[clang] [clang] Implement CWG1719 "Layout compatibility and cv-qualification revisited" (PR #82358)

Vlad Serebrennikov via cfe-commits cfe-commits at lists.llvm.org
Tue Feb 20 12:27:46 PST 2024 

https://github.com/Endilll updated https://github.com/llvm/llvm-project/pull/82358

>From 15d9428a47ea5d7ba3b4ffd2e766406b16d781e0 Mon Sep 17 00:00:00 2001
From: Vlad Serebrennikov <serebrennikov.vladislav at gmail.com>
Date: Tue, 20 Feb 2024 16:44:04 +0300
Subject: [PATCH 1/3] [clang] Implement CWG1719 "Layout compatibility and
 cv-qualification revisited"

This patch updates our internal notion of layout-compatible, which in turn fixes `__is_layout_compatible` intrinsic.
---
 clang/docs/ReleaseNotes.rst        |  9 ++++++---
 clang/lib/Sema/SemaChecking.cpp    | 13 +++++++------
 clang/test/CXX/drs/dr17xx.cpp      | 11 +++++------
 clang/test/SemaCXX/type-traits.cpp | 14 +++++++-------
 clang/www/cxx_dr_status.html       |  4 ++--
 5 files changed, 27 insertions(+), 24 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 8ea4a9a5a4256c..ffdc2a2629e6e9 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -98,9 +98,8 @@ C++20 Feature Support
 
 - Implemented the `__is_layout_compatible` intrinsic to support
   `P0466R5: Layout-compatibility and Pointer-interconvertibility Traits <https://wg21.link/P0466R5>`_.
-  Note: `CWG1719: Layout compatibility and cv-qualification revisited  <https://cplusplus.github.io/CWG/issues/1719.html>`_
-  and `CWG2759: [[no_unique_address] and common initial sequence <https://cplusplus.github.io/CWG/issues/2759.html>`_
-  are not yet implemented.
+  Note: `CWG2759: [[no_unique_address] and common initial sequence <https://cplusplus.github.io/CWG/issues/2759.html>`_
+  is not yet implemented.
 
 C++23 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
@@ -120,6 +119,10 @@ Resolutions to C++ Defect Reports
   in the template parameters, but is deduced from a previous argument.
   (`#78449: <https://github.com/llvm/llvm-project/issues/78449>`_).
 
+- ``const`` and ``volatile`` qualifiers are now ignored when evaluating
+  layout compatibility of two types.
+  (`CWG1719: Layout compatibility and cv-qualification revisited <https://cplusplus.github.io/CWG/issues/1719.html>`_).
+
 C Language Changes
 ------------------
 
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index d951c0fc2732d2..e8bfb215a5b4c5 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -19124,15 +19124,16 @@ static bool isLayoutCompatible(ASTContext &C, QualType T1, QualType T2) {
   if (T1.isNull() || T2.isNull())
     return false;
 
-  // C++11 [basic.types] p11:
-  // If two types T1 and T2 are the same type, then T1 and T2 are
-  // layout-compatible types.
-  if (C.hasSameType(T1, T2))
-    return true;
-
+  // C++20 [basic.types] p11:
+  // Two types cv1 T1 and cv2 T2 are layout-compatible types
+  // if T1 and T2 are the same type, layout-compatible enumerations (9.7.1),
+  // or layout-compatible standard-layout class types (11.4).
   T1 = T1.getCanonicalType().getUnqualifiedType();
   T2 = T2.getCanonicalType().getUnqualifiedType();
 
+  if (C.hasSameType(T1, T2))
+    return true;
+
   const Type::TypeClass TC1 = T1->getTypeClass();
   const Type::TypeClass TC2 = T2->getTypeClass();
 
diff --git a/clang/test/CXX/drs/dr17xx.cpp b/clang/test/CXX/drs/dr17xx.cpp
index e5cee19337ebd4..d933c244aada42 100644
--- a/clang/test/CXX/drs/dr17xx.cpp
+++ b/clang/test/CXX/drs/dr17xx.cpp
@@ -46,7 +46,7 @@ namespace dr1715 { // dr1715: 3.9
 #endif
 }
 
