Support: Implement StreamingMemoryObject::getPointer

The implementation is fairly obvious.  This is preparation for using
some blobs in bitcode.

For clarity (and perhaps future-proofing?), I moved the call to
JumpToBit in BitstreamCursor::readRecord ahead of calling
MemoryObject::getPointer, since JumpToBit can theoretically (a) read
bytes, which (b) invalidates the blob pointer.

This isn't strictly necessary the two memory objects we have:

  - The return of RawMemoryObject::getPointer is valid until the memory
    object is destroyed.

  - StreamingMemoryObject::getPointer is valid until the next chunk is
    read from the stream.  Since the JumpToBit call is only going ahead
    to a word boundary, we'll never load another chunk.

However, reordering makes it clear by inspection that the blob returned
by BitstreamCursor::readRecord will be valid.

I added some tests for StreamingMemoryObject::getPointer and
BitstreamCursor::readRecord.

llvm-svn: 264549

llvm/lib/Bitcode/Reader/BitstreamReader.cpp

@@ -260,7 +260,10 @@ unsigned BitstreamCursor::readRecord(unsigned AbbrevID,
       break;
     }
 
-    // Otherwise, inform the streamer that we need these bytes in memory.
+    // Otherwise, inform the streamer that we need these bytes in memory.  Skip
+    // over tail padding first, in case jumping to NewEnd invalidates the Blob
+    // pointer.
+    JumpToBit(NewEnd);
     const char *Ptr = (const char *)getPointerToBit(CurBitPos, NumElts);
 
     // If we can return a reference to the data, do so to avoid copying it.
@@ -271,8 +274,6 @@ unsigned BitstreamCursor::readRecord(unsigned AbbrevID,
       for (; NumElts; --NumElts)
         Vals.push_back((unsigned char)*Ptr++);
     }
-    // Skip over tail padding.
-    JumpToBit(NewEnd);
   }
 
   return Code;