Add a BinarySubstreamRef, and a method to read one.

This is essentially just a BinaryStreamRef packaged with an
offset and the logic for reading one is no different than the
logic for reading a BinaryStreamRef, except that we save the
current offset.

llvm-svn: 306122

include/llvm/Support/BinaryStreamReader.h

@@ -137,6 +137,15 @@ public:
   /// returns an appropriate error code.
   Error readStreamRef(BinaryStreamRef &Ref, uint32_t Length);
 
+  /// Read \p Length bytes from the underlying stream into \p Stream.  This is
+  /// equivalent to calling getUnderlyingStream().slice(Offset, Length).
+  /// Updates the stream's offset to point after the newly read object.  Never
+  /// causes a copy.
+  ///
+  /// \returns a success error code if the data was successfully read, otherwise
+  /// returns an appropriate error code.
+  Error readSubstream(BinarySubstreamRef &Stream, uint32_t Size);
+
   /// Get a pointer to an object of type T from the underlying stream, as if by
   /// memcpy, and store the result into \p Dest.  It is up to the caller to
   /// ensure that objects of type T can be safely treated in this manner.

include/llvm/Support/BinaryStreamRef.h

@@ -166,6 +166,11 @@ public:
                                    ArrayRef<uint8_t> &Buffer) const;
 };
 
+struct BinarySubstreamRef {
+  uint32_t Offset;            // Offset in the parent stream
+  BinaryStreamRef StreamData; // Stream Data
+};
+
 class WritableBinaryStreamRef
     : public BinaryStreamRefBase<WritableBinaryStreamRef,
                                  WritableBinaryStream> {

lib/Support/BinaryStreamReader.cpp

@@ -109,6 +109,12 @@ Error BinaryStreamReader::readStreamRef(BinaryStreamRef &Ref, uint32_t Length) {
   return Error::success();
 }
 
+Error BinaryStreamReader::readSubstream(BinarySubstreamRef &Stream,
+                                        uint32_t Size) {
+  Stream.Offset = getOffset();
+  return readStreamRef(Stream.StreamData, Size);
+}
+
 Error BinaryStreamReader::skip(uint32_t Amount) {
   if (Amount > bytesRemaining())
     return make_error<BinaryStreamError>(stream_error_code::stream_too_short);