func.cpp

#include "func.hpp"

#include <iostream>
#include <string.h>
#include <gmpxx.h>

void fff(mp_limb_t *x) {
    memset(x, 0, sizeof(mp_limb_t)*4);
}

void fff(mp_limb_t *x, size_t n) {
    memset(x, 0, sizeof(mp_limb_t)*n);
}

void ggg(mp_limb_t *x) {
    mpn_zero(x, 4);
}

void ggg(mp_limb_t *x, size_t n) {
    mpn_zero(x, n);
}

func.hpp

#pragma once

#include <gmpxx.h>

void fff(mp_limb_t*);
void fff(mp_limb_t*, size_t);
void ggg(mp_limb_t*);
void ggg(mp_limb_t*, size_t);

main.cpp

#include "func.hpp"
#include <iostream>
#include <time.h>
#include <gmpxx.h>

int main() {
    const int N = 10000000;
    time_t begin, end;
    mp_limb_t x[4];

    begin = clock();
    for (size_t i = 0; i < N; i++) {
        fff(x, 4);
    }
    end = clock();
    printf("memset(x, 4):\t%f [nsec]\n", double(end-begin)*1e9/N/CLOCKS_PER_SEC);

    begin = clock();
    for (size_t i = 0; i < N; i++) {
        ggg(x, 4);
    }
    end = clock();
    printf("mpn_zero(x, 4):\t%f [nsec]\n\n", double(end-begin)*1e9/N/CLOCKS_PER_SEC);


    begin = clock();
    for (size_t i = 0; i < N; i++) {
        fff(x);
    }
    end = clock();
    printf("memset(x):\t%f [nsec]\n", double(end-begin)*1e9/N/CLOCKS_PER_SEC);

    begin = clock();
    for (size_t i = 0; i < N; i++) {
        ggg(x);
    }
    end = clock();
    printf("mpn_zero(x):\t%f [nsec]\n\n", double(end-begin)*1e9/N/CLOCKS_PER_SEC);

}

Makefile

CXXFLAGS=-O3 -Wall -Wextra
LDFLAGS=-lgmpxx -lgmp

all: main

%.o: %.cpp
        $(CXX) $< $(CXXFLAGS) -c

main: main.o func.o
        $(CXX) main.o func.o $(CXXFLAGS) $(LDFLAGS)


.PHONY:
        clean

clean:
        $(RM) *.o a.out

result.txt

## CPUINFO
model name      : AMD Ryzen 7 3700X 8-Core Processor
cpu MHz         : 3600.000
/* movups命令が利用可能 */

$ make && repeat 20 ./a.out
...
memset(x, 4):   2.116500 [nsec]
mpn_zero(x, 4): 2.592400 [nsec]

memset(x):      1.173900 [nsec]
mpn_zero(x):    2.616200 [nsec]