From 6c17805ddc0f7c94298c9e253d298f87bad6490a Mon Sep 17 00:00:00 2001
From: Marco Bubke <marco.bubke@qt.io>
Date: Sat, 9 Sep 2023 11:53:32 +0200
Subject: [PATCH] Utils: Improve union layout

It is undefined behavior to access a uninitialized part of a union but
in this case it is not because both member have the same start member.
ControlBlock is in both at the address with the same layout and so it is
allowed by the C++ standard.

Change-Id: I3a165f67030d6726d2cb0cfbcbfb2622bcf7dcc4
Reviewed-by: Qt CI Patch Build Bot <ci_patchbuild_bot@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>
---
 src/libs/utils/smallstring.h       |  3 +++
 src/libs/utils/smallstringlayout.h | 34 ++++++++++++++++++------------
 2 files changed, 23 insertions(+), 14 deletions(-)

diff --git a/src/libs/utils/smallstring.h b/src/libs/utils/smallstring.h
index 2ef565bde2e..16e872d5afa 100644
--- a/src/libs/utils/smallstring.h
+++ b/src/libs/utils/smallstring.h
@@ -425,7 +425,10 @@ public:
         size_type newSize = oldSize + string.size();
 
         reserve(optimalCapacity(newSize));
+        QT_WARNING_PUSH
+        QT_WARNING_DISABLE_CLANG("-Wunsafe-buffer-usage")
         std::char_traits<char>::copy(data() + oldSize, string.data(), string.size());
+        QT_WARNING_POP
         setSize(newSize);
     }
 
diff --git a/src/libs/utils/smallstringlayout.h b/src/libs/utils/smallstringlayout.h
index a7f4fd3a188..03e764dfee3 100644
--- a/src/libs/utils/smallstringlayout.h
+++ b/src/libs/utils/smallstringlayout.h
@@ -11,6 +11,10 @@
 #include <memory>
 #include <type_traits>
 
+QT_WARNING_PUSH
+QT_WARNING_DISABLE_CLANG("-Wgnu-anonymous-struct")
+QT_WARNING_DISABLE_CLANG("-Wnested-anon-types")
+
 namespace Utils {
 
 namespace Internal {
@@ -82,7 +86,7 @@ struct alignas(16) StringDataLayout
     constexpr StringDataLayout() noexcept { reset(); }
 
     constexpr StringDataLayout(const char *string, size_type size) noexcept
-        : control{0, true, true}
+        : controlReference{0, true, true}
         , reference{{string}, size, 0}
     {}
 
@@ -130,18 +134,18 @@ struct alignas(16) StringDataLayout
 #endif
     }
 
-#pragma pack(push)
-#pragma pack(1)
-    ControlBlock<MaximumShortStringDataAreaSize> control;
     union {
-        char shortString[MaximumShortStringDataAreaSize];
         struct
         {
-            char dummy[sizeof(void *) - sizeof(ControlBlock<MaximumShortStringDataAreaSize>)];
+            ControlBlock<MaximumShortStringDataAreaSize> control;
+            char shortString[MaximumShortStringDataAreaSize];
+        };
+        struct
+        {
+            ControlBlock<MaximumShortStringDataAreaSize> controlReference;
             ReferenceLayout reference;
         };
     };
-#pragma pack(pop)
 };
 
 template<uint MaximumShortStringDataAreaSize>
@@ -158,7 +162,7 @@ struct alignas(16) StringDataLayout<MaximumShortStringDataAreaSize,
     constexpr StringDataLayout() noexcept { reset(); }
 
     constexpr StringDataLayout(const char *string, size_type size) noexcept
-        : control{0, true, true}
+        : controlReference{0, true, true}
         , reference{{string}, size, 0}
     {}
 
@@ -223,19 +227,21 @@ struct alignas(16) StringDataLayout<MaximumShortStringDataAreaSize,
 #endif
     }
 
-#pragma pack(push)
-#pragma pack(1)
-    ControlBlock<MaximumShortStringDataAreaSize> control;
     union {
-        char shortString[MaximumShortStringDataAreaSize];
         struct
         {
-            char dummy[sizeof(void *) - sizeof(ControlBlock<MaximumShortStringDataAreaSize>)];
+            ControlBlock<MaximumShortStringDataAreaSize> control;
+            char shortString[MaximumShortStringDataAreaSize];
+        };
+        struct
+        {
+            ControlBlock<MaximumShortStringDataAreaSize> controlReference;
             ReferenceLayout reference;
         };
     };
-#pragma pack(pop)
 };
 
 } // namespace Internal
 }  // namespace Utils
+
+QT_WARNING_POP