[CodeGen] Correctly model std::byte's aliasing properties

std::byte, when defined as an enum, needs to be given special treatment
with regards to its aliasing properties. An array of std::byte is
allowed to be used as storage for other types.

This fixes PR33916.

Differential Revision: https://reviews.llvm.org/D35824

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@309058 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/AST/Type.h

@@ -1752,6 +1752,7 @@ class Type : public ExtQualsTypeCommonBase {
   bool isTemplateTypeParmType() const;          // C++ template type parameter
   bool isNullPtrType() const;                   // C++11 std::nullptr_t
   bool isAlignValT() const;                     // C++17 std::align_val_t
+  bool isStdByteType() const;                   // C++17 std::byte
   bool isAtomicType() const;                    // C11 _Atomic()
 
 #define IMAGE_TYPE(ImgType, Id, SingletonId, Access, Suffix) \

lib/AST/Type.cpp

@@ -2313,6 +2313,15 @@ bool Type::isAlignValT() const {
   return false;
 }
 
+bool Type::isStdByteType() const {
+  if (auto *ET = getAs<EnumType>()) {
+    auto *II = ET->getDecl()->getIdentifier();
+    if (II && II->isStr("byte") && ET->getDecl()->isInStdNamespace())
+      return true;
+  }
+  return false;
+}
+
 bool Type::isPromotableIntegerType() const {
   if (const BuiltinType *BT = getAs<BuiltinType>())
     switch (BT->getKind()) {

lib/CodeGen/CodeGenTBAA.cpp

@@ -139,6 +139,12 @@ CodeGenTBAA::getTBAAInfo(QualType QTy) {
     }
   }
 
+  // C++1z [basic.lval]p10: "If a program attempts to access the stored value of
+  // an object through a glvalue of other than one of the following types the
+  // behavior is undefined: [...] a char, unsigned char, or std::byte type."
+  if (Ty->isStdByteType())
+    return MetadataCache[Ty] = getChar();
+
   // Handle pointers.
   // TODO: Implement C++'s type "similarity" and consider dis-"similar"
   // pointers distinct.

test/CodeGenCXX/std-byte.cpp

@@ -0,0 +1,41 @@
+// RUN: %clang_cc1 -std=c++1z -Werror -triple i386-unknown-unknown -emit-llvm -O1 -disable-llvm-passes -o - %s | FileCheck %s
+
+// std::byte should be considered equivalent to char for aliasing.
+
+namespace std {
+enum byte : unsigned char {};
+}
+
+// CHECK-LABEL: define void @test0(
+extern "C" void test0(std::byte *sb, int *i) {
+  // CHECK: store i8 0, i8* %{{.*}} !tbaa [[TAG_CHAR:!.*]]
+  *sb = std::byte{0};
+
+  // CHECK: store i32 1, i32* %{{.*}} !tbaa [[TAG_INT:!.*]]
+  *i = 1;
+}
+
+enum byte : unsigned char {};
+namespace my {
+enum byte : unsigned char {};
+namespace std {
+enum byte : unsigned char {};
+} // namespace std
+} // namespace my
+
+// Make sure we don't get confused with other enums named 'byte'.
+
+// CHECK-LABEL: define void @test1(
+extern "C" void test1(::byte *b, ::my::byte *mb, ::my::std::byte *msb) {
+  *b = ::byte{0};
+  *mb = ::my::byte{0};
+  *msb = ::my::std::byte{0};
+  // CHECK-NOT: store i8 0, i8* %{{.*}} !tbaa [[TAG_CHAR]]
+}
+
+// CHECK:  !"any pointer", [[TYPE_CHAR:!.*]],
+// CHECK: [[TYPE_CHAR]] = !{!"omnipotent char", [[TAG_CXX_TBAA:!.*]],
+// CHECK: [[TAG_CXX_TBAA]] = !{!"Simple C++ TBAA"}
+// CHECK: [[TAG_CHAR]] = !{[[TYPE_CHAR:!.*]], [[TYPE_CHAR]], i64 0}
+// CHECK: [[TAG_INT]] = !{[[TYPE_INT:!.*]], [[TYPE_INT]], i64 0}
+// CHECK: [[TYPE_INT]] = !{!"int", [[TYPE_CHAR]]