Issue #1738 - Part 1: Improve performance of JSON stringify

- Use some pointer voodoo and instead of stringbuffer append()
- Use a lookup table instead of char comparisons for chr < 256
- Stop using a Hashtable/MovableCellHasher for JSON CycleDetector

3 files changed, 111 insertions, 90 deletions

diff --git a/js/src/json.cpp b/js/src/json.cpp
index 5f1238f9f..73e37e237 100644
--- a/js/src/json.cpp
+++ b/js/src/json.cpp
@@ -39,78 +39,81 @@ const Class js::JSONClass = {
     JSCLASS_HAS_CACHED_PROTO(JSProto_JSON)
 };
 
-static inline bool
-IsQuoteSpecialCharacter(char16_t c)
-{
-    static_assert('\b' < ' ', "'\\b' must be treated as special below");
-    static_assert('\f' < ' ', "'\\f' must be treated as special below");
-    static_assert('\n' < ' ', "'\\n' must be treated as special below");
-    static_assert('\r' < ' ', "'\\r' must be treated as special below");
-    static_assert('\t' < ' ', "'\\t' must be treated as special below");
-
-    return c == '"' || c == '\\' || c < ' ';
-}
-
-/* ES5 15.12.3 Quote. */
-template <typename CharT>
-static bool
-Quote(StringBuffer& sb, JSLinearString* str)
+/* ES5 15.12.3 Quote.
+ * Requires that the destination has enough space allocated for src after escaping
+ * (that is, `2 + 6 * (srcEnd - srcBegin)` characters).
+ */
+template <typename SrcCharT, typename DstCharT>
+static MOZ_ALWAYS_INLINE RangedPtr<DstCharT>
+InfallibleQuote(RangedPtr<const SrcCharT> srcBegin, RangedPtr<const SrcCharT> srcEnd, RangedPtr<DstCharT> dstPtr)
 {
-    size_t len = str->length();
+    // Maps characters < 256 to the value that must follow the '\\' in the quoted string.
+    // Entries with 'u' are handled as \\u00xy, and entries with 0 are not escaped in any way.
+    // Characters >= 256 are all assumed to be unescaped.
+    static const Latin1Char escapeLookup[256] = {
+        'u', 'u', 'u', 'u', 'u', 'u', 'u', 'u', 'b', 't',
+        'n', 'u', 'f', 'r', 'u', 'u', 'u', 'u', 'u', 'u',
+        'u', 'u', 'u', 'u', 'u', 'u', 'u', 'u', 'u', 'u',
+        'u', 'u', 0,   0,  '\"', 0,   0,   0,   0,   0,
+        0,   0,   0,   0,   0,   0,   0,   0,   0,   0,
+        0,   0,   0,   0,   0,   0,   0,   0,   0,   0,
+        0,   0,   0,   0,   0,   0,   0,   0,   0,   0,
+        0,   0,   0,   0,   0,   0,   0,   0,   0,   0,
+        0,   0,   0,   0,   0,   0,   0,   0,   0,   0,
+        0,   0,  '\\', // rest are all zeros
+    };
 
     /* Step 1. */
-    if (!sb.append('"'))
-        return false;
+    *dstPtr++ = '"';
 
     /* Step 2. */
-    JS::AutoCheckCannotGC nogc;
-    const RangedPtr<const CharT> buf(str->chars<CharT>(nogc), len);
-    for (size_t i = 0; i < len; ++i) {
-        /* Batch-append maximal character sequences containing no escapes. */
-        size_t mark = i;
-        do {
-            if (IsQuoteSpecialCharacter(buf[i]))
-                break;
-        } while (++i < len);
-        if (i > mark) {
-            if (!sb.appendSubstring(str, mark, i - mark))
-                return false;
-            if (i == len)
-                break;
+    while (srcBegin != srcEnd) {
+        SrcCharT c = *srcBegin++;
+        size_t escapeIndex = c % sizeof(escapeLookup);
+        Latin1Char escaped = escapeLookup[escapeIndex];
+        if (MOZ_LIKELY((escapeIndex != size_t(c)) || !escaped)) {
+            *dstPtr++ = c;
+            continue;
         }
-
-        char16_t c = buf[i];
-        if (c == '"' || c == '\\') {
-            if (!sb.append('\\') || !sb.append(c))
-                return false;
-        } else if (c == '\b' || c == '\f' || c == '\n' || c == '\r' || c == '\t') {
-           char16_t abbrev = (c == '\b')
-                         ? 'b'
-                         : (c == '\f')
-                         ? 'f'
-                         : (c == '\n')
-                         ? 'n'
-                         : (c == '\r')
-                         ? 'r'
-                         : 't';
-           if (!sb.append('\\') || !sb.append(abbrev))
-               return false;
-        } else {
+        *dstPtr++ = '\\';
+        *dstPtr++ = escaped;
+        if (escaped == 'u') {
             MOZ_ASSERT(c < ' ');
-            if (!sb.append("\\u00"))
-                return false;
             MOZ_ASSERT((c >> 4) < 10);
             uint8_t x = c >> 4, y = c % 16;
-            if (!sb.append(Latin1Char('0' + x)) ||
-                !sb.append(Latin1Char(y < 10 ? '0' + y : 'a' + (y - 10))))
-            {
-                return false;
-            }
+            *dstPtr++ = '0';
+            *dstPtr++ = '0';
+            *dstPtr++ = '0' + x;
+            *dstPtr++ = y < 10 ? '0' + y : 'a' + (y - 10);
         }
     }
 
