Start MCAsmStreamer implementation.

[oota-llvm.git] / include / llvm / Use.h

diff --git a/include/llvm/Use.h b/include/llvm/Use.h
index 32cdd1b95ddb362d29f32d081bd5598bd381b364..489dbc50a041251ba870b0dadab7067bd13e6dfd 100644 (file)
--- a/include/llvm/Use.h
+++ b/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
@@ -29,6 +38,17 @@ class Use;
 /// Tag - generic tag type for (at least 32 bit) pointers
 enum Tag { noTag, tagOne, tagTwo, tagThree };
 
+// Use** is only 4-byte aligned.
+template<>
+class PointerLikeTypeTraits<Use**> {
+public:
+  static inline void *getAsVoidPointer(Use** P) { return P; }
+  static inline Use **getFromVoidPointer(void *P) {
+    return static_cast<Use**>(P);
+  }
+  enum { NumLowBitsAvailable = 2 };
+};
+
 //===----------------------------------------------------------------------===//
 //                                  Use Class
 //===----------------------------------------------------------------------===//