Make QTextDecoder use our qt_from_latin1 code

Disassembly shows the Intel compiler does expand to SIMD, but a much
worse code than ours. Clang 3.4 does generate a compact SIMD version,
probably of the same quality as our hand-written code. And GCC 4.7
through 4.9 don't generate SIMD at all.

So let's use the most efficient version.

Change-Id: I418e201a774ac0df1fb2b7a7d9589df7c9b655db
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

src/corelib/codecs/qtextcodec.cpp

@@ -993,6 +993,8 @@ QString QTextDecoder::toUnicode(const char *chars, int len)
     return c->toUnicode(chars, len, &state);
 }
 
+// in qstring.cpp:
+void qt_from_latin1(ushort *dst, const char *str, size_t size);
 
 /*! \overload
 
@@ -1005,12 +1007,10 @@ void QTextDecoder::toUnicode(QString *target, const char *chars, int len)
     case 106: // utf8
         static_cast<const QUtf8Codec*>(c)->convertToUnicode(target, chars, len, &state);
         break;
-    case 4: { // latin1
+    case 4: // latin1
         target->resize(len);
-        ushort *data = (ushort*)target->data();
+        qt_from_latin1((ushort*)target->data(), chars, len);
-        for (int i = len; i >=0; --i)
+        break;
-            data[i] = (uchar) chars[i];
-    } break;
     default:
         *target = c->toUnicode(chars, len, &state);
     }

src/corelib/tools/qstring.cpp

@@ -210,7 +210,7 @@ inline RetType UnrollTailLoop<0>::exec(int, RetType returnIfExited, Functor1, Fu
 #endif
 
 // conversion between Latin 1 and UTF-16
-static void qt_from_latin1(ushort *dst, const char *str, size_t size)
+void qt_from_latin1(ushort *dst, const char *str, size_t size)
 {
     /* SIMD:
      * Unpacking with SSE has been shown to improve performance on recent CPUs