     /* Steps 3-4. */
-    return sb.append('"');
+    *dstPtr++ = '"';
+    return dstPtr;
+}
+
+template <typename SrcCharT, typename CharVectorT>
+static bool
+Quote(CharVectorT& sb, JSLinearString* str)
+{
+    // We resize the backing buffer to the maximum size we could possibly need,
+    // write the escaped string into it, and shrink it back to the size we ended
+    // up needing.
+    size_t len = str->length();
+    size_t sbInitialLen = sb.length();
+    if (!sb.growByUninitialized(len * 6 + 2))
+        return false;
+
+    typedef typename CharVectorT::ElementType DstCharT;
+
+    JS::AutoCheckCannotGC nogc;
+    RangedPtr<const SrcCharT> srcBegin{str->chars<SrcCharT>(nogc), len};
+    RangedPtr<DstCharT> dstBegin{sb.begin(), sb.begin(), sb.end()};
+    RangedPtr<DstCharT> dstEnd = InfallibleQuote(srcBegin, srcBegin + len, dstBegin + sbInitialLen);
+    size_t newSize = dstEnd - dstBegin;
+    sb.shrinkTo(newSize);
+    return true;
 }
 
 static bool
@@ -120,14 +123,23 @@ Quote(JSContext* cx, StringBuffer& sb, JSString* str)
     if (!linear)
         return false;
 
-    return linear->hasLatin1Chars()
-           ? Quote<Latin1Char>(sb, linear)
-           : Quote<char16_t>(sb, linear);
+    // Check if either has non-latin1 before calling ensure, so that the buffer's
+    // hasEnsured flag is set if the converstion to twoByte was automatic.
+    if (!sb.isUnderlyingBufferLatin1() || linear->hasTwoByteChars()) {
+        if (!sb.ensureTwoByteChars())
+            return false;
+    }
+    if (linear->hasTwoByteChars())
+        return Quote<char16_t>(sb.rawTwoByteBuffer(), linear);
+
+    return sb.isUnderlyingBufferLatin1()
+           ? Quote<Latin1Char>(sb.latin1Chars(), linear)
+           : Quote<Latin1Char>(sb.rawTwoByteBuffer(), linear);
 }
 
 namespace {
 
-using ObjectSet = GCHashSet<JSObject*, MovableCellHasher<JSObject*>, SystemAllocPolicy>;
+using ObjectVector = GCVector<JSObject*, 8>;
 
 class StringifyContext
 {
@@ -138,7 +150,7 @@ class StringifyContext
       : sb(sb),
         gap(gap),
         replacer(cx, replacer),
-        stack(cx),
+        stack(cx, ObjectVector(cx)),
         propertyList(propertyList),
         depth(0),
         maybeSafely(maybeSafely)
@@ -147,14 +159,10 @@ class StringifyContext
         MOZ_ASSERT_IF(maybeSafely, gap.empty());
     }
 
-    bool init() {
-        return stack.init(8);
-    }
-
     StringBuffer& sb;
     const StringBuffer& gap;
     RootedObject replacer;
-    Rooted<ObjectSet> stack;
+    Rooted<ObjectVector> stack;
     const AutoIdVector& propertyList;
     uint32_t depth;
     bool maybeSafely;
@@ -302,29 +310,34 @@ class CycleDetector
 {
   public:
     CycleDetector(StringifyContext* scx, HandleObject obj)
-      : stack(&scx->stack), obj_(obj) {
-    }
-
-    bool foundCycle(JSContext* cx) {
-        auto addPtr = stack.lookupForAdd(obj_);
-        if (addPtr) {
-            JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_JSON_CYCLIC_VALUE);
-            return false;
-        }
-        if (!stack.add(addPtr, obj_)) {
-            ReportOutOfMemory(cx);
-            return false;
+      : stack_(&scx->stack)
+      , obj_(obj)
+      , appended_(false) 
+    {}
+
+    MOZ_ALWAYS_INLINE bool foundCycle(JSContext* cx) {
+        JSObject* obj = obj_;
+        for (JSObject* obj2 : stack_) {
+            if (MOZ_UNLIKELY(obj == obj2)) {
+                JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_JSON_CYCLIC_VALUE);
+                return false;
+            }
         }
-        return true;
+        appended_ = stack_.append(obj);
+        return appended_;
     }
 
