cxxrtl: add a representation of simulation timestamps.

While the VCD format separates the timescale and the timestep (likely
to allow representing the timestep with a small integer type), time in
CXXRTL is represented using a uniform 96-bit number, which allows for
a ±100 year range at femtosecond resolution.

The implementation uses `value<96>`, which provides fast arithmetic and
comparison operations, as well as conversion to/from a more common
representation of integer seconds plus femtoseconds.

backends/cxxrtl/runtime/cxxrtl/cxxrtl_time.h

@@ -0,0 +1,229 @@
+/*
+ *  yosys -- Yosys Open SYnthesis Suite
+ *
+ *  Copyright (C) 2023  Catherine <whitequark@whitequark.org>
+ *
+ *  Permission to use, copy, modify, and/or distribute this software for any
+ *  purpose with or without fee is hereby granted.
+ *
+ *  THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ *  WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ *  MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ *  ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ *  WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ *  ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ *  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ *
+ */
+
+#ifndef CXXRTL_TIME_H
+#define CXXRTL_TIME_H
+
+#include <cinttypes>
+#include <string>
+
+#include <cxxrtl/cxxrtl.h>
+
+namespace cxxrtl {
+
+// A timestamp or a difference in time, stored as a 96-bit number of femtoseconds (10e-15 s). The dynamic range and
+// resolution of this format can represent any VCD timestamp within 136 years, without the need for a timescale.
+class time {
+public:
+	static constexpr size_t bits = 96; // 3 chunks
+
+private:
+	static constexpr value<bits> resolution = value<bits> {
+		chunk_t(1000000000000000ull & 0xffffffffull), chunk_t(1000000000000000ull >> 32), 0u
+	};
+	static constexpr size_t resolution_digits = 15;
+
+	// Signed number of femtoseconds from the beginning of time.
+	value<bits> raw;
+
+public:
+	constexpr time() {}
+
+	explicit constexpr time(const value<bits> &raw) : raw(raw) {}
+	explicit operator const value<bits> &() const { return raw; }
+
+	static constexpr time maximum() {
+		return time(value<bits> { 0xffffffffu, 0xffffffffu, 0x7fffffffu });
+	}
+
+	time(int32_t secs, int64_t femtos) {
+		value<32> secs_val;
+		secs_val.set((uint32_t)secs);
+		value<64> femtos_val;
+		femtos_val.set((uint64_t)femtos);
+		raw = secs_val.sext<bits>().mul<bits>(resolution).add(femtos_val.sext<bits>());
+	}
+
+	bool is_zero() const {
+		return raw.is_zero();
+	}
+
+	// Extracts the sign of the value.
+	bool is_negative() const {
+		return raw.is_neg();
+	}
+
+	// Extracts the absolute number of whole seconds. Negative if the value is negative.
+	int32_t secs() const {
+		return raw.sdivmod(resolution).first.trunc<32>().get<uint32_t>();
+	}
+
+	// Extracts the absolute number of femtoseconds in the fractional second. Negative if the value is negative.
+	int64_t femtos() const {
+		return raw.sdivmod(resolution).second.trunc<64>().get<uint64_t>();
+	}
+
+	bool operator==(const time &other) const {
+		return raw == other.raw;
+	}
+
+	bool operator!=(const time &other) const {
+		return raw != other.raw;
+	}
+
+	bool operator>(const time &other) const {
+		return other.raw.scmp(raw);
+	}
+
+	bool operator>=(const time &other) const {
+		return !raw.scmp(other.raw);
+	}
+
+	bool operator<(const time &other) const {
+		return raw.scmp(other.raw);
+	}
+
+	bool operator<=(const time &other) const {
+		return !other.raw.scmp(raw);
+	}
+
+	time operator+(const time &other) const {
+		return time(raw.add(other.raw));
+	}
+
+	time &operator+=(const time &other) {
+		*this = *this + other;
+		return *this;
+	}
+
+	time operator-() const {
+		return time(raw.neg());
+	}
+
+	time operator-(const time &other) const {
+		return *this + (-other);
+	}
+
+	time &operator-=(const time &other) {
+		*this = *this - other;
+		return *this;
+	}
+
+	operator std::string() const {
+		char buf[38]; // len(f"-{2**64}.{10**15-1}") + 1 == 38
+		int32_t secs = this->secs();
+		int64_t femtos = this->femtos();
+		snprintf(buf, sizeof(buf), "%s%" PRIi32 ".%015" PRIi64,
+			is_negative() ? "-" : "", secs >= 0 ? secs : -secs, femtos >= 0 ? femtos : -femtos);
+		return buf;
+	}
+
+#if __cplusplus >= 201603L
+	[[nodiscard("ignoring parse errors")]]
+#endif
+	bool parse(const std::string &str) {
+		enum {
+			parse_sign_opt,
+			parse_integral,
+			parse_fractional,
+		} state = parse_sign_opt;
+		bool negative = false;
+		int32_t integral = 0;
+		int64_t fractional = 0;
+		size_t frac_digits = 0;
+		for (auto chr : str) {
+			switch (state) {
+				case parse_sign_opt:
+					state = parse_integral;
+					if (chr == '+' || chr == '-') {
+						negative = (chr == '-');
+						break;
+					}
+					/* fallthrough */
+				case parse_integral:
+					if (chr >= '0' && chr <= '9') {
+						integral *= 10;
+						integral += chr - '0';
+					} else if (chr == '.') {
+						state = parse_fractional;
+					} else {
+						return false;
+					}
+					break;
+				case parse_fractional:
+					if (chr >= '0' && chr <= '9' && frac_digits < resolution_digits) {
+						fractional *= 10;
+						fractional += chr - '0';
+						frac_digits++;
+					} else {
+						return false;
+					}
+					break;
+			}
+		}
+		if (frac_digits == 0)
+			return false;
+		while (frac_digits++ < resolution_digits)
+			fractional *= 10;
+		*this = negative ? -time { integral, fractional} : time { integral, fractional };
+		return true;
+	}
+};
+
+// Out-of-line definition required until C++17.
+constexpr value<time::bits> time::resolution;
+
+std::ostream &operator<<(std::ostream &os, const time &val) {
+	os << (std::string)val;
+	return os;
+}
+
+// These literals are (confusingly) compatible with the ones from `std::chrono`: the `std::chrono` literals do not
+// have an underscore (e.g. 1ms) and the `cxxrtl::time` literals do (e.g. 1_ms). This syntactic difference is
+// a requirement of the C++ standard. Despite being compatible the literals should not be mixed in the same namespace.
+namespace time_literals {
+
+time operator""_s(unsigned long long seconds) {
+	return time { (int32_t)seconds, 0 };
+}
+
+time operator""_ms(unsigned long long milliseconds) {
+	return time { 0, (int64_t)milliseconds * 1000000000000 };
+}
+
+time operator""_us(unsigned long long microseconds) {
+	return time { 0, (int64_t)microseconds * 1000000000 };
+}
+
+time operator""_ns(unsigned long long nanoseconds) {
+	return time { 0, (int64_t)nanoseconds * 1000000 };
+}
+
+time operator""_ps(unsigned long long picoseconds) {
+	return time { 0, (int64_t)picoseconds * 1000 };
+}
+
+time operator""_fs(unsigned long long femtoseconds) {
+	return time { 0, (int64_t)femtoseconds };
+}
+
+};
+
+};
+
+#endif