[PATCH] D35824: [Sema] Implicitly apply the may_alias attribute to std::byte

[David Majnemer via Phabricator via cfe-commits]

majnemer created this revision.

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.


https://reviews.llvm.org/D35824

Files:
  lib/Sema/SemaDecl.cpp
  test/CodeGenCXX/std-byte.cpp


Index: test/CodeGenCXX/std-byte.cpp
===================================================================
--- /dev/null
+++ test/CodeGenCXX/std-byte.cpp
@@ -0,0 +1,25 @@
+// 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 {};
+}
+
+void test0(std::byte *sb, int *i) {
+  // CHECK: store i8 0, i8* %{{.*}}, !tbaa [[TAG_STD_BYTE:!.*]]
+  // PATH: store i8 0, i8* %{{.*}}, !tbaa [[TAG_STD_BYTE:!.*]]
+  *sb = std::byte{0};
+
+  // CHECK: store i32 1, i32* %{{.*}}, !tbaa [[TAG_INT:!.*]]
+  // PATH: store i32 1, i32* %{{.*}}, !tbaa [[TAG_INT:!.*]]
+  *i = 1;
+}
+
+// CHECK:  !"any pointer", [[TYPE_CHAR:!.*]],
+// CHECK: [[TYPE_CHAR]] = !{!"omnipotent char", [[TAG_CXX_TBAA:!.*]],
+// CHECK: [[TAG_CXX_TBAA]] = !{!"Simple C++ TBAA"}
+// CHECK: [[TAG_STD_BYTE]] = !{[[TYPE_STD_BYTE:!.*]], [[TYPE_STD_BYTE]], i64 0}
+// CHECK: [[TYPE_STD_BYTE]] = !{!"_ZTSSt4byte", [[TYPE_CHAR]], i64 0}
+// CHECK: [[TAG_INT]] = !{[[TYPE_INT:!.*]], [[TYPE_INT]], i64 0}
+// CHECK: [[TYPE_INT]] = !{!"int", [[TYPE_CHAR]]
Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp
+++ lib/Sema/SemaDecl.cpp
@@ -13227,6 +13227,7 @@
   DeclContext *DC = CurContext;
   bool isStdBadAlloc = false;
   bool isStdAlignValT = false;
+  bool isStdByte = false;
 
   RedeclarationKind Redecl = ForRedeclaration;
   if (TUK == TUK_Friend || TUK == TUK_Reference)
@@ -13444,6 +13445,8 @@
       isStdAlignValT = true;
       if (Previous.empty() && StdAlignValT)
         Previous.addDecl(getStdAlignValT());
+    } else if (Name->isStr("byte")) {
+      isStdByte = true;
     }
   }
 
@@ -13845,6 +13848,13 @@
     if (isStdAlignValT && (!StdAlignValT || getStdAlignValT()->isImplicit()))
       StdAlignValT = cast<EnumDecl>(New);
 
+    // C++1z [intro.object]p3: If a complete object is created (5.3.4) in
+    // storage associated with another object e of type "array of N unsigned
+    // char" or "array of N std::byte", that array provides storage for the
+    // created object [...].
+    if (isStdByte)
+      New->addAttr(MayAliasAttr::CreateImplicit(Context, New->getLocation()));
+
     // If this is an undefined enum, warn.
     if (TUK != TUK_Definition && !Invalid) {
       TagDecl *Def;


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