[RFC] Introduce MemoryBuffer::getSlice.

include/llvm/Support/MemoryBuffer.h

public:
 
   /// getFileOrSTDIN - Open the specified file as a MemoryBuffer, or open stdin
   /// if the Filename is "-".  If an error occurs, this returns null and sets
   /// ec.
   static std::error_code getFileOrSTDIN(StringRef Filename,
                                         std::unique_ptr<MemoryBuffer> &Result,
                                         int64_t FileSize = -1);
 
+  /// \brief Returns a slice of the given MemoryBuffer, to which ownership is
+  /// transferred. Note that the slice must be null terminated if
+  /// RequiresNullTerminator is true.
+  static MemoryBuffer *getSlice(std::unique_ptr<MemoryBuffer> Underlying,
+                                uint64_t Offset, uint64_t Len,
+                                bool RequiresNullTerminator);
+
   //===--------------------------------------------------------------------===//
   // Provided for performance analysis.
   //===--------------------------------------------------------------------===//
 
   /// The kind of memory backing used to support the MemoryBuffer.
   enum BufferKind {
     MemoryBuffer_Malloc,
     MemoryBuffer_MMap

lib/Support/MemoryBuffer.cpp

MemoryBuffer::getFileOrSTDIN(StringRef Filename,
                              int64_t FileSize) {
   if (Filename == "-")
     return getSTDIN(Result);
   return getFile(Filename, Result, FileSize);
 }
 
 
 //===----------------------------------------------------------------------===//
+// MemoryBufferSlice implementation.
+//===----------------------------------------------------------------------===//
+
+namespace {
+/// MemoryBufferSlice - slice of an owned underlying MemoryBuffer.
+class MemoryBufferSlice : public MemoryBuffer {
+  std::unique_ptr<MemoryBuffer> Underlying;
+
+public:
+  MemoryBufferSlice(std::unique_ptr<MemoryBuffer> U, uint64_t Offset,
+                    uint64_t Len, bool RequiresNullTerminator)
+      : Underlying(std::move(U)) {
+    init(Underlying->getBufferStart() + Offset,
+         Underlying->getBufferStart() + Offset + Len, RequiresNullTerminator);
+  }
+
+  const char *getBufferIdentifier() const override {
+    return Underlying->getBufferIdentifier();
+  }
+
+  BufferKind getBufferKind() const override {
+    return Underlying->getBufferKind();
+  }
+};
+}
+
+/// getSlice - returns a slice of the given MemoryBuffer, to which ownership is
+/// transferred. Note that the slice must be null terminated if
+/// RequiresNullTerminator is true!
+MemoryBuffer *MemoryBuffer::getSlice(std::unique_ptr<MemoryBuffer> Underlying,
+                                     uint64_t Offset, uint64_t Len,
+                                     bool RequiresNullTerminator) {
+  assert(Offset + Len < Underlying->getBufferSize());
+  return new MemoryBufferSlice(std::move(Underlying), Offset, Len,
+                               RequiresNullTerminator);
+}
+
+
+//===----------------------------------------------------------------------===//
 // MemoryBuffer::getFile implementation.
 //===----------------------------------------------------------------------===//
 
 namespace {
 /// \brief Memory maps a file descriptor using sys::fs::mapped_file_region.
 ///
 /// This handles converting the offset into a legal offset on the platform.
 class MemoryBufferMMapFile : public MemoryBuffer {