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◆ main openmsx / src / utils / DivModBySame.hh
2.5 KiB · C++ · last changed chore(): initial commit 1 month ago source blame history raw
e8555c89 Andy McCall 1 
#ifndef DIVISIONBYCONST_HH
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#define DIVISIONBYCONST_HH
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#include "narrow.hh"
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#include <cassert>
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#include <cstdint>
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#include <utility>
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namespace openmsx {
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/** Helper class to divide multiple times by the same number.
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 * Binary division can be performed by:
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 *   - multiplication by a magic number
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 *   - followed by an addition of a magic number
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 *   - followed by a right shift over some magic number of bits
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 * These magic constants only depend on the divisor, but they are quite
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 * expensive to calculate.
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 * However if you know you will divide many times by the same number this
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 * algorithm does make sense and can result in a big speedup, especially on
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 * CPUs which lack a hardware division instruction (like ARM).
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 *
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 * If the divisor is a compile-time constant, it's even faster to use
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 * the DivModByConst utility class.
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 */
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class DivModBySame
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{
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public:
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	void setDivisor(uint32_t divisor);
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	[[nodiscard]] uint32_t getDivisor() const { return divisor; }
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	[[nodiscard]] uint32_t div(uint64_t dividend) const
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	{
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	#if defined __x86_64 && !defined _MSC_VER
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		auto t = narrow_cast<uint64_t>((__uint128_t(dividend) * m + a) >> 64);
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		return narrow_cast<uint32_t>(t >> s);
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	#else
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		return divInC(dividend);
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	#endif
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	}
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	[[nodiscard]] uint32_t divInC(uint64_t dividend) const
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	{
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		uint64_t t1 = uint64_t(uint32_t(dividend)) * uint32_t(m);
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		uint64_t t2 = (dividend >> 32) * uint32_t(m);
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		uint64_t t3 = uint32_t(dividend) * (m >> 32);
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		uint64_t t4 = (dividend >> 32) * (m >> 32);
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		uint64_t s1 = uint64_t(uint32_t(a)) + uint32_t(t1);
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		uint64_t s2 = (s1 >> 32) + (a >> 32) + (t1 >> 32) + t2;
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		uint64_t s3 = uint64_t(uint32_t(s2)) + uint32_t(t3);
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		uint64_t s4 = (s3 >> 32) + (s2 >> 32) + (t3 >> 32) + t4;
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		uint64_t result = s4 >> s;
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	#ifdef DEBUG
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		// we don't even want this overhead in devel builds
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		assert(result == uint32_t(result));
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	#endif
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		return uint32_t(result);
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	}
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	[[nodiscard]] std::pair<uint32_t, uint32_t> divMod(uint64_t dividend) const
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	{
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		assert(uint32_t(divisor) == divisor); // must fit in 32-bit
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		uint64_t q = div(dividend);
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		assert(uint32_t(q) == q); // must fit in 32 bit
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		// result fits in 32-bit, so no 64-bit calculations required
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		uint32_t r = uint32_t(dividend) - uint32_t(q) * uint32_t(divisor);
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		return {uint32_t(q), r};
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	}
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	[[nodiscard]] uint32_t mod(uint64_t dividend) const
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	{
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		auto [q, r] = divMod(dividend);
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		(void)q;
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		return r;
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	}
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private:
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	uint64_t m = 0;
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	uint64_t a = 0;
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	uint32_t s = 0;
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	uint32_t divisor= 0; // only used by mod() and getDivisor()
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};
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} // namespace openmsx
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#endif // DIVISIONBYCONST_HH