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Speedup; check for the expected separator before the lexer's token switch (#5592)

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* Check for the expected separator before the lexer's token switch

After a key the parser expects ':', after a value usually ','. Test for
that character first instead of going through scan()'s switch, which
compiles to an indirect jump. Any other character takes the old path,
so tokens and error messages are unchanged.

Parsing 6.3% faster with GCC 15.2 and 2.7% with Clang 22.1 (geomean of
the ParseString, ParseFile and ParseIndented benchmarks).

Signed-off-by: Michiel van Slobbe <michiel.van.slobbe@gmail.com>

* Improvement: address PR comments

Signed-off-by: Michiel van Slobbe <michiel.van.slobbe@gmail.com>

* fix: address comments

Signed-off-by: Michiel van Slobbe <michiel.van.slobbe@gmail.com>

* Fix clang-tidy bugprone-signed-char-misuse in scan_expecting

Convert the expected separator through unsigned char before storing it as
char_int_type. The generated code is unchanged.

Signed-off-by: Michiel van Slobbe <michiel.van.slobbe@gmail.com>

* Use raw string literals in the separator comment tests

Signed-off-by: Michiel van Slobbe <michiel.van.slobbe@gmail.com>

---------

Signed-off-by: Michiel van Slobbe <michiel.van.slobbe@gmail.com>
Co-authored-by: Michiel van Slobbe <michiel.van.slobbe@gmail.com>
This commit is contained in:
Michiel van Slobbe
2026-10-06 08:36:40 +02:00
committed by GitHub
co-authored by Michiel van Slobbe
parent 0490778fc3
commit 5ecb704f6b
4 changed files with 140 additions and 10 deletions
Download Patch File Download Diff File Expand all files Collapse all files
include/nlohmann/detail/input/lexer.hpp
+33 -1
View File
@@ -2021,6 +2021,39 @@ scan_number_done:
// read the next character and ignore whitespace
skip_whitespace();
return scan_after_whitespace();
}
/*!
@brief scan the next token when the caller expects a separator (':' or
',') most of the time
After an object key the next token is almost always ':', after a value
inside an object or array almost always ','. Testing for that character
first is a compare and a well-predicted branch, where the switch in
scan_after_whitespace() is an indirect jump through a table. Anything else
goes through the switch, so the result is the same as scan()'s.
May only be called after scan() has run once (the BOM check is skipped).
*/
token_type scan_expecting(token_type expected_type)
{
JSON_ASSERT(expected_type == token_type::name_separator || expected_type == token_type::value_separator);
JSON_ASSERT(position.chars_read_total > 0);
const char_int_type expected_char = static_cast<unsigned char>((expected_type == token_type::name_separator) ? ':' : ',');
skip_whitespace();
if (JSON_HEDLEY_LIKELY(current == expected_char))
{
return expected_type;
}
return scan_after_whitespace();
}
private:
/// the part of scan() after the leading whitespace: skip comments and
/// scan the token that starts with current
token_type scan_after_whitespace()
{
// ignore comments
while (ignore_comments && current == '/')
{
@@ -2100,7 +2133,6 @@ scan_number_done:
}
}
private:
/// input adapter
InputAdapterType ia;
include/nlohmann/detail/input/parser.hpp
+11 -4
View File
@@ -260,7 +260,7 @@ class parser
}
// parse separator (:)
if (JSON_HEDLEY_UNLIKELY(get_token() != token_type::name_separator))
if (JSON_HEDLEY_UNLIKELY(!get_token_expecting(token_type::name_separator)))
{
return sax->parse_error(m_lexer.get_position(),
m_lexer.get_token_string(),
@@ -423,7 +423,7 @@ class parser
