C言語のループを展開して高速化する

// ©2021 Yuichiro Nakada
// gcc unrolling.c -o unrolling
#include <stdio.h>

#define DO16(e) e e e e e e e e e e e e e e e e
#define FOR16(n, e) { int c=n/16; n-=c*16; for (; c>0; c--) { DO16(e) }; do { e } while (--n>0); }
#define DO4(e) e e e e
#define FOR4(n, e) { int c=n/4; n-=c*4; for (; c>0; c--) { DO4(e) }; do { e } while (--n>0); }

int main()
{
	int n = 0;
	int len = 17;
	FOR4(len,
		printf("%d\n", n);
		n++;
	);

	n = 0;
	len = 3;
	FOR4(len,
		printf("%d\n", n);
		n++;
	);

	n = 0;
	len = 17;
	FOR16(len,
		printf("%d\n", n);
		n++;
	);
}

$ ./unrolling 
0
1
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9
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16
0
1
2
0
1
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9
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16