cpp b/llvm/lib/Support/Debug.cpp
index 50abe01..12b38c7 100644
--- a/llvm/lib/Support/Debug.cpp
+++ b/llvm/lib/Support/Debug.cpp
@@ -25,6 +25,9 @@
 
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
+#include "llvm/Support/circular_raw_ostream.h"
+#include "llvm/System/Signals.h"
+
 using namespace llvm;
 
 // All Debug.h functionality is a no-op in NDEBUG mode.
@@ -37,6 +40,16 @@
 Debug("debug", cl::desc("Enable debug output"), cl::Hidden,
       cl::location(DebugFlag));
 
+// -debug-buffer-size - Buffer the last N characters of debug output
+//until program termination.
+static cl::opt<unsigned>
+DebugBufferSize("debug-buffer-size",
+                cl::desc("Buffer the last N characters of debug output"
+                         "until program termination. "
+                         "[default 0 -- immediate print-out]"),
+                cl::Hidden,
+                cl::init(0));
+
 static std::string CurrentDebugType;
 static struct DebugOnlyOpt {
   void operator=(const std::string &Val) const {
@@ -50,6 +63,18 @@
           cl::Hidden, cl::value_desc("debug string"),
           cl::location(DebugOnlyOptLoc), cl::ValueRequired);
 
+// Signal handlers - dump debug output on termination.
+static void debug_user_sig_handler(void *Cookie)
+{
+  // This is a bit sneaky.  Since this is under #ifndef NDEBUG, we
+  // know that debug mode is enabled and dbgs() really is a
+  // circular_raw_ostream.  If NDEBUG is defined, then dbgs() ==
+  // errs() but this will never be invoked.
+  llvm::circular_raw_ostream *dbgout =
+    static_cast<llvm::circular_raw_ostream *>(&llvm::dbgs());
+  dbgout->flushBufferWithBanner();
+}
+
 // isCurrentDebugType - Return true if the specified string is the debug type
 // specified on the command line, or if none was specified on the command line
 // with the -debug-only=X option.
@@ -66,9 +91,38 @@
   CurrentDebugType = Type;
 }
 
+/// dbgs - Return a circular-buffered debug stream.
+raw_ostream &llvm::dbgs() {
+  // Do one-time initialization in a thread-safe way.
+  static struct dbgstream {
+    circular_raw_ostream strm;
+
+    dbgstream() :
+        strm(errs(), "*** Debug Log Output ***\n",
+             (!EnableDebugBuffering || !DebugFlag) ? 0 : DebugBufferSize) {
+      if (EnableDebugBuffering && DebugFlag && DebugBufferSize != 0)
+        // TODO: Add a handler for SIGUSER1-type signals so the user can
+        // force a debug dump.
+        sys::AddSignalHandler(&debug_user_sig_handler, 0);
+      // Otherwise we've already set the debug stream buffer size to
+      // zero, disabling buffering.
+    }
+  } thestrm;
+
+  return thestrm.strm;
+}
+
 #else
 // Avoid "has no symbols" warning.
 namespace llvm {
-int Debug_dummy = 0;
+  /// dbgs - Return dbgs().
+  raw_ostream &dbgs() {
+    return dbgs();
+  }
 }
+
 #endif
+
+/// EnableDebugBuffering - Turn on signal handler installation.
+///
+bool llvm::EnableDebugBuffering = false;