{
// comma -> next value
// or end of array (ignore_trailing_commas = true)
if (get_token() == token_type::value_separator)
if (get_token_expecting(token_type::value_separator))
{
// parse a new value
get_token();
@@ -463,7 +463,7 @@ class parser
// comma -> next value
// or end of object (ignore_trailing_commas = true)
if (get_token() == token_type::value_separator)
if (get_token_expecting(token_type::value_separator))
{
get_token();
@@ -484,7 +484,7 @@ class parser
}
// parse separator (:)
if (JSON_HEDLEY_UNLIKELY(get_token() != token_type::name_separator))
if (JSON_HEDLEY_UNLIKELY(!get_token_expecting(token_type::name_separator)))
{
return sax->parse_error(m_lexer.get_position(),
m_lexer.get_token_string(),
@@ -528,6 +528,13 @@ class parser
return last_token = m_lexer.scan();
}
/// get next token from lexer; true if it is the separator @a expected_type
/// (name_separator or value_separator), which it usually is
bool get_token_expecting(token_type expected_type)
{
return (last_token = m_lexer.scan_expecting(expected_type)) == expected_type;
}
std::string exception_message(const token_type expected, const std::string& context)
{
std::string error_msg = "syntax error ";
single_include/nlohmann/json.hpp
+44 -5
View File
@@ -12313,6 +12313,39 @@ scan_number_done:
// read the next character and ignore whitespace
skip_whitespace();
return scan_after_whitespace();
}
/*!
@brief scan the next token when the caller expects a separator (':' or
',') most of the time
After an object key the next token is almost always ':', after a value
inside an object or array almost always ','. Testing for that character
first is a compare and a well-predicted branch, where the switch in
scan_after_whitespace() is an indirect jump through a table. Anything else
goes through the switch, so the result is the same as scan()'s.
May only be called after scan() has run once (the BOM check is skipped).
*/
token_type scan_expecting(token_type expected_type)
{
JSON_ASSERT(expected_type == token_type::name_separator || expected_type == token_type::value_separator);
JSON_ASSERT(position.chars_read_total > 0);
const char_int_type expected_char = static_cast<unsigned char>((expected_type == token_type::name_separator) ? ':' : ',');
skip_whitespace();
if (JSON_HEDLEY_LIKELY(current == expected_char))
{
return expected_type;
}
return scan_after_whitespace();
}
private:
/// the part of scan() after the leading whitespace: skip comments and
/// scan the token that starts with current
token_type scan_after_whitespace()
{
// ignore comments
while (ignore_comments && current == '/')
{
@@ -12392,7 +12425,6 @@ scan_number_done:
}
}
private:
/// input adapter
InputAdapterType ia;
@@ -18422,7 +18454,7 @@ class parser
}
// parse separator (:)
if (JSON_HEDLEY_UNLIKELY(get_token() != token_type::name_separator))
if (JSON_HEDLEY_UNLIKELY(!get_token_expecting(token_type::name_separator)))
{
return sax->parse_error(m_lexer.get_position(),
m_lexer.get_token_string(),
@@ -18585,7 +18617,7 @@ class parser
{
// comma -> next value
// or end of array (ignore_trailing_commas = true)
if (get_token() == token_type::value_separator)
if (get_token_expecting(token_type::value_separator))
{
// parse a new value
get_token();
@@ -18625,7 +18657,7 @@ class parser
// comma -> next value
// or end of object (ignore_trailing_commas = true)
if (get_token() == token_type::value_separator)
if (get_token_expecting(token_type::value_separator))
{
get_token();
@@ -18646,7 +18678,7 @@ class parser
}
// parse separator (:)
if (JSON_HEDLEY_UNLIKELY(get_token() != token_type::name_separator))
