Adds a JSON parser and a benchmark (json-bench) to catch performance regressions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@146735 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/JSONParser.cpp

@@ -0,0 +1,221 @@
+//===--- JsonParser.cpp - Simple JSON parser ------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+//  This file implements a JSON parser.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/JSONParser.h"
+
+#include "llvm/ADT/Twine.h"
+#include "llvm/Support/Casting.h"
+
+namespace llvm {
+
+JSONParser::JSONParser(StringRef Input)
+  : Input(Input), Position(Input.begin()) {}
+
+JSONValue *JSONParser::parseRoot() {
+  if (Position != Input.begin())
+    report_fatal_error("Cannot resuse JSONParser.");
+  if (isWhitespace())
+    nextNonWhitespace();
+  if (errorIfAtEndOfFile("'[' or '{' at start of JSON text"))
+    return 0;
+  switch (*Position) {
+    case '[':
+      return new (ValueAllocator.Allocate<JSONArray>(1)) JSONArray(this);
+    case '{':
+      return new (ValueAllocator.Allocate<JSONObject>(1)) JSONObject(this);
+    default:
+      setExpectedError("'[' or '{' at start of JSON text", *Position);
+      return 0;
+  }
+}
+
+bool JSONParser::validate() {
+  return parseRoot()->skip();
+}
+
+// Sets the current error to:
+// "Error while parsing JSON: expected <Expected>, but found <Found>".
+void JSONParser::setExpectedError(StringRef Expected, StringRef Found) {
+  ErrorMessage = ("Error while parsing JSON: expected " +
+    Expected + ", but found " + Found + ".").str();
+}
+
+// Sets the current error to:
+// "Error while parsing JSON: expected <Expected>, but found <Found>".
+void JSONParser::setExpectedError(StringRef Expected, char Found) {
+  setExpectedError(Expected, StringRef(&Found, 1));
+}
+
+// If there is no character available, returns true and sets the current error
+// to: "Error while parsing JSON: expected <Expected>, but found EOF.".
+bool JSONParser::errorIfAtEndOfFile(StringRef Expected) {
+  if (Position == Input.end()) {
+    setExpectedError(Expected, "EOF");
+    return true;
+  }
+  return false;
+}
+
+// Sets the current error if the current character is not C to:
+// "Error while parsing JSON: expected 'C', but got <current character>".
+bool JSONParser::errorIfNotAt(char C, StringRef Message) {
+  if (Position == Input.end() || *Position != C) {
+    std::string Expected =
+      ("'" + StringRef(&C, 1) + "' " + Message).str();
+    if (Position == Input.end())
+      setExpectedError(Expected, "EOF");
+    else
+      setExpectedError(Expected, *Position);
+    return true;
+  }
+  return false;
+}
+
+// Forbidding inlining improves performance by roughly 20%.
+// FIXME: Remove once llvm optimizes this to the faster version without hints.
+LLVM_ATTRIBUTE_NOINLINE static bool
+wasEscaped(StringRef::iterator First, StringRef::iterator Position);
+
+// Returns whether a character at 'Position' was escaped with a leading '\'.
+// 'First' specifies the position of the first character in the string.
+static bool wasEscaped(StringRef::iterator First,
+                       StringRef::iterator Position) {
+  assert(Position - 1 >= First);
+  StringRef::iterator I = Position - 1;
+  // We calulate the number of consecutive '\'s before the current position
+  // by iterating backwards through our string.
+  while (I >= First && *I == '\\') --I;
+  // (Position - 1 - I) now contains the number of '\'s before the current
+  // position. If it is odd, the character at 'Positon' was escaped.
+  return (Position - 1 - I) % 2 == 1;
+}
+
+// Parses a JSONString, assuming that the current position is on a quote.
