Replace usage of Q_DECL_ALIGN with C++11 alignas keyword

The macro is not documented, so can be considered private API.
Pre-C++11 compilers that don't support alignas will no longer be
supported with Qt 6.

The macro definition for the standard case of compilers supporting
the alignof keyword is left in place.

Task-number: QTBUG-76414
Change-Id: I7d722e4faf09ae998a972d3ed914de808ab316d7
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

3 files changed, 5 insertions, 13 deletions

diff --git a/src/corelib/global/qcompilerdetection.h b/src/corelib/global/qcompilerdetection.h
index b173dcf522..ad3d2be69b 100644
--- a/src/corelib/global/qcompilerdetection.h
+++ b/src/corelib/global/qcompilerdetection.h
@@ -91,7 +91,6 @@
 #  define Q_OUTOFLINE_TEMPLATE inline
 #  define Q_COMPILER_MANGLES_RETURN_TYPE
 #  define Q_FUNC_INFO __FUNCSIG__
-#  define Q_DECL_ALIGN(n) __declspec(align(n))
 #  define Q_ASSUME_IMPL(expr) __assume(expr)
 #  define Q_UNREACHABLE_IMPL() __assume(0)
 #  define Q_NORETURN __declspec(noreturn)
@@ -223,7 +222,6 @@
 #  define Q_FUNC_INFO       __PRETTY_FUNCTION__
 #  define Q_TYPEOF(expr)    __typeof__(expr)
 #  define Q_DECL_DEPRECATED __attribute__ ((__deprecated__))
-#  define Q_DECL_ALIGN(n)   __attribute__((__aligned__(n)))
 #  define Q_DECL_UNUSED     __attribute__((__unused__))
 #  define Q_LIKELY(expr)    __builtin_expect(!!(expr), true)
 #  define Q_UNLIKELY(expr)  __builtin_expect(!!(expr), false)
@@ -271,7 +269,6 @@
 #    error "Compiler not supported"
 #  elif __xlC__ >= 0x0600
 #    define Q_TYPEOF(expr)      __typeof__(expr)
-#    define Q_DECL_ALIGN(n)     __attribute__((__aligned__(n)))
 #    define Q_PACKED            __attribute__((__packed__))
 #  endif
 
@@ -447,7 +444,6 @@
    /* see http://developers.sun.com/sunstudio/support/Ccompare.html */
 #    if __SUNPRO_CC >= 0x590
 #      define Q_TYPEOF(expr)    __typeof__(expr)
-#      define Q_DECL_ALIGN(n)   __attribute__((__aligned__(n)))
 #    endif
 #    if __SUNPRO_CC >= 0x550
 #      define Q_DECL_EXPORT     __global
@@ -484,9 +480,6 @@
 #      define Q_DECL_EXPORT     __declspec(dllexport)
 #      define Q_DECL_IMPORT     __declspec(dllimport)
 #    endif
-#    if __HP_aCC-0 >= 061200
-#      define Q_DECL_ALIGN(n) __attribute__((aligned(n)))
-#    endif
 #    if __HP_aCC-0 >= 062000
 #      define Q_DECL_EXPORT     __attribute__((visibility("default")))
 #      define Q_DECL_HIDDEN     __attribute__((visibility("hidden")))
@@ -1127,9 +1120,8 @@
 # define Q_ALIGNOF(x)  alignof(x)
 #endif
 
-#if defined(Q_COMPILER_ALIGNAS)
-#  undef Q_DECL_ALIGN
-#  define Q_DECL_ALIGN(n)   alignas(n)
+#ifndef Q_DECL_ALIGN
+# define Q_DECL_ALIGN(n)   alignas(n)
 #endif
 
 #if QT_HAS_CPP_ATTRIBUTE(nodiscard) && !defined(Q_CC_CLANG)         // P0188R1
diff --git a/src/corelib/global/qendian.cpp b/src/corelib/global/qendian.cpp
index 7fd6e13d3b..c56a7ffbf7 100644
--- a/src/corelib/global/qendian.cpp
+++ b/src/corelib/global/qendian.cpp
@@ -765,7 +765,7 @@ QT_BEGIN_NAMESPACE
 
 #if defined(__SSSE3__)
 using ShuffleMask = uchar[16];
-Q_DECL_ALIGN(16) static const ShuffleMask shuffleMasks[3] = {
+alignas(16) static const ShuffleMask shuffleMasks[3] = {
     // 16-bit
     {1, 0, 3, 2,  5, 4, 7, 6,  9, 8, 11, 10,  13, 12, 15, 14},
     // 32-bit
diff --git a/src/corelib/global/qrandom.cpp b/src/corelib/global/qrandom.cpp
index 3cbd40b772..563e5eb7ed 100644
--- a/src/corelib/global/qrandom.cpp
+++ b/src/corelib/global/qrandom.cpp
@@ -355,7 +355,7 @@ struct QRandomGenerator::SystemAndGlobalGenerators
     //    the state in case another thread tries to lock the mutex. It's not
     //    a common scenario, but since sizeof(QRandomGenerator) >= 2560, the
     //    overhead is actually acceptable.
-    // 2) We use both Q_DECL_ALIGN and std::aligned_storage<..., 64> because
+    // 2) We use both alignas and std::aligned_storage<..., 64> because
     //    some implementations of std::aligned_storage can't align to more
     //    than a primitive type's alignment.
     // 3) We don't store the entire system QRandomGenerator, only the space
@@ -364,7 +364,7 @@ struct QRandomGenerator::SystemAndGlobalGenerators
     QBasicMutex globalPRNGMutex;
     struct ShortenedSystem { uint type; } system_;
     SystemGenerator sys;
-    Q_DECL_ALIGN(64) std::aligned_storage<sizeof(QRandomGenerator64), 64>::type global_;
+    alignas(64) std::aligned_storage<sizeof(QRandomGenerator64), 64>::type global_;
 
 #ifdef Q_COMPILER_CONSTEXPR
     constexpr SystemAndGlobalGenerators()