json: add json parsing support

This patch implements JSON parsing support.

It's based on the parsing parts of the patch I posted here:
https://gcc.gnu.org/legacy-ml/gcc-patches/2017-08/msg00417.html
with the parsing moved to a separate source file and header, heavily
rewritten to capture source location information for JSON values, and
to capture errors via a result template.

I also added optional support for C and C++ style comments, which is
extremely useful in DejaGnu tests.

gcc/ChangeLog:
	* Makefile.in (OBJS-libcommon): Add json-parsing.o.
	* json-parsing.cc: New file.
	* json-parsing.h: New file.
	* json.cc (selftest::assert_print_eq): Remove "static".
	* json.h (json::array::begin): New.
	(json::array::end): New.
	(json::array::length): New.
	(json::array::get): New.
	(is_a_helper <json::value *>::test): New.
	(is_a_helper <const json::value *>::test): New.
	(is_a_helper <json::object *>::test): New.
	(is_a_helper <const json::object *>::test): New.
	(is_a_helper <json::array *>::test): New.
	(is_a_helper <const json::array *>::test): New.
	(is_a_helper <json::float_number *>::test): New.
	(is_a_helper <const json::float_number *>::test): New.
	(is_a_helper <json::integer_number *>::test): New.
	(is_a_helper <const json::integer_number *>::test): New.
	(is_a_helper <json::string *>::test): New.
	(is_a_helper <const json::string *>::test): New.
	(selftest::assert_print_eq): New decl.
	* selftest-run-tests.cc (selftest::run_tests): Call
	selftest::json_parser_cc_tests.
	* selftest.h (selftest::json_parser_cc_tests): New decl.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>

gcc/Makefile.in

@@ -1838,7 +1838,7 @@ OBJS-libcommon = diagnostic-spec.o diagnostic.o diagnostic-color.o \
 	diagnostic-show-locus.o \
 	edit-context.o \
 	pretty-print.o intl.o \
-	json.o \
+	json.o json-parsing.o \
 	sbitmap.o \
 	vec.o input.o hash-table.o ggc-none.o memory-block.o \
 	selftest.o selftest-diagnostic.o sort.o \

gcc/json-parsing.h

@@ -0,0 +1,113 @@
+/* JSON parsing
+   Copyright (C) 2017-2022 Free Software Foundation, Inc.
+   Contributed by David Malcolm <dmalcolm@redhat.com>.
+
+This file is part of GCC.
+
+GCC is free software; you can redistribute it and/or modify it under
+the terms of the GNU General Public License as published by the Free
+Software Foundation; either version 3, or (at your option) any later
+version.
+
+GCC is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or
+FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+for more details.
+
+You should have received a copy of the GNU General Public License
+along with GCC; see the file COPYING3.  If not see
+<http://www.gnu.org/licenses/>.  */
+
+#ifndef GCC_JSON_PARSING_H
+#define GCC_JSON_PARSING_H
+
+#include "json.h"
+
+namespace json
+{
+
+/* Declarations for parsing JSON to a json::value * tree.  */
+
+/* Abstract base class for recording what the locations of JSON values
+   were as they parsed.  */
+
+class location_map
+{
+public:
+  /* A point within the JSON input file.  */
+  struct point
+  {
+    size_t m_unichar_idx; /* zero-based.  */
+    int m_line;   /* one-based.  */
+    int m_column; /* zero-based unichar count.  */
+  };
+
+  /* A range of points within the JSON input file.
+     Both endpoints are part of the range.  */
+  struct range
+  {
+    point m_start;
+    point m_end;
+  };
+
+  virtual ~location_map () {}
+  virtual void record_range_for_value (json::value *jv, const range &r) = 0;
+  virtual void on_finished_parsing () {}
+};
+
+/* Class for recording an error within a JSON file.  */
+
+class error
+{
+public:
+  error (const location_map::range &r, char *msg)
+  : m_range (r), m_msg (msg)
+  {
+  }
+  ~error ()
+  {
+    free (m_msg);
+  }
+
+  const location_map::range &get_range () const { return m_range; }
+  const char *get_msg () const { return m_msg; }
+
+private:
+  location_map::range m_range;
+  char *m_msg;
+};
+
+/* Class for the result of an operation: either a value or an error
+   (or both null for the case of "successful nullptr").
+   The types must be default-constructible.  */
+
+template <typename ValueType, typename ErrorType>
+struct result
+{
+  result (ValueType val) : m_val (std::move (val)), m_err () {}
+  result (ErrorType err) : m_val (), m_err (std::move (err)) {}
+
+  ValueType m_val;
+  ErrorType m_err;
+};
+
+/* Typedef for the result of parsing JSON: ownership of either a
+   json::value * or of a json::error *.  */
+typedef result<std::unique_ptr<value>,
+	       std::unique_ptr<error>> parser_result_t;
+
+/* Functions for parsing JSON buffers.  */
+
+extern parser_result_t
+parse_utf8_string (size_t length,
+		   const char *utf8_buf,
+		   bool allow_comments,
+		   location_map *out_loc_map);
+extern parser_result_t
+parse_utf8_string (const char *utf8,
+		   bool allow_comments,
+		   location_map *out_loc_map);
+
+} // namespace json
+
+#endif  /* GCC_JSON_PARSING_H  */

gcc/json.cc

@@ -383,7 +383,7 @@ namespace selftest {
 
 /* Verify that JV->print () prints EXPECTED_JSON.  */
 
-static void
+void
 assert_print_eq (const location &loc,
 		 const json::value &jv,
 		 bool formatted,

gcc/json.h

@@ -36,8 +36,8 @@ along with GCC; see the file COPYING3.  If not see
    and http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-404.pdf
    and https://tools.ietf.org/html/rfc7159
 
