qalgorithms.h: fix mistake that MSVC has constexpr bitops in C++17 mode

<bit> exists in C++20 and is properly both constexpr and optimized. But
in C++17 mode, we don't have constexpr bitops and instead elect to have
performance at runtime instead. But somewhere along the line, either
when they were added, when C++20 <bit> support was, or in any of the
bugfixes for other compilers, the nesting of #ifdef got messed up and we
declared that we had constexpr builtins for MSVC in C++17 too.

The macro QT_HAS_CONSTEXPR_BUILTINS isn't supposed to be used by anyone
else... but we ended up not being able to use it ourselves either. So
I'm renaming it to a more precise label.

Change-Id: I9671dee8ceb64aa9b9cafffd1741b9b4060c9753
Reviewed-by: Mårten Nordheim <[email protected]>
Reviewed-by: Edward Welbourne <[email protected]>
(cherry picked from commit e69d80e14d24001f93442ab59b8babb7e9df0092)
Reviewed-by: Qt Cherry-pick Bot <[email protected]>

2 files changed, 6 insertions, 17 deletions

diff --git a/src/corelib/tools/qalgorithms.h b/src/corelib/tools/qalgorithms.h
index 94aa2da012e..8889effecee 100644
--- a/src/corelib/tools/qalgorithms.h
+++ b/src/corelib/tools/qalgorithms.h
@@ -41,19 +41,11 @@ inline void qDeleteAll(const Container &c)
 */
 namespace QAlgorithmsPrivate {
 
-#ifdef Q_CC_CLANG
-// Clang had a bug where __builtin_ctz/clz/popcount were not marked as constexpr.
-#  if (defined __apple_build_version__ &&  __clang_major__ >= 7) || (Q_CC_CLANG >= 307)
-#    define QT_HAS_CONSTEXPR_BUILTINS
-#  endif
-#elif defined(Q_CC_MSVC) && !defined(Q_PROCESSOR_ARM)
-#  define QT_HAS_CONSTEXPR_BUILTINS
+#if defined(__cpp_lib_bitops) && __cpp_lib_bitops >= 201907L
+// We use C++20 <bit> operations instead which ensures constexpr bit ops
+#  define QT_HAS_CONSTEXPR_BITOPS
 #elif defined(Q_CC_GNU)
-#  define QT_HAS_CONSTEXPR_BUILTINS
-#endif
-
-#if defined QT_HAS_CONSTEXPR_BUILTINS
-#if defined(Q_CC_GNU) || defined(Q_CC_CLANG)
+#  define QT_HAS_CONSTEXPR_BITOPS
 #  define QT_HAS_BUILTIN_CTZS
 constexpr Q_ALWAYS_INLINE uint qt_builtin_ctzs(quint16 v) noexcept
 {
@@ -169,9 +161,7 @@ Q_ALWAYS_INLINE uint qt_builtin_clzs(quint16 v) noexcept
 //    architecture), but unlike the other compilers, MSVC has no option
 //    to generate code for those processors.
 // So it's an acceptable compromise.
-#if defined(__cpp_lib_bitops) && __cpp_lib_bitops >= 201907L
-// We use C++20 <bit> operations instead which ensures constexpr popcount
-#elif defined(__AVX__) || defined(__SSE4_2__) || defined(__POPCNT__)
+#if defined(__AVX__) || defined(__SSE4_2__) || defined(__POPCNT__)
 #define QT_POPCOUNT_CONSTEXPR
 #define QT_POPCOUNT_RELAXED_CONSTEXPR
 #define QALGORITHMS_USE_BUILTIN_POPCOUNT
@@ -200,7 +190,6 @@ Q_ALWAYS_INLINE uint qt_builtin_popcountll(quint64 v) noexcept
 #endif // __AVX__ || __SSE4_2__ || __POPCNT__
 
 #endif // MSVC
-#endif // QT_HAS_CONSTEXPR_BUILTINS
 
 #ifndef QT_POPCOUNT_CONSTEXPR
 #define QT_POPCOUNT_CONSTEXPR constexpr
diff --git a/tests/auto/widgets/kernel/qsizepolicy/tst_qsizepolicy.cpp b/tests/auto/widgets/kernel/qsizepolicy/tst_qsizepolicy.cpp
index 5b64f122e20..f59471a6e67 100644
--- a/tests/auto/widgets/kernel/qsizepolicy/tst_qsizepolicy.cpp
+++ b/tests/auto/widgets/kernel/qsizepolicy/tst_qsizepolicy.cpp
@@ -95,7 +95,7 @@ void tst_QSizePolicy::constExpr()
     {
       // QTBUG-69983: For ControlType != QSizePolicy::DefaultType, qCountTrailingZeroBits()
       // is used, which MSVC 15.8.1 does not consider constexpr due to built-ins
-#  if defined(QT_HAS_CONSTEXPR_BUILTINS) && (!defined(Q_CC_MSVC) || _MSC_VER < 1915)
+#  if defined(QT_HAS_CONSTEXPR_BITOPS)
       constexpr auto sp = QSizePolicy(QSizePolicy::Minimum, QSizePolicy::Fixed, QSizePolicy::CheckBox);
 #  else
       constexpr auto sp = QSizePolicy(QSizePolicy::Minimum, QSizePolicy::Expanding, QSizePolicy::DefaultType);