Implement CodeGen support for the may_alias attribute.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@121734 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/CodeGenTBAA.cpp

@@ -77,8 +77,29 @@ llvm::MDNode *CodeGenTBAA::getTBAAInfoForNamedType(llvm::StringRef NameStr,
   return llvm::MDNode::get(VMContext, Ops, llvm::array_lengthof(Ops) - !Flags);
 }
 
+static bool TypeHasMayAlias(QualType QTy) {
+  // Tagged types have declarations, and therefore may have attributes.
+  if (const TagType *TTy = dyn_cast<TagType>(QTy))
+    return TTy->getDecl()->hasAttr<MayAliasAttr>();
+
+  // Typedef types have declarations, and therefore may have attributes.
+  if (const TypedefType *TTy = dyn_cast<TypedefType>(QTy)) {
+    if (TTy->getDecl()->hasAttr<MayAliasAttr>())
+      return true;
+    // Also, their underlying types may have relevant attributes.
+    return TypeHasMayAlias(TTy->desugar());
+  }
+
+  return false;
+}
+
 llvm::MDNode *
 CodeGenTBAA::getTBAAInfo(QualType QTy) {
+  // If the type has the may_alias attribute (even on a typedef), it is
+  // effectively in the general char alias class.
+  if (TypeHasMayAlias(QTy))
+    return getChar();
+
   Type *Ty = Context.getCanonicalType(QTy).getTypePtr();
 
   if (llvm::MDNode *N = MetadataCache[Ty])

test/CodeGen/may-alias.c

@@ -0,0 +1,21 @@
+// RUN: %clang_cc1 -Werror -triple i386-unknown-unknown -emit-llvm -O1 -disable-llvm-optzns -o %t %s
+// RUN: FileCheck < %t %s
+
+// Types with the may_alias attribute should be considered equivalent
+// to char for aliasing.
+
+typedef int __attribute__((may_alias)) aliasing_int;
+
+void test0(aliasing_int *ai, int *i)
+{
+  *ai = 0;
+  *i = 1;
+}
+
+// CHECK: store i32 0, i32* %tmp, !tbaa !1
+// CHECK: store i32 1, i32* %tmp1, !tbaa !3
+
+// CHECK: !0 = metadata !{metadata !"any pointer", metadata !1}
+// CHECK: !1 = metadata !{metadata !"omnipotent char", metadata !2}
+// CHECK: !2 = metadata !{metadata !"Simple C/C++ TBAA", null}
+// CHECK: !3 = metadata !{metadata !"int", metadata !1}