[PATCH] [Support] Add MemoryBuffer::getFileSlice()

[Rafael Ávila de Espíndola]

================
Comment at: include/llvm/Support/MemoryBuffer.h:131
@@ +130,3 @@
+  static ErrorOr<std::unique_ptr<MemoryBuffer>>
+  getFileSlice(const Twine &Filename, uint64_t Offset, uint64_t Length);
+
----------------
s/Length/MapSize/

Pass MapSize before Offset.

Alternatively, update getOpenFileSlice to have this order/name. My only preference is that they match.

================
Comment at: lib/Support/MemoryBuffer.cpp:335
@@ -327,1 +334,3 @@
 
+  // If we don't know the file size, use fstat to find out.  fstat on an open
+  // file descriptor is cheaper than stat on a random path.
----------------
Why was it necessary to move this?

Currently if the user passes the MapSize and we don't require a null terminator, we never call stat in here, since we don't need the file size.

I now notice that we call stat in mapped_file_region, which is a regression from when we transitioned to mapped_file_region :-(

In any case , if we can avoid an extra stat we should.

http://reviews.llvm.org/D5423