QStringView: De-inline the length calculation so we can use SSE2

Performance is more important in this case than the theoretical benefit
of constexpr. This commit implements the SSE2 search for 16-bit null and
it might be possible to implement the equivalent for AArch64
(investigation required). It also adds a fallback to wcslen() for
systems where wchar_t is short (non-x86 Windows or 32-bit x86 build with
-no-sse2).

We can re-add the constexpr loop once the C++ language has a way of
overloading constexpr and non-constexpr. GCC has a non-standard way to
do that with __builtin_constant_p, which is also implemented in this
commit, but note that the inline function is still not constexpr.

Change-Id: I6e9274c1e7444ad48c81fffd14dcaacafda5ebdc
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 12 insertions, 10 deletions

diff --git a/src/corelib/tools/qstringview.h b/src/corelib/tools/qstringview.h
index 24be441b00..764da71d0b 100644
--- a/src/corelib/tools/qstringview.h
+++ b/src/corelib/tools/qstringview.h
@@ -143,20 +143,22 @@ private:
     {
         return qssize_t(N - 1);
     }
+
     template <typename Char>
-    static Q_DECL_RELAXED_CONSTEXPR qssize_t lengthHelperPointer(const Char *str) Q_DECL_NOTHROW
+    static qssize_t lengthHelperPointer(const Char *str) Q_DECL_NOTHROW
     {
-        qssize_t result = 0;
-        while (*str++)
-            ++result;
-        return result;
+#if defined(Q_CC_GNU) && !defined(Q_CC_CLANG) && !defined(Q_CC_INTEL)
+        if (__builtin_constant_p(*str)) {
+            qssize_t result = 0;
+            while (*str++)
+                ++result;
+        }
+#endif
+        return qustrlen(reinterpret_cast<const ushort *>(str));
     }
-    static Q_DECL_RELAXED_CONSTEXPR qssize_t lengthHelperPointer(const QChar *str) Q_DECL_NOTHROW
+    static qssize_t lengthHelperPointer(const QChar *str) Q_DECL_NOTHROW
     {
-        qssize_t result = 0;
-        while (!str++->isNull())
-            ++result;
-        return result;
+        return qustrlen(reinterpret_cast<const ushort *>(str));
     }
 
     template <typename Char>