Here's a cleanup version:

  #ifndef _GNU_SOURCE
  #define _GNU_SOURCE 1
  #endif
  
  #include <errno.h>    /* ERANGE errno */
  #include <inttypes.h> /* strtoumax(3) */
  #include <limits.h>   /* LONG_MAX */
  #include <stdint.h>   /* uintmax_t */
  #include <stdio.h>    /* fflush(3) fprintf(3) fread(3) fwrite(3) tmpfile(3) */
  #include <stdlib.h>   /* EXIT_FAILURE */
  #include <string.h>   /* strlen(3) */
  
  #include <err.h>      /* err(3) errx(3) */
  #include <fcntl.h>    /* loff_t splice(2) */
  #include <sys/time.h> /* struct timeval gettimeofday(2) */
  #include <unistd.h>   /* _SC_PAGESIZE pread(2) sysconf(3) write(2) */
  
  #ifndef HAVE_SPLICE
  #define HAVE_SPLICE __linux__
  #endif
  
  #define MIN(a, b) (((a) < (b))? (a) : (b))
  #define MAX(a, b) (((a) > (b))? (a) : (b))
  
  #ifndef howmany
  #define howmany(x, y)  (((x) + ((y) - 1)) / (y))
  #endif
  
  #define UMAX_PREC (sizeof (uintmax_t) * CHAR_BIT) /* NB: assumes no padding */
  #define UMAX_HALF ((UMAX_PREC + 1) / 2)
  #define UMAX_LO(n) ((n) & ((UINTMAX_C(1) << UMAX_HALF) - 1))
  #define UMAX_HI(n) ((n) >> UMAX_HALF)
  
  static inline _Bool
  add_overflow(uintmax_t *r, const uintmax_t a, const uintmax_t b)
  {
          if (~a < b)
                  return 1;
          *r = a + b;
          return 0;
  }
  
  /*
   * Implement multiplication using a polynomial with four multiplications and
   * three additions, except we can optimize out some operations.
   */
  static inline _Bool
  mul_overflow(uintmax_t *_r, const uintmax_t _a, const uintmax_t _b)
  {
    uintmax_t a[2] = { UMAX_LO(_a), UMAX_HI(_a) };
    uintmax_t b[2] = { UMAX_LO(_b), UMAX_HI(_b) };
    uintmax_t r[2];
  
    /* if both are non-0, we'd always overflow */
    if (a[1] && b[1])
      return 1;
  
    /* either a[1] or b[1] must be 0 here, so no intermediate overflow */
    r[1] = (a[1] * b[0]) + (a[0] * b[1]);
  
    /* if the result has MSW bits set, we'd overflow */
    if (UMAX_HI(r[1]))
      return 1;
  
    r[0] = a[0] * b[0];
  
    return add_overflow(_r, r[0], r[1] << UMAX_HALF);
  }
  
  #if 0
  static uintmax_t
  add(uintmax_t a, uintmax_t b)
  {
    uintmax_t r;
    if (add_overflow(&r, a, b))
      errx(1, "arithmetic overflow (%ju * %ju)", a, b);
    return r;
  }
  #endif
  
  static uintmax_t
  mul(uintmax_t a, uintmax_t b)
  {
    uintmax_t r;
    if (mul_overflow(&r, a, b))
      errx(1, "arithmetic overflow (%ju * %ju)", a, b);
    return r;
  }
  
  static uintmax_t
  toumax(const char *p)
  {
    char *pe;
    uintmax_t n;
  
    errno = 0;
    if (UINTMAX_MAX == (n = strtoumax(p, &pe, 10)) && errno != 0)
      err(1, "%s", p);
    if (*pe != '\0' || p == pe)
      errx(1, "%s: invalid number", p);
  
    return n;
  }
  
  static uintmax_t
  gcd(uintmax_t a, uintmax_t b)
  {
    uintmax_t c;
  
    while (a != 0) {
      c = a;
      a = b % a;
      b = c;
    }
  
    return b;
  }
  
  static uintmax_t
  lcm(uintmax_t a, uintmax_t b)
  {
    return mul(a, b) / gcd(a, b);
  }
  