     ~CycleDetector() {
-        stack.remove(obj_);
+        if (MOZ_LIKELY(appended_)) {
+            MOZ_ASSERT(stack_.back() == obj_);
+            stack_.popBack();
+        }
     }
 
   private:
-    MutableHandle<ObjectSet> stack;
+    MutableHandle<ObjectVector> stack_;
     HandleObject obj_;
+    bool appended_;
 };
 
 /* ES5 15.12.3 JO. */
@@ -747,8 +760,6 @@ js::Stringify(JSContext* cx, MutableHandleValue vp, JSObject* replacer_, const V
     /* Step 12. */
     StringifyContext scx(cx, sb, gap, replacer, propertyList,
                          stringifyBehavior == StringifyBehavior::RestrictedSafe);
-    if (!scx.init())
-        return false;
     if (!PreprocessValue(cx, wrapper, HandleId(emptyId), vp, &scx))
         return false;
     if (IsFilteredValue(vp))
diff --git a/js/src/tests/ecma_5/JSON/stringify.js b/js/src/tests/ecma_5/JSON/stringify.js
index 1a7e9b150..3b2147420 100644
--- a/js/src/tests/ecma_5/JSON/stringify.js
+++ b/js/src/tests/ecma_5/JSON/stringify.js
@@ -34,6 +34,9 @@ assertStringify({'mmm\\mmm':"hmm"}, '{"mmm\\\\mmm":"hmm"}');
 assertStringify({'mmm\\mmm\\mmm':"hmm"}, '{"mmm\\\\mmm\\\\mmm":"hmm"}');
 assertStringify({"mm\u000bmm":"hmm"}, '{"mm\\u000bmm":"hmm"}');
 assertStringify({"mm\u0000mm":"hmm"}, '{"mm\\u0000mm":"hmm"}');
+assertStringify({"\u0000\u000b":""}, '{"\\u0000\\u000b":""}');
+assertStringify({"\u000b\ufdfd":"hmm"}, '{"\\u000b\ufdfd":"hmm"}');
+assertStringify({"\u000b\ufdfd":"h\xfc\ufdfdm"}, '{"\\u000b\ufdfd":"h\xfc\ufdfdm"}');
 
 var x = {"free":"variable"};
 assertStringify(x, '{"free":"variable"}');
diff --git a/js/src/vm/StringBuffer.h b/js/src/vm/StringBuffer.h
index f2437384a..502a3bc6f 100644
--- a/js/src/vm/StringBuffer.h
+++ b/js/src/vm/StringBuffer.h
@@ -61,12 +61,8 @@ class StringBuffer
     MOZ_ALWAYS_INLINE bool isLatin1() const { return cb.constructed<Latin1CharBuffer>(); }
     MOZ_ALWAYS_INLINE bool isTwoByte() const { return !isLatin1(); }
 
-    MOZ_ALWAYS_INLINE Latin1CharBuffer& latin1Chars() { return cb.ref<Latin1CharBuffer>(); }
     MOZ_ALWAYS_INLINE TwoByteCharBuffer& twoByteChars() { return cb.ref<TwoByteCharBuffer>(); }
 
-    MOZ_ALWAYS_INLINE const Latin1CharBuffer& latin1Chars() const {
-        return cb.ref<Latin1CharBuffer>();
-    }
     MOZ_ALWAYS_INLINE const TwoByteCharBuffer& twoByteChars() const {
         return cb.ref<TwoByteCharBuffer>();
     }
@@ -84,6 +80,12 @@ class StringBuffer
         cb.construct<Latin1CharBuffer>(cx);
     }
 
+    MOZ_ALWAYS_INLINE Latin1CharBuffer& latin1Chars() { return cb.ref<Latin1CharBuffer>(); }
+
+    MOZ_ALWAYS_INLINE const Latin1CharBuffer& latin1Chars() const {
+        return cb.ref<Latin1CharBuffer>();
+    }
+
     void clear() {
         if (isLatin1())
             latin1Chars().clear();
@@ -134,6 +136,11 @@ class StringBuffer
         return append(Latin1Char(c));
     }
 
+    TwoByteCharBuffer& rawTwoByteBuffer() {
+        MOZ_ASSERT(hasEnsuredTwoByteChars_);
+        return twoByteChars();
+    }
+
     inline MOZ_MUST_USE bool append(const char16_t* begin, const char16_t* end);
 
     MOZ_MUST_USE bool append(const char16_t* chars, size_t len) {