From a7f21f1767600656c344cc310da57645ebc0f62a Mon Sep 17 00:00:00 2001
From: Dan Gohman <gohman@apple.com>
Date: Sun, 16 Aug 2009 21:26:11 +0000
Subject: [PATCH] Add a getOffsetOf, for building a target-independent
 expression for offsetof, similar to getSizeOf for sizeof.

llvm-svn: 79208
---
 include/llvm/Constants.h |  5 +++++
 lib/VMCore/Constants.cpp | 12 ++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/include/llvm/Constants.h b/include/llvm/Constants.h
index e26bb62c357..a66773c0b53 100644
--- a/include/llvm/Constants.h
+++ b/include/llvm/Constants.h
@@ -602,6 +602,11 @@ public:
   /// independent way (Note: the return type is an i64).
   ///
   static Constant* getSizeOf(const Type* Ty);
+
+  /// getOffsetOf constant expr - computes the offset of a field in a target
+  /// independent way (Note: the return type is an i64).
+  ///
+  static Constant* getOffsetOf(const StructType* Ty, unsigned FieldNo);
   
   static Constant* getNeg(Constant* C);
   static Constant* getFNeg(Constant* C);
diff --git a/lib/VMCore/Constants.cpp b/lib/VMCore/Constants.cpp
index 647bc1225a2..f820033932d 100644
--- a/lib/VMCore/Constants.cpp
+++ b/lib/VMCore/Constants.cpp
@@ -1391,6 +1391,18 @@ Constant* ConstantExpr::getAlignOf(const Type* Ty) {
                  Type::getInt32Ty(Ty->getContext()));
 }
 
+Constant* ConstantExpr::getOffsetOf(const StructType* STy, unsigned FieldNo) {
+  // offsetof is implemented as: (i64) gep (Ty*)null, 0, FieldNo
+  // Note that a non-inbounds gep is used, as null isn't within any object.
+  Constant *GEPIdx[] = {
+    ConstantInt::get(Type::getInt64Ty(STy->getContext()), 0),
+    ConstantInt::get(Type::getInt32Ty(STy->getContext()), FieldNo)
+  };
+  Constant *GEP = getGetElementPtr(
+                Constant::getNullValue(PointerType::getUnqual(STy)), GEPIdx, 2);
+  return getCast(Instruction::PtrToInt, GEP,
+                 Type::getInt64Ty(STy->getContext()));
+}
 
 Constant *ConstantExpr::getCompare(unsigned short pred, 
                             Constant *C1, Constant *C2) {