Simplify QLocale::formattedDataSize()

- Introduce symbolic constants for the magic numbers 3 (= log10(base))
  and 10 (= log2(base)).

- Add and use QtPrivate::log2i() instead of manual bit fiddling. This
  makes the two cases nicely symmetric now.

Amends 9d23aebb271ea534a66cb3aceb2e63d9a1c870d6.

Pick-to: 6.9 6.8 6.5
Coverity-Id: 474294
Change-Id: I657f34878bc3a9b67b03a4d014b91e6d4de31e13
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

src/corelib/text/qlocale.cpp

@@ -5052,9 +5052,11 @@ QString QLocale::formattedDataSize(qint64 bytes, int precision, DataSizeFormats
     if (!bytes) {
         power = 0;
     } else if (format & DataSizeBase1000) {
-        power = int(std::log10(QtPrivate::qUnsignedAbs(bytes)) / 3);
-    } else { // Compute log2(bytes) / 10:
-        power = int((63 - qCountLeadingZeroBits(QtPrivate::qUnsignedAbs(bytes))) / 10);
+        constexpr auto log10_1000 = 3; // std::log10(1000U)
+        power = int(std::log10(QtPrivate::qUnsignedAbs(bytes))) / log10_1000;
+    } else {
+        constexpr auto log2_1024 = 10; // QtPrivate::log2i(1024U);
+        power = QtPrivate::log2i(QtPrivate::qUnsignedAbs(bytes)) / log2_1024;
         base = 1024;
     }
     // Only go to doubles if we'll be using a quantifier:

src/corelib/tools/qalgorithms.h

@@ -14,6 +14,7 @@
 #if __has_include(<bit>) && __cplusplus > 201703L
 #include <bit>
 #endif
+#include <type_traits>
 
 #ifdef Q_CC_MSVC
 #include <intrin.h>
@@ -455,6 +456,21 @@ Result qJoin(InputIterator first, InputIterator last, Result init, const Separat
     return init;
 }
 
+namespace QtPrivate {
+
+template <typename T>
+constexpr
+std::enable_if_t<std::conjunction_v<std::is_integral<T>, std::is_unsigned<T>>, int>
+log2i(T x)
+{
+    // Integral -> int version of std::log2():
+    Q_ASSERT(x > 0); // Q_PRE
+    // C++20: return std::bit_width(x) - 1
+    return int(sizeof(T) * 8 - 1 - qCountLeadingZeroBits(x));
+}
+
+} // namespace QtPrivate
+
 QT_END_NAMESPACE
 
 #endif // QALGORITHMS_H