QOffsetStringArray: simplify makeOffsetStringArray()

Instead of building two arrays, a short one for size_t, and then a
full-length one for the final narrowed MinifiedOffsetType, construct
the final array directly.

This should also speed up compiling qOffsetStringArray() expressions,
Clang's -ftime-trace, for ninja tst_qoffsetstringarray, with a dirty
qoffsetstringarray_p.h (which isn't included in the PCH, so only
recompiles TUs that actually include it) shows a slight improvement:

 -   37 ms: qOffsetStringArray<$> (7 times, avg 5 ms)
 +   35 ms: qOffsetStringArray<$> (7 times, avg 5 ms)

I had to disable one static_assert because it looks to be running over
some GCC limitation and concluding the array is not constexpr (even
though it is).

qhttpheaders.cpp:486:29: error: non-constant condition for static assertion
qhttpheaders.cpp:486:29: error: the value of ‘headerNames’ is not usable in a constant expression

Change-Id: I0d6109645075e5b533391ea04ea845538ad496fe
Reviewed-by: Ahmad Samir <[email protected]>

2 files changed, 9 insertions, 10 deletions

diff --git a/src/corelib/tools/qoffsetstringarray_p.h b/src/corelib/tools/qoffsetstringarray_p.h
index e022024a53a..63e901caf1a 100644
--- a/src/corelib/tools/qoffsetstringarray_p.h
+++ b/src/corelib/tools/qoffsetstringarray_p.h
@@ -141,21 +141,18 @@ template <typename Char, size_t N> struct StaticString
 template <typename Char, typename StringExtractor, typename... T>
 constexpr auto makeOffsetStringArray(StringExtractor extractString, const T &... entries)
 {
-    constexpr size_t Count = sizeof...(T);
     constexpr size_t StringLength = (sizeof(extractString(T{})) + ...) / sizeof(Char);
-    using MinifiedOffsetType = decltype(QtPrivate::minifyValue<StringLength>());
-
-    size_t offset = 0;
-    std::array fullOffsetList = { offset += (sizeof(extractString(T{})) / sizeof(Char))... };
+    using OffsetType = decltype(QtPrivate::minifyValue<StringLength>());
 
     // prepend the first offset (zero) pointing to the *start* of the first element
-    std::array<MinifiedOffsetType, Count + 1> minifiedOffsetList = {};
-    q20::transform(fullOffsetList.begin(), fullOffsetList.end(),
-                   minifiedOffsetList.begin() + 1,
-                   [] (auto e) { return MinifiedOffsetType(e); });
+    size_t offset = 0;
+    std::array offsetList = {
+        OffsetType(0),
+        OffsetType(offset += sizeof(extractString(T{})) / sizeof(Char))...
+    };
 
     std::array staticString = QtPrivate::makeStaticString<Char, StringLength>(extractString, entries...);
-    return QOffsetStringArray(staticString, minifiedOffsetList);
+    return QOffsetStringArray(staticString, offsetList);
 }
 } // namespace QtPrivate
 
diff --git a/src/network/access/qhttpheaders.cpp b/src/network/access/qhttpheaders.cpp
index e988f098b55..1528b02e259 100644
--- a/src/network/access/qhttpheaders.cpp
+++ b/src/network/access/qhttpheaders.cpp
@@ -483,9 +483,11 @@ static constexpr quint8 orderedHeaderNameIndexes[] = {
     171, // x-frame-options
 };
 static_assert(std::size(orderedHeaderNameIndexes) == size_t(headerNames.count()));
+#if !defined(Q_CC_GNU_ONLY) || Q_CC_GNU_ONLY >= 1000
 static_assert(q20::is_sorted(std::begin(orderedHeaderNameIndexes),
                              std::end(orderedHeaderNameIndexes),
                              ByIndirectHeaderName{}));
+#endif
 
 /*!
     \enum QHttpHeaders::WellKnownHeader