Re: Ile cykli zajmuje mnożenie liczb 64-bitowych?

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#217887

From: osobli...
Date: Mon, 15 May 2023 13:10

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A gdyby użyć tego:

https://godbolt.org

Jak wrzucam sobie tam kod:

#include <cstdint>

 inline uint64_t rotl(const uint64_t x, int k) {
	return (x << k) | (x >> (64 - k));
}


uint64_t s[2] = {1,2};

uint64_t next(void) {
	const uint64_t s0 = s[0];
	uint64_t s1 = s[1];
	const uint64_t result = s0 + s1;

	s1 ^= s0;
	s[0] = rotl(s0, 24) ^ s1 ^ (s1 << 16); // a, b
	s[1] = rotl(s1, 37); // c

	return result;
}

To ma sens analiza tego:

movq s(%rip), %rax
movq s+8(%rip), %rsi
movq %rax, %rdx
movq %rax, %rcx
addq %rsi, %rax
xorq %rsi, %rdx
rolq $24, %rcx
movq %rdx, %rdi
xorq %rdx, %rcx
rorq $27, %rdx
salq $16, %rdi
movq %rdx, s+8(%rip)
xorq %rdi, %rcx
movq %rcx, s(%rip)
ret

Jak rozumiem wszystko to są standardowe nazwy instrukcji? Więc wystarczy zsumować ich cykle? Mogę założyć, że movq to 2 cykle, xorq to 1 cykl, salq to 1 cykl?

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