MSVC: improve QASV(const char(&str)[N]) compilation time

The lengthHelperContainer() implementation for sizeof(Char) == 1 case
is using qstrnlen() for the non-constexpr case.
qstrnlen() is an inline function which is effectively a nullptr check
and a memchr() call.

For some reason, on MSVC this combination resulted in very slow
compilation for the user projects, where each call to
QObject::setObjectName() was hitting this codepath.

Fix it by replacing the qstrnlen() call with strnlen_s() for MSVC.
It seems that for now all versions of MSVC are affected. However,
introduce a new Q_COMPILER_SLOW_QSTRNLEN_COMPILATION definition,
which will allow us to check for the compiler version later on.
For now this definition is set for all MSVC versions unconditionally.

Fixes: QTBUG-124376
Pick-to: 6.7 6.7.1
Change-Id: Id769bef1e950ffa756acf7af39d362fd8b112019
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

2 files changed, 12 insertions, 1 deletions

diff --git a/src/corelib/global/qcompilerdetection.h b/src/corelib/global/qcompilerdetection.h
index 9c46ea0efb..0230b5a784 100644
--- a/src/corelib/global/qcompilerdetection.h
+++ b/src/corelib/global/qcompilerdetection.h
@@ -857,6 +857,8 @@
 #    if _MSC_VER < 1936
 #      define Q_COMPILER_LACKS_THREE_WAY_COMPARE_SYMMETRY
 #    endif
+// QTBUG-124376: MSVC is slow at compiling qstrnlen()
+#    define Q_COMPILER_SLOW_QSTRNLEN_COMPILATION
 #  endif /* __cplusplus */
 #endif // defined(Q_CC_MSVC) && !defined(Q_CC_CLANG)
 
diff --git a/src/corelib/text/qstringalgorithms.h b/src/corelib/text/qstringalgorithms.h
index 538ae892b0..71a1dbd526 100644
--- a/src/corelib/text/qstringalgorithms.h
+++ b/src/corelib/text/qstringalgorithms.h
@@ -178,12 +178,21 @@ lengthHelperContainer(const Char (&str)[N])
     return lengthHelperContainerLoop(str);
 }
 
+inline qsizetype qstrnlen_helper(const char *str, size_t maxlen)
+{
+#if !defined(Q_COMPILER_SLOW_QSTRNLEN_COMPILATION)
+    return qstrnlen(str, maxlen);
+#else
+    return strnlen_s(str, maxlen);
+#endif
+}
+
 template <typename Char, size_t N> [[nodiscard]] constexpr inline
 std::enable_if_t<sizeof(Char) == 1, qsizetype> lengthHelperContainer(const Char (&str)[N])
 {
 #ifdef QT_SUPPORTS_IS_CONSTANT_EVALUATED
     if (!q20::is_constant_evaluated())
-        return qstrnlen(reinterpret_cast<const char *>(str), N);
+        return qstrnlen_helper(reinterpret_cast<const char *>(str), N);
 #endif
 
     return lengthHelperContainerLoop(str);