-namespace dr1719 { // dr1719: no
+namespace dr1719 { // dr1719: 19
 #if __cplusplus >= 201103L
 struct CStruct {
   int one;
@@ -66,11 +66,10 @@ struct CStructWithQualifiers {
 static_assert(__is_layout_compatible(CStruct, const CStruct2), "");
 static_assert(__is_layout_compatible(CStruct, volatile CStruct2), "");
 static_assert(__is_layout_compatible(const CStruct, volatile CStruct2), "");
-// FIXME: all of the following pairs of types are layout-compatible
-static_assert(!__is_layout_compatible(int, const int), "");
-static_assert(!__is_layout_compatible(int, volatile int), "");
-static_assert(!__is_layout_compatible(const int, volatile int), "");
-static_assert(!__is_layout_compatible(CStruct, CStructWithQualifiers), "");
+static_assert(__is_layout_compatible(int, const int), "");
+static_assert(__is_layout_compatible(int, volatile int), "");
+static_assert(__is_layout_compatible(const int, volatile int), "");
+static_assert(__is_layout_compatible(CStruct, CStructWithQualifiers), "");
 #endif
 } // namespace dr1719
 
diff --git a/clang/test/SemaCXX/type-traits.cpp b/clang/test/SemaCXX/type-traits.cpp
index 6ff04b6c8c7223..94889d654abeca 100644
--- a/clang/test/SemaCXX/type-traits.cpp
+++ b/clang/test/SemaCXX/type-traits.cpp
@@ -1711,13 +1711,13 @@ void is_layout_compatible(int n)
 {
   static_assert(__is_layout_compatible(void, void), "");
   static_assert(!__is_layout_compatible(void, int), "");
-  static_assert(!__is_layout_compatible(void, const void), ""); // FIXME: this is CWG1719
-  static_assert(!__is_layout_compatible(void, volatile void), ""); // FIXME: this is CWG1719
-  static_assert(!__is_layout_compatible(const int, volatile int), ""); // FIXME: this is CWG1719
+  static_assert(__is_layout_compatible(void, const void), "");
+  static_assert(__is_layout_compatible(void, volatile void), "");
+  static_assert(__is_layout_compatible(const int, volatile int), "");
   static_assert(__is_layout_compatible(int, int), "");
-  static_assert(!__is_layout_compatible(int, const int), ""); // FIXME: this is CWG1719
-  static_assert(!__is_layout_compatible(int, volatile int), ""); // FIXME: this is CWG1719
-  static_assert(!__is_layout_compatible(const int, volatile int), ""); // FIXME: this is CWG1719
+  static_assert(__is_layout_compatible(int, const int), "");
+  static_assert(__is_layout_compatible(int, volatile int), "");
+  static_assert(__is_layout_compatible(const int, volatile int), "");
   static_assert(!__is_layout_compatible(int, unsigned int), "");
   static_assert(!__is_layout_compatible(char, unsigned char), "");
   static_assert(!__is_layout_compatible(char, signed char), "");
@@ -1758,7 +1758,7 @@ void is_layout_compatible(int n)
   static_assert(!__is_layout_compatible(CppStructNonStandardByVirtBase, CppStructNonStandardByVirtBase2), "");
   static_assert(!__is_layout_compatible(CppStructNonStandardBySameBase, CppStructNonStandardBySameBase2), "");
   static_assert(!__is_layout_compatible(CppStructNonStandardBy2ndVirtBase, CppStructNonStandardBy2ndVirtBase2), "");
-  static_assert(!__is_layout_compatible(CStruct, CStructWithQualifiers), ""); // FIXME: this is CWG1719
+  static_assert(__is_layout_compatible(CStruct, CStructWithQualifiers), "");
   static_assert(__is_layout_compatible(CStruct, CStructNoUniqueAddress) == bool(__has_cpp_attribute(no_unique_address)), ""); // FIXME: this is CWG2759
   static_assert(__is_layout_compatible(CStructNoUniqueAddress, CStructNoUniqueAddress2) == bool(__has_cpp_attribute(no_unique_address)), ""); // FIXME: this is CWG2759
   static_assert(__is_layout_compatible(CStruct, CStructAlignment), "");
diff --git a/clang/www/cxx_dr_status.html b/clang/www/cxx_dr_status.html
index e9b18b1e283e66..38e2cb63142662 100755
--- a/clang/www/cxx_dr_status.html
+++ b/clang/www/cxx_dr_status.html
@@ -7812,7 +7812,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/1334.html">1334</a></td>
     <td>NAD</td>
     <td>Layout compatibility and cv-qualification</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="unreleased" align="center">Superseded by <a href="#1719">1719</a></td>
   </tr>
   <tr id="1335">
     <td><a href="https://cplusplus.github.io/CWG/issues/1335.html">1335</a></td>
@@ -10122,7 +10122,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/1719.html">1719</a></td>
     <td>CD4</td>
     <td>Layout compatibility and cv-qualification revisited</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="unreleased" align="center">Clang 19</td>
   </tr>
   <tr id="1720">
     <td><a href="https://cplusplus.github.io/CWG/issues/1720.html">1720</a></td>

>From f84eb1aacd061c6838e87726d352bdb8eeb683cd Mon Sep 17 00:00:00 2001
From: Vlad Serebrennikov <serebrennikov.vladislav at gmail.com>
Date: Tue, 20 Feb 2024 23:23:14 +0300
Subject: [PATCH 2/3] Address Aaron's feedback

---
 clang/docs/ReleaseNotes.rst        | 4 ++--
 clang/test/CXX/drs/dr17xx.cpp      | 1 +
 clang/test/SemaCXX/type-traits.cpp | 4 ++++
 3 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index ffdc2a2629e6e9..b3c412ab570148 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -119,8 +119,8 @@ Resolutions to C++ Defect Reports
   in the template parameters, but is deduced from a previous argument.
   (`#78449: <https://github.com/llvm/llvm-project/issues/78449>`_).
 