-   Supports creating a DOM-like tree of json::value *, and then dumping
+   Supports parsing text into a DOM-like tree of json::value *, directly
-   json::value * to text.  */
+   creating such trees, and dumping json::value * to text.  */
 
 /* TODO: `libcpp/mkdeps.cc` wants JSON writing support for p1689r5 output;
    extract this code and move to libiberty.  */
@@ -175,6 +175,13 @@ class array : public value
   size_t size () const { return m_elements.length (); }
   value *operator[] (size_t i) const { return m_elements[i]; }
 
+  value **begin () { return m_elements.begin (); }
+  value **end () { return m_elements.end (); }
+  const value * const *begin () const { return m_elements.begin (); }
+  const value * const *end () const { return m_elements.end (); }
+  size_t length () const { return m_elements.length (); }
+  value *get (size_t idx) const { return m_elements[idx]; }
+
  private:
   auto_vec<value *> m_elements;
 };
@@ -252,4 +259,115 @@ class literal : public value
 
 } // namespace json
 
+template <>
+template <>
+inline bool
+is_a_helper <json::value *>::test (json::value *)
+{
+  return true;
+}
+
+template <>
+template <>
+inline bool
+is_a_helper <const json::value *>::test (const json::value *)
+{
+  return true;
+}
+
+template <>
+template <>
+inline bool
+is_a_helper <json::object *>::test (json::value *jv)
+{
+  return jv->get_kind () == json::JSON_OBJECT;
+}
+
+template <>
+template <>
+inline bool
+is_a_helper <const json::object *>::test (const json::value *jv)
+{
+  return jv->get_kind () == json::JSON_OBJECT;
+}
+
+template <>
+template <>
+inline bool
+is_a_helper <json::array *>::test (json::value *jv)
+{
+  return jv->get_kind () == json::JSON_ARRAY;
+}
+
+template <>
+template <>
+inline bool
+is_a_helper <const json::array *>::test (const json::value *jv)
+{
+  return jv->get_kind () == json::JSON_ARRAY;
+}
+
+template <>
+template <>
+inline bool
+is_a_helper <json::float_number *>::test (json::value *jv)
+{
+  return jv->get_kind () == json::JSON_FLOAT;
+}
+
+template <>
+template <>
+inline bool
+is_a_helper <const json::float_number *>::test (const json::value *jv)
+{
+  return jv->get_kind () == json::JSON_FLOAT;
+}
+
+template <>
+template <>
+inline bool
+is_a_helper <json::integer_number *>::test (json::value *jv)
+{
+  return jv->get_kind () == json::JSON_INTEGER;
+}
+
+template <>
+template <>
+inline bool
+is_a_helper <const json::integer_number *>::test (const json::value *jv)
+{
+  return jv->get_kind () == json::JSON_INTEGER;
+}
+
+template <>
+template <>
+inline bool
+is_a_helper <json::string *>::test (json::value *jv)
+{
+  return jv->get_kind () == json::JSON_STRING;
+}
+
+template <>
+template <>
+inline bool
+is_a_helper <const json::string *>::test (const  json::value *jv)
+{
+  return jv->get_kind () == json::JSON_STRING;
+}
+
+#if CHECKING_P
+
+namespace selftest {
+
+class location;
+
+extern void assert_print_eq (const location &loc,
+			     const json::value &jv,
+			     bool formatted,
+			     const char *expected_json);
+
+} // namespace selftest
+
+#endif /* #if CHECKING_P */
+
 #endif  /* GCC_JSON_H  */

gcc/selftest-run-tests.cc

@@ -75,6 +75,7 @@ selftest::run_tests ()
   opt_suggestions_cc_tests ();
   opts_cc_tests ();
   json_cc_tests ();
+  json_parser_cc_tests ();
   cgraph_cc_tests ();
   optinfo_emit_json_cc_tests ();
   ordered_hash_map_tests_cc_tests ();

gcc/selftest.h

@@ -240,6 +240,7 @@ extern void hash_set_tests_cc_tests ();
 extern void input_cc_tests ();
 extern void json_cc_tests ();
 extern void lazy_diagnostic_path_cc_tests ();
+extern void json_parser_cc_tests ();
 extern void optinfo_emit_json_cc_tests ();
 extern void opts_cc_tests ();
 extern void opts_diagnostic_cc_tests ();