fizzbuzz using ymm + non-overlapping writes of uint32

fizzbuzz-overwrite32.c

#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <immintrin.h>
#include <unistd.h>

#define inline __attribute__((always_inline))

static char tens[48] __attribute__((aligned(64)));
static size_t tenslen = 0;
static int n, regen = 1;
static unsigned tens32;

static char outbuf[8000], *outp = outbuf;

static void flush(void)
{
    write(1, outbuf, outp - outbuf);
    outp = outbuf;
}

static void flush_and_exit(void)
{
    flush();
    exit(0);
}

static void plus10(void)
{
    if (tenslen == 0)
        tenslen = 1;

    size_t i = 15;
    for (i = 15;; i--) {
       if (__builtin_expect(++tens[i] <= '9', 1))
           break;
       tens[i] = '0';
       if (16 - i == tenslen)
           ++tenslen;
        if (i <= 12)
            regen = 2;
    }
    if (i == 15) {
        tens32 += 0x1000000; /* avoid partial forward */
    } else {
        tens32 = *(unsigned *)(tens + 12);
    }
}

static inline void emit30(int maxval)
{
#define EMIT(s1, s2, s3) \
    do { \
        if (++n > maxval) flush_and_exit(); \
        if (__builtin_expect(regen, 0)) { \
            static const char s[] __attribute__((aligned(64))) = "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0" s1 s2 s3 "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"; \
            _mm256_storeu_si256((__m256i *)outp, \
                                _mm256_or_si256(tens256, \
                                                _mm256_loadu_si256((__m256i *)(s + 31 - tenslen)))); \
        } else { \
            *(unsigned *)(outp + tenslen - 4) = tens32; \
        } \
        outp += tenslen + 2; \
        if (sizeof(s2) != 1) { \
            if (++n > maxval) flush_and_exit(); \
            outp += sizeof(s2) - 1; \
            if (sizeof(s3) != 1) { \
                if (++n > maxval) flush_and_exit(); \
                outp += sizeof(s3) - 1; \
            } \
        } \
    } while (0)

    __m256i tens256;

    if (__builtin_expect(regen, 0))
        tens256 = _mm256_loadu_si256((__m256i *)(tens + 16 - tenslen));

    EMIT("1\n", "", "");
    EMIT("2\n", "fizz\n", "");
    EMIT("4\n", "buzz\n", "fizz\n");
    EMIT("7\n", "", "");
    EMIT("8\n", "fizz\n", "buzz\n");

    plus10();
    if (__builtin_expect(regen, 0))
        tens256 = _mm256_loadu_si256((__m256i *)(tens + 16 - tenslen));

    EMIT("1\n", "fizz\n", "");
    EMIT("3\n", "", "");
    EMIT("4\n", "fizzbuzz\n", "");
    EMIT("6\n", "", "");
    EMIT("7\n", "fizz\n", "");
    EMIT("9\n", "buzz\n", "fizz\n");

    plus10();
    if (__builtin_expect(regen, 0))
        tens256 = _mm256_loadu_si256((__m256i *)(tens + 16 - tenslen));

    EMIT("2\n", "", "");
    EMIT("3\n", "fizz\n", "buzz\n");
    EMIT("6\n", "fizz\n", "");
    EMIT("8\n", "", "");
    EMIT("9\n", "fizzbuzz\n", "");

    plus10();
}

int main(int argc, char **argv)
{
    int maxval;

    if (argc < 2 || sscanf(argv[1], "%d", &maxval) != 1) {
        fprintf(stderr, "usage: %s <max-value>\n", argv[0]);
        exit(1);
    }
    memset(tens, '0', 16);

    int prevgen = 1;
    while (n + 30 < maxval) {
        emit30(INT_MAX);
        if (__builtin_expect(outp - outbuf >= sizeof(outbuf) - 500, 0)) {
            flush();
            if (regen != 0 && n >= 10000 && prevgen >= 10000)
                --regen;
            prevgen = n + 1;
        }
    }
    emit30(maxval);

    return 0;
}