-- ``const`` and ``volatile`` qualifiers are now ignored when evaluating
-  layout compatibility of two types.
+- Type qualifications are now ignored when evaluating layout compatibility
+  of two types.
   (`CWG1719: Layout compatibility and cv-qualification revisited <https://cplusplus.github.io/CWG/issues/1719.html>`_).
 
 C Language Changes
diff --git a/clang/test/CXX/drs/dr17xx.cpp b/clang/test/CXX/drs/dr17xx.cpp
index d933c244aada42..d3cb5e58f06b32 100644
--- a/clang/test/CXX/drs/dr17xx.cpp
+++ b/clang/test/CXX/drs/dr17xx.cpp
@@ -70,6 +70,7 @@ static_assert(__is_layout_compatible(int, const int), "");
 static_assert(__is_layout_compatible(int, volatile int), "");
 static_assert(__is_layout_compatible(const int, volatile int), "");
 static_assert(__is_layout_compatible(CStruct, CStructWithQualifiers), "");
+static_assert(__is_layout_compatible(int[], const volatile int[]), "");
 #endif
 } // namespace dr1719
 
diff --git a/clang/test/SemaCXX/type-traits.cpp b/clang/test/SemaCXX/type-traits.cpp
index 94889d654abeca..d8d1425cd838a3 100644
--- a/clang/test/SemaCXX/type-traits.cpp
+++ b/clang/test/SemaCXX/type-traits.cpp
@@ -1718,6 +1718,10 @@ void is_layout_compatible(int n)
   static_assert(__is_layout_compatible(int, const int), "");
   static_assert(__is_layout_compatible(int, volatile int), "");
   static_assert(__is_layout_compatible(const int, volatile int), "");
+  static_assert(__is_layout_compatible(int *, int * __restrict), "");
+  // Note: atomic qualification matters for layout compatibility.
+  static_assert(!__is_layout_compatible(int, _Atomic int), "");
+  static_assert(__is_layout_compatible(_Atomic(int), _Atomic int), "");
   static_assert(!__is_layout_compatible(int, unsigned int), "");
   static_assert(!__is_layout_compatible(char, unsigned char), "");
   static_assert(!__is_layout_compatible(char, signed char), "");

>From 68e20c8a5004fcfdfe56bf3e637db656713b151f Mon Sep 17 00:00:00 2001
From: Vlad Serebrennikov <serebrennikov.vladislav at gmail.com>
Date: Tue, 20 Feb 2024 23:27:28 +0300
Subject: [PATCH 3/3] Add alignas test

---
 clang/test/SemaCXX/type-traits.cpp | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/clang/test/SemaCXX/type-traits.cpp b/clang/test/SemaCXX/type-traits.cpp
index d8d1425cd838a3..2c35d5ee19a4c6 100644
--- a/clang/test/SemaCXX/type-traits.cpp
+++ b/clang/test/SemaCXX/type-traits.cpp
@@ -1609,7 +1609,12 @@ struct CStructNoUniqueAddress2 {
   [[no_unique_address]] int two;
 };
 
-struct CStructAlignment {
+struct alignas(64) CStructAlignment {
+  int one;
+  int two;
+};
+
+struct CStructAlignedMembers {
   int one;
   alignas(16) int two;
 };
@@ -1766,6 +1771,7 @@ void is_layout_compatible(int n)
   static_assert(__is_layout_compatible(CStruct, CStructNoUniqueAddress) == bool(__has_cpp_attribute(no_unique_address)), ""); // FIXME: this is CWG2759
   static_assert(__is_layout_compatible(CStructNoUniqueAddress, CStructNoUniqueAddress2) == bool(__has_cpp_attribute(no_unique_address)), ""); // FIXME: this is CWG2759
   static_assert(__is_layout_compatible(CStruct, CStructAlignment), "");
+  static_assert(__is_layout_compatible(CStruct, CStructAlignedMembers), ""); // FIXME: alignment of members impact common initial sequence
   static_assert(__is_layout_compatible(CStructWithBitfelds, CStructWithBitfelds), "");
   static_assert(__is_layout_compatible(CStructWithBitfelds, CStructWithBitfelds2), "");
   static_assert(!__is_layout_compatible(CStructWithBitfelds, CStructWithBitfelds3), "");

More information about the cfe-commits mailing list