QString::fromLatin1: improve the sub-16-character case

For both the [4, 7] and [8,15] length cases, we can perform the same
technique: perform two overlapped loads, zero-extend, then perform two
overlapped stores. The 8-character case could be done in a single
load/store pair, but is not worth the extra conditionals. And it should
have the exact same performance numbers whether we use non-overlapping
4-character operations or completely-overlapping 8-character ones (I
*think* the full overlap is actually better).

The 4-character operation is new in this commit. That reduces the
non-vectorized, unrolled to at most 3 characters.

Change-Id: Ib42b3adc93bf4d43bd55fffd16c257ada774236a
Reviewed-by: Lars Knoll <lars@knoll.priv.no>

1 files changed, 38 insertions, 11 deletions

diff --git a/tests/auto/corelib/text/qstring/tst_qstring.cpp b/tests/auto/corelib/text/qstring/tst_qstring.cpp
index 65c9e086cc..0d0c38d51f 100644
--- a/tests/auto/corelib/text/qstring/tst_qstring.cpp
+++ b/tests/auto/corelib/text/qstring/tst_qstring.cpp
@@ -5295,19 +5295,46 @@ void tst_QString::fromLatin1Roundtrip()
     QCOMPARE(latin1.isEmpty(), unicode.isEmpty());
     QCOMPARE(latin1.size(), unicode.size());
 
-    if (!latin1.isEmpty())
-        while (latin1.size() < 128) {
-            latin1 += latin1;
-            unicode += unicode;
-        }
+    auto roundtripTest = [&]() {
+        // fromLatin1
+        QString fromLatin1 = QString::fromLatin1(latin1, latin1.length());
+        QCOMPARE(fromLatin1.length(), unicode.length());
+        QCOMPARE(fromLatin1, unicode);
+
+        // and back:
+        QByteArray toLatin1 = unicode.toLatin1();
+        QCOMPARE(toLatin1.length(), latin1.length());
+        QCOMPARE(toLatin1, latin1);
+    };
 
-    // fromLatin1
-    QCOMPARE(QString::fromLatin1(latin1, latin1.size()).size(), unicode.size());
-    QCOMPARE(QString::fromLatin1(latin1, latin1.size()), unicode);
+    roundtripTest();
 
-    // and back:
-    QCOMPARE(unicode.toLatin1().size(), latin1.size());
-    QCOMPARE(unicode.toLatin1(), latin1);
+    if (latin1.isEmpty())
+        return;
+
+    if (QTest::currentTestFailed()) QFAIL("failed");
+    while (latin1.length() < 16) {
+        latin1 += latin1;
+        unicode += unicode;
+    }
+    roundtripTest();
+
+    // double again (length will be > 32)
+    if (QTest::currentTestFailed()) QFAIL("failed");
+    latin1 += latin1;
+    unicode += unicode;
+    roundtripTest();
+
+    // double again (length will be > 64)
+    if (QTest::currentTestFailed()) QFAIL("failed");
+    latin1 += latin1;
+    unicode += unicode;
+    roundtripTest();
+
+    if (QTest::currentTestFailed()) QFAIL("failed");
+    latin1 += latin1;
+    unicode += unicode;
+    roundtripTest();
 }
 
 void tst_QString::toLatin1Roundtrip_data()