Bug 1301742 - Improve UTF-16 to UTF-8 conversion speed. r=froydnj

This adds an SSE2 optimized path for UTF-16 strings that can be trivially
converted to UTF-8 (they contain no characters > 0X7F). It has a
speedup for the case where many of the leading characters are < 0X80 and
should have the same performance in the worst-case scenario where the first
character is > 0X7F.

xpcom/string/nsReadableUtilsSSE2.cpp

@@ -0,0 +1,70 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include <emmintrin.h>
+
+#include "nsReadableUtilsImpl.h"
+
+namespace mozilla {
+namespace SSE2 {
+
+static inline bool
+is_zero (__m128i x)
+{
+  return
+    _mm_movemask_epi8(_mm_cmpeq_epi8(x, _mm_setzero_si128())) == 0xffff;
+}
+
+int32_t
+FirstNonASCII(const char16_t* aBegin, const char16_t* aEnd)
+{
+  const size_t kNumUnicharsPerVector = sizeof(__m128i) / sizeof(char16_t);
+  typedef NonASCIIParameters<sizeof(size_t)> p;
+  const size_t kMask = p::mask();
+  const uintptr_t kXmmAlignMask = 0xf;
+  const uint16_t kShortMask = 0xff80;
+  const size_t kNumUnicharsPerWord = p::numUnicharsPerWord();
+
+  const char16_t* idx = aBegin;
+
+  // Align ourselves to a 16-byte boundary as required by _mm_load_si128
+  for (; idx != aEnd && ((uintptr_t(idx) & kXmmAlignMask) != 0); idx++) {
+    if (!IsASCII(*idx)) {
+      return idx - aBegin;
+    }
+  }
+
+  // Check one XMM register (16 bytes) at a time.
+  const char16_t* vectWalkEnd = aligned(aEnd, kXmmAlignMask);
+  __m128i vectmask = _mm_set1_epi16(static_cast<int16_t>(kShortMask));
+  for (; idx != vectWalkEnd; idx += kNumUnicharsPerVector) {
+    const __m128i vect = *reinterpret_cast<const __m128i*>(idx);
+    if (!is_zero(_mm_and_si128(vect, vectmask))) {
+      return idx - aBegin;
+    }
+  }
+
+  // Check one word at a time.
+  const char16_t* wordWalkEnd = aligned(aEnd, p::alignMask());
+  for(; idx != wordWalkEnd; idx += kNumUnicharsPerWord) {
+    const size_t word = *reinterpret_cast<const size_t*>(idx);
+    if (word & kMask) {
+      return idx - aBegin;
+    }
+  }
+
+  // Take care of the remainder one character at a time.
+  for (; idx != aEnd; idx++) {
+    if (!IsASCII(*idx)) {
+      return idx - aBegin;
+    }
+  }
+
+  return -1;
+}
+
+} // namespace SSE2
+} // namespace mozilla