QString: remove the old unaligned 4-byte compares in ucstrncmp()

This is very old code, predating the public Qt history (Qt 4.5). It
predates all other SIMD code in qstring.cpp, actually. Now that we do
have implementations for MIPS DSP, ARM Neon and x86 SSE2, this content
has very little value. It would be relevant for other architectures Qt
still supports (POWER and RISC-V come to mind), but I guess the
compiler's auto-vectorizer functionality can do a better job than this
content.

Change-Id: I0e5f6bec596a4a78bd3bfffd16c90733fb0d8f22
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

1 files changed, 9 insertions, 60 deletions

diff --git a/src/corelib/text/qstring.cpp b/src/corelib/text/qstring.cpp
index 57bed1240f..0b5aa5a885 100644
--- a/src/corelib/text/qstring.cpp
+++ b/src/corelib/text/qstring.cpp
@@ -1096,25 +1096,15 @@ extern "C" int qt_ucstrncmp_mips_dsp_asm(const char16_t *a,
 // Unicode case-sensitive compare two same-sized strings
 static int ucstrncmp(const QChar *a, const QChar *b, size_t l)
 {
-#ifdef __OPTIMIZE_SIZE__
-    const QChar *end = a + l;
-    while (a < end) {
-        if (int diff = (int)a->unicode() - (int)b->unicode())
-            return diff;
-        ++a;
-        ++b;
-    }
-    return 0;
-#else
-#if defined(__mips_dsp)
+#ifndef __OPTIMIZE_SIZE__
+#  if defined(__mips_dsp)
     static_assert(sizeof(uint) == sizeof(size_t));
     if (l >= 8) {
         return qt_ucstrncmp_mips_dsp_asm(reinterpret_cast<const char16_t*>(a),
                                          reinterpret_cast<const char16_t*>(b),
                                          l);
     }
-#endif // __mips_dsp
-#ifdef __SSE2__
+#  elif defined(__SSE2__)
     const QChar *end = a + l;
     qptrdiff offset = 0;
 
@@ -1182,8 +1172,7 @@ static int ucstrncmp(const QChar *a, const QChar *b, size_t l)
         return a[offset + i].unicode() - b[offset + i].unicode();
     };
     return UnrollTailLoop<3>::exec(l, 0, lambda, lambda);
-#endif
-#ifdef __ARM_NEON__
+#  elif defined(__ARM_NEON__)
     if (l >= 8) {
         const QChar *end = a + l;
         const uint16x8_t mask = { 1, 1 << 1, 1 << 2, 1 << 3, 1 << 4, 1 << 5, 1 << 6, 1 << 7 };
@@ -1206,54 +1195,14 @@ static int ucstrncmp(const QChar *a, const QChar *b, size_t l)
         return a[i].unicode() - b[i].unicode();
     };
     return UnrollTailLoop<7>::exec(l, 0, lambda, lambda);
-#endif // __ARM_NEON__
-    if (!l)
-        return 0;
-
-    // check alignment
-    if ((reinterpret_cast<quintptr>(a) & 2) == (reinterpret_cast<quintptr>(b) & 2)) {
-        // both addresses have the same alignment
-        if (reinterpret_cast<quintptr>(a) & 2) {
-            // both addresses are not aligned to 4-bytes boundaries
-            // compare the first character
-            if (*a != *b)
-                return a->unicode() - b->unicode();
-            --l;
-            ++a;
-            ++b;
-
-            // now both addresses are 4-bytes aligned
-        }
-
-        // both addresses are 4-bytes aligned
-        // do a fast 32-bit comparison
-        const quint32 *da = reinterpret_cast<const quint32 *>(a);
-        const quint32 *db = reinterpret_cast<const quint32 *>(b);
-        const quint32 *e = da + (l >> 1);
-        for ( ; da != e; ++da, ++db) {
-            if (*da != *db) {
-                a = reinterpret_cast<const QChar *>(da);
-                b = reinterpret_cast<const QChar *>(db);
-                if (*a != *b)
-                    return a->unicode() - b->unicode();
-                return a[1].unicode() - b[1].unicode();
-            }
-        }
+#  endif // MIPS DSP or __SSE2__ or __ARM_NEON__
+#endif // __OPTIMIZE_SIZE__
 
-        // do we have a tail?
-        a = reinterpret_cast<const QChar *>(da);
-        b = reinterpret_cast<const QChar *>(db);
-        return (l & 1) ? a->unicode() - b->unicode() : 0;
-    } else {
-        // one of the addresses isn't 4-byte aligned but the other is
-        const QChar *e = a + l;
-        for ( ; a != e; ++a, ++b) {
-            if (*a != *b)
-                return a->unicode() - b->unicode();
-        }
+    for (size_t i = 0; i < l; ++i) {
+        if (int diff = a[i].unicode() - b[i].unicode())
+            return diff;
     }
     return 0;
-#endif
 }
 
 static int ucstrncmp(const QChar *a, const uchar *c, size_t l)