8 bit positional popcount with AVX2 without too much code

harness.c

#define _XOPEN_SOURCE 700
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

extern void pospopcnt_reg(int accum[8], const char *buf, size_t len);
extern void pospopcnt_mem(int accum[8], const char *buf, size_t len);

extern void
pospopcnt_naive(int accum[8], const char *buf, size_t len)
{
	size_t i;
	int j;

	for (i = 0; i < len; i++)
		for (j = 0; j < 8; j++)
			accum[j] += !!(buf[i] & 1 << j);
}

/*
 * Compute the difference of two struct timespec.
 */
static struct timespec
tsdiff(struct timespec a, struct timespec b)
{
	a.tv_sec -= b.tv_sec;
	a.tv_nsec -= b.tv_nsec;
	if (a.tv_nsec < 0) {
		a.tv_sec -= 1;
		a.tv_nsec += 1000000000;
	}

	return (a);
}

/* perform a benchmark */
static void benchmark(const char *buf, size_t len, const char *name, void (*pospopcnt)(int[8], const char *, size_t))
{
	struct timespec diff, start, end;
	double dur;
	long i, n = 1;
	int accum[8] = {0};

	do {

		clock_gettime(CLOCK_REALTIME, &start);
		for (i = 0; i < n; i++)
			pospopcnt(accum, buf, len);
		clock_gettime(CLOCK_REALTIME, &end);
		diff = tsdiff(end, start);
		n <<= 1;
	} while (diff.tv_sec == 0);

	n >>= 1;
	dur = diff.tv_sec + diff.tv_nsec / 1000000000.0;
	dur /= n;
	printf("%s\t%g B/s\n", name, len / dur);
}

extern int
main(int argc, char *argv[])
{
	size_t len = 8192;
	int naive_accum[8], asm_accum[8];
	FILE *random;

	char *buf;

	if (argc > 1)
		len = atoll(argv[1]) + 31 & ~31LL;

	buf = malloc(len);
	if (buf == NULL) {
		perror("malloc");
		return (EXIT_FAILURE);
	}

	random = fopen("/dev/urandom", "rb");
	if (random == NULL) {
		perror("/dev/urandom");
		return (EXIT_FAILURE);
	}

	fread(buf, 1, len, random);

	memset(naive_accum, 0, sizeof naive_accum);
	memset(asm_accum, 0, sizeof asm_accum);

	pospopcnt_reg(asm_accum, buf, len);
	pospopcnt_naive(naive_accum, buf, len);

	if (memcmp(asm_accum, naive_accum, 8*4) != 0)
		printf("mismatch\n");

	benchmark(buf, len, "naive", pospopcnt_naive);
	benchmark(buf, len, "addmem", pospopcnt_mem);
	benchmark(buf, len, "addreg", pospopcnt_reg);
}

pospopcnt.s

	# pospopcnt(accum, buf, len)
	# len must be a multiple of 32
	.globl pospopcnt_mem
	.type pospopcnt_mem, @function
pospopcnt_mem:
	# rdi: accum
	# rsi: buf
	# rdx: len
	test	%rdx, %rdx		# any work to do?
	jnz	0f
	ret

	.align	16
0:	vmovdqu	(%rsi), %ymm0
	add	$32, %rsi
	vpmovmskb %ymm0, %eax
	popcnt	%eax, %eax
	add	%eax, 7*4(%rdi)
	vpaddd	%ymm0, %ymm0, %ymm0
	vpmovmskb %ymm0, %eax
	popcnt	%eax, %eax
	add	%eax, 6*4(%rdi)
	vpaddd	%ymm0, %ymm0, %ymm0
	vpmovmskb %ymm0, %eax
	popcnt	%eax, %eax
	add	%eax, 5*4(%rdi)
	vpaddd	%ymm0, %ymm0, %ymm0
	vpmovmskb %ymm0, %eax
	popcnt	%eax, %eax
	add	%eax, 4*4(%rdi)
	vpaddd	%ymm0, %ymm0, %ymm0
	vpmovmskb %ymm0, %eax
	popcnt	%eax, %eax
	add	%eax, 3*4(%rdi)
	vpaddd	%ymm0, %ymm0, %ymm0
	vpmovmskb %ymm0, %eax
	popcnt	%eax, %eax
	add	%eax, 2*4(%rdi)
	vpaddd	%ymm0, %ymm0, %ymm0
	vpmovmskb %ymm0, %eax
	popcnt	%eax, %eax
	add	%eax, 1*4(%rdi)
	vpaddd	%ymm0, %ymm0, %ymm0
	vpmovmskb %ymm0, %eax
	popcnt	%eax, %eax
	add	%eax, 0*4(%rdi)
	sub	$32, %rdx
	jnz	0b
	vzeroupper
	ret

	.globl pospopcnt_reg
	.type pospopcnt_reg, @function
pospopcnt_reg:
	# rdi: accum
	# rsi: buf
	# rdx: len
	test	%rdx, %rdx		# any work to do?
	jnz	1f
	ret

1:	push	%rbp
	push	%rbx
	push	%r12

	mov	0*4(%rdi), %r8d
	mov	1*4(%rdi), %r9d
	mov	2*4(%rdi), %r10d
	mov	3*4(%rdi), %r11d
	mov	4*4(%rdi), %r12d
	mov	5*4(%rdi), %ebx
	mov	6*4(%rdi), %ecx
	mov	7*4(%rdi), %ebp

	.align	16
0:	vmovdqu	(%rsi), %ymm0
	add	$32, %rsi
	vpmovmskb %ymm0, %eax
	popcnt	%eax, %eax
	add	%eax, %ebp
	vpaddd	%ymm0, %ymm0, %ymm0
	vpmovmskb %ymm0, %eax
	popcnt	%eax, %eax
	add	%eax, %ecx
	vpaddd	%ymm0, %ymm0, %ymm0
	vpmovmskb %ymm0, %eax
	popcnt	%eax, %eax
	add	%eax, %ebx
	vpaddd	%ymm0, %ymm0, %ymm0
	vpmovmskb %ymm0, %eax
	popcnt	%eax, %eax
	add	%eax, %r12d
	vpaddd	%ymm0, %ymm0, %ymm0
	vpmovmskb %ymm0, %eax
	popcnt	%eax, %eax
	add	%eax, %r11d
	vpaddd	%ymm0, %ymm0, %ymm0
	vpmovmskb %ymm0, %eax
	popcnt	%eax, %eax
	add	%eax, %r10d
	vpaddd	%ymm0, %ymm0, %ymm0
	vpmovmskb %ymm0, %eax
	popcnt	%eax, %eax
	add	%eax, %r9d
	vpaddd	%ymm0, %ymm0, %ymm0
	vpmovmskb %ymm0, %eax
	popcnt	%eax, %eax
	add	%eax, %r8d
	sub	$32, %rdx
	jnz	0b

	mov	%r8d,  0*4(%rdi)
	mov	%r9d,  1*4(%rdi)
	mov	%r10d, 2*4(%rdi)
	mov	%r11d, 3*4(%rdi)
	mov	%r12d, 4*4(%rdi)
	mov	%ebx,  5*4(%rdi)
	mov	%ecx,  6*4(%rdi)
	mov	%ebp,  7*4(%rdi)

	vzeroupper
	pop	%r12
	pop	%rbx
	pop	%rbp
	ret