  static size_t
  xgetpagesize(void)
  {
    long n = sysconf(_SC_PAGESIZE);
    if (n <= 0)
      err(1, "sysconf");
    return n;
  }
  
  static double
  gtod(void)
  {
    struct timeval now;
    gettimeofday(&now, NULL);
    return now.tv_sec + (now.tv_usec / 1000000.0);
  }
  
  int
  main(int argc, char **argv)
  {
    _Bool cflag = 0, nflag = 0;
    uintmax_t count = UINTMAX_MAX, lines = 0, total = 0;
    int optc;
  
    while (-1 != (optc = getopt(argc, argv, "c:n:"))) {
      switch (optc) {
      case 'c':
        if (nflag)
          errx(1, "-c and -n mutually exclusive");
        count = toumax(optarg);
        cflag = 1;
        break;
                  case 'n':
                    if (cflag)
          errx(1, "-c and -n mutually exclusive");
        lines = toumax(optarg);
        nflag = 1;
        break;
      default:
        return EXIT_FAILURE;
      }
    }
    argc -= optind;
    argv += optind;
  
    const char *const line = (argc > 0)? argv[0] : "y";
    const size_t linesize = strlen(line) + 1;
    const size_t pagesize = xgetpagesize();
    const size_t blocksize = MAX(mul(pagesize, 16), lcm(linesize, pagesize));
    const uintmax_t filesize = mul(blocksize, howmany((1UL << 20), blocksize));
  
    if (nflag)
      count = mul(linesize, lines);
  
  //  fprintf(stderr, "linesize:  %zu\n", linesize);
  //  fprintf(stderr, "pagesize:  %zu\n", xgetpagesize());
  //  fprintf(stderr, "blocksize: %ju\n", blocksize);
  //  fprintf(stderr, "filesize:  %ju\n", filesize);
  
    FILE *fh;
    if (!(fh = tmpfile()))
      err(1, "tmpfile");
    for (size_t i = 0; i < filesize; i += linesize) {
      int n = fprintf(fh, "%s\n", line);
      if (n == -1)
        err(1, "fprintf");
      if ((size_t)n != linesize)
        errx(1, "wrote %d bytes, expected %zu", n, linesize);
    }
    if (0 != fflush(fh))
      err(1, "fflush");
  
    double begin = gtod();
  #if HAVE_SPLICE
    int fd = fileno(fh);
    _Bool eof = 0;
    do {
      loff_t p = 0;
      size_t r;
      ssize_t n;
  
      /* NB: no LOFF_MAX available */
      _Static_assert(sizeof p <= sizeof (long), "unexpected type for loff_t");
      if (filesize > LONG_MAX)
        errx(1, "filesize too large (%ju > %jd)", filesize, (intmax_t)LONG_MAX);
  
      while ((size_t)p < filesize && count >= blocksize && !eof) {
        r = MIN(blocksize, count);
        r -= r % blocksize;
        n = splice(fd, &p, STDOUT_FILENO, NULL, r, SPLICE_F_MOVE);
        if (n == -1)
          err(1, "splice");
        p += n;
        count -= n;
        total += n;
        eof = n == 0;
      }
    } while (!eof && (count || (!cflag && !nflag)));
  #endif
    {
      char buf[BUFSIZ], *p, *pe;
      size_t n;
  
      while (count) {
        rewind(fh);
        if (!(n = fread(buf, 1, MIN(sizeof buf, count), fh))) {
          if (ferror(fh))
            err(1, "fread");
          break;
        }
  
        p = buf;
        pe = &buf[n];
        while (p < pe) {
          if (!(n = fwrite(p, 1, (size_t)(pe - p), stdout)))
            err(1, "fwrite");
          p += n;
          count -= n;
          total += n;
        }
      }
    }
    if (0 != fflush(stdout))
      err(1, "fflush");
  
    double elapsed = gtod() - begin;
    fprintf(stderr, "%.3f GB/s (%zu bytes in %.3f seconds)\n", ((double)total / elapsed) / (1UL<<30), total, elapsed);
  
    return 0;
  }