GCC vs repne scasl: wstrlen bench

main.c

#include <stddef.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>

extern size_t wstrlen_c(unsigned int *);
extern size_t wstrlen_asm(unsigned int *);

int main() {
    unsigned int *data = malloc(1001 * sizeof(int));
    memset(data, 'h', 1000 * sizeof(int));
    data[1000] = 0;

    size_t l = WSTRLEN(data);
    if (l != 1000)
        printf("bad: wanted 1000, got %lu\n", l);

    size_t sum = 0;
    for (int i = 0; i < 1000000; i++)
        sum += WSTRLEN(data);
    return sum;
}

Makefile

.PHONY: all bench

all: bench

bench: main_c main_asm
	@echo "Running the C version..."
	@bash -c 'time ./main_c'
	@echo
	@echo "Running the asm version..."
	@bash -c 'time ./main_asm'

wstrlen_c.o: wstrlen_c.c
	gcc -O3 -c wstrlen_c.c

wstrlen_asm.o: wstrlen_asm.s
	gcc -O3 -c wstrlen_asm.s

main_c: wstrlen_c.o main.c
	gcc -Wall -O3 -DWSTRLEN=wstrlen_c main.c wstrlen_c.o -o main_c

main_asm: wstrlen_asm.o main.c
	gcc -Wall -O3 -DWSTRLEN=wstrlen_asm main.c wstrlen_asm.o -o main_asm

output.txt

Running the C version...

real	0m0.250s
user	0m0.250s
sys	0m0.000s

Running the asm version...

real	0m0.505s
user	0m0.504s
sys	0m0.000s

wstrlen_asm.s

.text
.globl wstrlen_asm
wstrlen_asm:
    xor %eax, %eax
    xor %ecx, %ecx
    dec %rcx
    mov %ax, %es
    cld
    repne scasl
    not %ecx
    dec %ecx
    mov %ecx, %eax
    ret

wstrlen_c.c

#include <stddef.h>

size_t wstrlen_c(unsigned int *str) {
    size_t len = 0;
    while (str[len])
        len++;
    return len;
}