if (JSON_HEDLEY_UNLIKELY(!get_token_expecting(token_type::name_separator)))
{
return sax->parse_error(m_lexer.get_position(),
m_lexer.get_token_string(),
@@ -18690,6 +18722,13 @@ class parser
return last_token = m_lexer.scan();
}
/// get next token from lexer; true if it is the separator @a expected_type
/// (name_separator or value_separator), which it usually is
bool get_token_expecting(token_type expected_type)
{
return (last_token = m_lexer.scan_expecting(expected_type)) == expected_type;
}
std::string exception_message(const token_type expected, const std::string& context)
{
std::string error_msg = "syntax error ";
tests/src/unit-class_parser.cpp
+52
View File
@@ -2317,6 +2317,58 @@ TEST_CASE("parser class")
#endif
}
SECTION("comments before separators")
{
// The parser first checks for the expected ':' or ',' and only then
// falls back to the full token switch, which skips comments. A comment
// directly before a separator takes that fallback.
json _;
SECTION("ignored")
{
const std::vector<std::pair<std::string, json>> inputs =
{
{"{\"a\" /* c */ : 1}", {{"a", 1}}},
{"{\"a\" // c\n: 1}", {{"a", 1}}},
{R"({"a": 1, "b" /* c */ : 2})", {{"a", 1}, {"b", 2}}},
{R"({"a": 1 /* c */ , "b": 2})", {{"a", 1}, {"b", 2}}},
{"{\"a\": 1 // c\n, \"b\": 2}", {{"a", 1}, {"b", 2}}},
{"[1 /* c */ , 2]", {1, 2}},
{"[1 // c\n, 2]", {1, 2}},
{"{\"a\" /* c */ /* d */ : [1 // c\n , 2 /**/ ] /**/ , \"b\" : 3}", {{"a", {1, 2}}, {"b", 3}}}
};
for (const auto& input : inputs)
{
CAPTURE(input.first)
CHECK(json::parse(input.first, nullptr, true, true) == input.second);
CHECK(json::accept(input.first, true));
}
}
SECTION("ignored, with trailing commas")
{
CHECK(json::parse(std::string("[1 /* c */ , ]"), nullptr, true, true, true) == json({1}));
CHECK(json::parse(std::string("{\"a\": 1 /* c */ , }"), nullptr, true, true, true) == json({{"a", 1}}));
CHECK_THROWS_WITH_AS(_ = json::parse(std::string("[1 /* c */ , ]"), nullptr, true, true),
"[json.exception.parse_error.101] parse error at line 1, column 14: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal", json::parse_error);
CHECK_THROWS_WITH_AS(_ = json::parse(std::string("{\"a\": 1 /* c */ , }"), nullptr, true, true),
"[json.exception.parse_error.101] parse error at line 1, column 19: syntax error while parsing object key - unexpected '}'; expected string literal", json::parse_error);
}
SECTION("not ignored")
{
CHECK_THROWS_WITH_AS(_ = json::parse(std::string("{\"a\" /* c */ : 1}")),
"[json.exception.parse_error.101] parse error at line 1, column 6: syntax error while parsing object separator - invalid literal; last read: '\"a\" /'; expected ':'", json::parse_error);
CHECK_THROWS_WITH_AS(_ = json::parse(std::string("{\"a\": 1, \"b\" /* c */ : 2}")),
"[json.exception.parse_error.101] parse error at line 1, column 14: syntax error while parsing object separator - invalid literal; last read: '\"b\" /'; expected ':'", json::parse_error);
CHECK_THROWS_WITH_AS(_ = json::parse(std::string("{\"a\": 1 /* c */ , \"b\": 2}")),
"[json.exception.parse_error.101] parse error at line 1, column 9: syntax error while parsing object - invalid literal; last read: '1 /'; expected '}'", json::parse_error);
CHECK_THROWS_WITH_AS(_ = json::parse(std::string("[1 /* c */ , 2]")),
"[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing array - invalid literal; last read: '1 /'; expected ']'", json::parse_error);
CHECK(!json::accept(std::string("[1 /* c */ , 2]")));
}
}
#if JSON_DIAGNOSTIC_POSITIONS
// Macro for all test cases for start_pos and end_pos
#define SETUP_TESTCASES() \