1 files changed, 158 insertions, 0 deletions
diff --git a/js/src/vm/InlineCharBuffer-inl.h b/js/src/vm/InlineCharBuffer-inl.h
new file mode 100644
index 0000000000..ac006a0d98
--- /dev/null
+++ b/js/src/vm/InlineCharBuffer-inl.h
@@ -0,0 +1,158 @@
+/* -*- 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/. */
+
+#ifndef vm_InlineCharBuffer_inl_h
+#define vm_InlineCharBuffer_inl_h
+
+#include "vm/StringType-inl.h"
+
+namespace js {
+
+template <typename CharT>
+struct MaximumInlineLength;
+
+template <>
+struct MaximumInlineLength<Latin1Char> {
+  static constexpr size_t value = JSFatInlineString::MAX_LENGTH_LATIN1;
+};
+
+template <>
+struct MaximumInlineLength<char16_t> {
+  static constexpr size_t value = JSFatInlineString::MAX_LENGTH_TWO_BYTE;
+};
+
+// Character buffer class used for ToLowerCase and ToUpperCase operations, as
+// well as other string operations where the final string length is known in
+// advance.
+//
+// Case conversion operations normally return a string with the same length as
+// the input string. To avoid over-allocation, we optimistically allocate an
+// array with same size as the input string and only when we detect special
+// casing characters, which can change the output string length, we reallocate
+// the output buffer to the final string length.
+//
+// As a further mean to improve runtime performance, the character buffer
+// contains an inline storage, so we don't need to heap-allocate an array when
+// a JSInlineString will be used for the output string.
+//
+// Why not use mozilla::Vector instead? mozilla::Vector doesn't provide enough
+// fine-grained control to avoid over-allocation when (re)allocating for exact
+// buffer sizes. This led to visible performance regressions in µ-benchmarks.
+template <typename CharT>
+class MOZ_NON_PARAM InlineCharBuffer {
+  static constexpr size_t InlineCapacity = MaximumInlineLength<CharT>::value;
+
+  CharT inlineStorage[InlineCapacity];
+  UniquePtr<CharT[], JS::FreePolicy> heapStorage;
+
+#ifdef DEBUG
+  // In debug mode, we keep track of the requested string lengths to ensure
+  // all character buffer methods are called in the correct order and with
+  // the expected argument values.
+  size_t lastRequestedLength = 0;
+
+  void assertValidRequest(size_t expectedLastLength, size_t length) {
+    MOZ_ASSERT(length >= expectedLastLength, "cannot shrink requested length");
+    MOZ_ASSERT(lastRequestedLength == expectedLastLength);
+    lastRequestedLength = length;
+  }
+#else
+  void assertValidRequest(size_t expectedLastLength, size_t length) {}
+#endif
+
+ public:
+  CharT* get() { return heapStorage ? heapStorage.get() : inlineStorage; }
+
+  bool maybeAlloc(JSContext* cx, size_t length) {
+    assertValidRequest(0, length);
+
+    if (length <= InlineCapacity) {
+      return true;
+    }
+
+    MOZ_ASSERT(!heapStorage, "heap storage already allocated");
+    heapStorage =
+        cx->make_pod_arena_array<CharT>(js::StringBufferArena, length);
+    return !!heapStorage;
+  }
+
+  bool maybeRealloc(JSContext* cx, size_t oldLength, size_t newLength) {
+    assertValidRequest(oldLength, newLength);
+
+    if (newLength <= InlineCapacity) {
+      return true;
+    }
+
+    if (!heapStorage) {
+      heapStorage =
+          cx->make_pod_arena_array<CharT>(js::StringBufferArena, newLength);
+      if (!heapStorage) {
+        return false;
+      }
+
+      MOZ_ASSERT(oldLength <= InlineCapacity);
+      mozilla::PodCopy(heapStorage.get(), inlineStorage, oldLength);
+      return true;
+    }
+
+    CharT* oldChars = heapStorage.release();
+    CharT* newChars = cx->pod_arena_realloc(js::StringBufferArena, oldChars,
+                                            oldLength, newLength);
+    if (!newChars) {
+      js_free(oldChars);
+      return false;
+    }
+
+    heapStorage.reset(newChars);
+    return true;
+  }
+
+  JSString* toStringDontDeflate(JSContext* cx, size_t length,
+                                js::gc::Heap heap = js::gc::Heap::Default) {
+    MOZ_ASSERT(length == lastRequestedLength);
+
+    if (JSInlineString::lengthFits<CharT>(length)) {
+      MOZ_ASSERT(
+          !heapStorage,
+          "expected only inline storage when length fits in inline string");
+
+      if (JSString* str = TryEmptyOrStaticString(cx, inlineStorage, length)) {
+        return str;
+      }
+
+      mozilla::Range<const CharT> range(inlineStorage, length);
+      return NewInlineString<CanGC>(cx, range, heap);
+    }
+
+    MOZ_ASSERT(heapStorage,
+               "heap storage was not allocated for non-inline string");
+
+    return NewStringDontDeflate<CanGC>(cx, std::move(heapStorage), length,
+                                       heap);
+  }
+
+  JSString* toString(JSContext* cx, size_t length,
+                     js::gc::Heap heap = js::gc::Heap::Default) {
+    MOZ_ASSERT(length == lastRequestedLength);
+
+    if (JSInlineString::lengthFits<CharT>(length)) {
+      MOZ_ASSERT(
+          !heapStorage,
+          "expected only inline storage when length fits in inline string");
+
+      return NewStringCopyN<CanGC>(cx, inlineStorage, length, heap);
+    }
+
+    MOZ_ASSERT(heapStorage,
+               "heap storage was not allocated for non-inline string");
+
+    return NewString<CanGC>(cx, std::move(heapStorage), length, heap);
+  }
+};
+
+} /* namespace js */
+
+#endif /* vm_InlineCharBuffer_inl_h */