+JSONString *JSONParser::parseString() {
+  assert(Position != Input.end());
+  assert(!isWhitespace());
+  if (errorIfNotAt('"', "at start of string"))
+    return 0;
+  StringRef::iterator First = Position + 1;
+
+  // Benchmarking shows that this loop is the hot path of the application with
+  // about 2/3rd of the runtime cycles. Since escaped quotes are not the common
+  // case, and multiple escaped backslashes before escaped quotes are very rare,
+  // we pessimize this case to achieve a smaller inner loop in the common case.
+  // We're doing that by having a quick inner loop that just scans for the next
+  // quote. Once we find the quote we check the last character to see whether
+  // the quote might have been escaped. If the last character is not a '\', we
+  // know the quote was not escaped and have thus found the end of the string.
+  // If the immediately preceding character was a '\', we have to scan backwards
+  // to see whether the previous character was actually an escaped backslash, or
+  // an escape character for the quote. If we find that the current quote was
+  // escaped, we continue parsing for the next quote and repeat.
+  // This optimization brings around 30% performance improvements.
+  do {
+    // Step over the current quote.
+    ++Position;
+    // Find the next quote.
+    while (Position != Input.end() && *Position != '"')
+      ++Position;
+    if (errorIfAtEndOfFile("\" at end of string"))
+      return 0;
+    // Repeat until the previous character was not a '\' or was an escaped
+    // backslash.
+  } while (*(Position - 1) == '\\' && wasEscaped(First, Position));
+
+  return new (ValueAllocator.Allocate<JSONString>())
+      JSONString(StringRef(First, Position - First));
+}
+
+
+// Advances the position to the next non-whitespace position.
+void JSONParser::nextNonWhitespace() {
+  do {
+    ++Position;
+  } while (isWhitespace());
+}
+
+// Checks if there is a whitespace character at the current position.
+bool JSONParser::isWhitespace() {
+  return Position != Input.end() && (*Position == ' ' || *Position == '\t' ||
+                                     *Position == '\n' || *Position == '\r');
+}
+
+bool JSONParser::failed() const {
+  return !ErrorMessage.empty();
+}
+
+std::string JSONParser::getErrorMessage() const {
+  return ErrorMessage;
+}
+
+bool JSONAtom::skip() const {
+  switch (MyKind) {
+  case JK_Array:        return cast<JSONArray>(this)->skip();
+  case JK_Object:       return cast<JSONObject>(this)->skip();
+  case JK_String:       return cast<JSONString>(this)->skip();
+  case JK_KeyValuePair: return cast<JSONKeyValuePair>(this)->skip();
+  }
+  llvm_unreachable("Impossible enum value.");
+}
+
+// Parses a JSONValue, assuming that the current position is at the first
+// character of the value.
+JSONValue *JSONParser::parseValue() {
+  assert(Position != Input.end());
+  assert(!isWhitespace());
+  switch (*Position) {
+    case '[':
+      return new (ValueAllocator.Allocate<JSONArray>(1)) JSONArray(this);
+    case '{':
+      return new (ValueAllocator.Allocate<JSONObject>(1)) JSONObject(this);
+    case '"':
+      return parseString();
+    default:
+      setExpectedError("'[', '{' or '\"' at start of value", *Position);
+      return 0;
+  }
+}
+
+// Parses a JSONKeyValuePair, assuming that the current position is at the first
+// character of the key, value pair.
+JSONKeyValuePair *JSONParser::parseKeyValuePair() {
+  assert(Position != Input.end());
+  assert(!isWhitespace());
+
+  JSONString *Key = parseString();
+  if (Key == 0)
+    return 0;
+
+  nextNonWhitespace();
+  if (errorIfNotAt(':', "between key and value"))
+    return 0;
+
+  nextNonWhitespace();
+  const JSONValue *Value = parseValue();
+  if (Value == 0)
+    return 0;
+
+  return new (ValueAllocator.Allocate<JSONKeyValuePair>(1))
+    JSONKeyValuePair(Key, Value);
+}
+
+template <> JSONValue *JSONParser::parseElement() {
+  return parseValue();
+}
+
+template <> JSONKeyValuePair *JSONParser::parseElement() {
+  return parseKeyValuePair();
+}
+
+} // end namespace llvm