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Optimize small string parsing

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This commit is contained in:
Victor Zverovich 2020-05-19 12:56:06 -07:00
parent 2aa2526f64
commit a265e25b79
1 changed files with 47 additions and 24 deletions
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71
include/fmt/format.h
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@ -288,7 +288,6 @@ template <> constexpr int num_bits<fallback_uintptr>() {
std::numeric_limits<unsigned char>::digits);
}
// A workaround for gcc 4.8 to make void_t work in a SFINAE context.
template <typename... Ts> struct void_t_impl { using type = void; };
@ -2508,9 +2507,55 @@ template <typename Handler, typename Char> struct id_adapter {
Handler& handler;
};
template <typename Char, typename Handler>
FMT_CONSTEXPR FMT_INLINE const Char* parse_replacement_field(
const Char* begin, const Char* end, Handler&& handler) {
++begin;
if (begin == end) return handler.on_error("invalid format string"), end;
if (static_cast<char>(*begin) == '}') {
handler.on_arg_id();
handler.on_replacement_field(begin);
} else if (*begin == '{') {
handler.on_text(begin, begin + 1);
} else {
begin = parse_arg_id(begin, end, id_adapter<Handler, Char>{handler});
Char c = begin != end ? *begin : Char();
if (c == '}') {
handler.on_replacement_field(begin);
} else if (c == ':') {
begin = handler.on_format_specs(begin + 1, end);
if (begin == end || *begin != '}')
return handler.on_error("unknown format specifier"), end;
} else {
return handler.on_error("missing '}' in format string"), end;
}
}
return begin + 1;
}
template <bool IS_CONSTEXPR, typename Char, typename Handler>
FMT_CONSTEXPR void parse_format_string(basic_string_view<Char> format_str,
Handler&& handler) {
auto begin = format_str.data();
auto end = begin + format_str.size();
if (end - begin < 32) {
// Use a simple loop instead of memchr for small strings.
const Char* p = begin;
while (p != end) {
auto c = *p++;
if (c == '{') {
handler.on_text(begin, p - 1);
begin = p = parse_replacement_field(p - 1, end, handler);
} else if (c == '}') {
if (p == end || *p != '}')
return handler.on_error("unmatched '}' in format string");
handler.on_text(begin, p);
begin = ++p;
}
}
handler.on_text(begin, end);
return;
}
struct writer {
FMT_CONSTEXPR void operator()(const Char* begin, const Char* end) {
if (begin + 1 >= end) {
@ -2532,8 +2577,6 @@ FMT_CONSTEXPR void parse_format_string(basic_string_view<Char> format_str,
}
Handler& handler_;
} write{handler};
auto begin = format_str.data();
auto end = begin + format_str.size();
while (begin != end) {
// Doing two passes with memchr (one for '{' and another for '}') is up to
// 2.5x faster than the naive one-pass implementation on big format strings.
@ -2541,27 +2584,7 @@ FMT_CONSTEXPR void parse_format_string(basic_string_view<Char> format_str,
if (*begin != '{' && !find<IS_CONSTEXPR>(begin + 1, end, '{', p))
return write(begin, end);
write(begin, p);
++p;
if (p == end) return handler.on_error("invalid format string");
if (static_cast<char>(*p) == '}') {
handler.on_arg_id();
handler.on_replacement_field(p);
} else if (*p == '{') {
handler.on_text(p, p + 1);
} else {
p = parse_arg_id(p, end, id_adapter<Handler, Char>{handler});
Char c = p != end ? *p : Char();
if (c == '}') {
handler.on_replacement_field(p);
} else if (c == ':') {
p = handler.on_format_specs(p + 1, end);
if (p == end || *p != '}')
return handler.on_error("unknown format specifier");
} else {
return handler.on_error("missing '}' in format string");
}
}
begin = p + 1;
begin = parse_replacement_field(p, end, handler);
}
}