Add a comment to refer to the section of the programmer's manual that explains what the pointer tagging in Use is for.

llvm-svn: 70521

1 files changed, 9 insertions, 0 deletions

diff --git a/llvm/include/llvm/Use.h b/llvm/include/llvm/Use.h
index 53df699e703..489dbc50a04 100644
--- a/llvm/include/llvm/Use.h
+++ b/llvm/include/llvm/Use.h
@@ -11,6 +11,15 @@
 // instruction or some other User instance which refers to a Value.  The Use
 // class keeps the "use list" of the referenced value up to date.
 //
+// Pointer tagging is used to efficiently find the User corresponding
+// to a Use without having to store a User pointer in every Use. A
+// User is preceded in memory by all the Uses corresponding to its
+// operands, and the low bits of one of the fields (Prev) of the Use
+// class are used to encode offsets to be able to find that User given
+// a pointer to any Use. For details, see:
+//
+//   http://www.llvm.org/docs/ProgrammersManual.html#UserLayout
+//
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_USE_H