Fastest way to sum integers in text file

Makefile

NAME := sum1Gints
CC := gcc -std=c89
INPUTFILE := /tmp/1G-int-per-line

.PHONY: clean run

run: $(NAME)
	/usr/bin/time ./$< $(INPUTFILE)

# Recommended gcc warning options for C, from:
# http://stackoverflow.com/a/1667114
%-debug: %.c
	$(CC) -pedantic -Wall \
-Wno-missing-braces -Wextra -Wno-missing-field-initializers -Wformat=2 \
-Wswitch-default -Wswitch-enum -Wcast-align -Wpointer-arith \
-Wbad-function-cast -Wstrict-overflow=5 -Wstrict-prototypes -Winline \
-Wundef -Wnested-externs -Wcast-qual -Wshadow -Wunreachable-code \
-Wlogical-op -Wfloat-equal -Wstrict-aliasing=2 -Wredundant-decls \
-Wold-style-definition -Werror \
-ggdb3 \
-O0 \
-fno-omit-frame-pointer -ffloat-store -fno-common -fstrict-aliasing \
-lm -o $@ $^

%: %.c
	$(CC) -pedantic -O3 -DNDEBUG -flto -o $@ $^

clean:
	rm ./$(NAME) ./$(NAME)-debug -f

# $@ - current target
# $* '%'-part (works if there *is* '%' in specification)
# $< first dependence
# $^ all dependencies (without duplicates)

sum1Gints-maaartinus-mmap.c

/** To measure time performance, run:

   $ make NAME=sum1Gints-maaartinus-mmap

   To clear file caches (as a root):

   # free && sync && echo 3 > /proc/sys/vm/drop_caches && free

   Algorithm from:

   http://stackoverflow.com/questions/25606833/fastest-way-to-sum-integers-in-text-file/25607155#comment40046670_25607155

mmap code is from the mmap man-page

*/
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <inttypes.h>
#include <ctype.h>

#define report_error_and_exit(msg) \
    do { perror(msg); exit(EXIT_FAILURE); } while (0)

int
main(int argc, char *argv[])
{
    if (argc == 2) {
      uint64_t total_sum = 0; /* sum of all (nonnegative) integers in the file */

      int fd = -1;
      struct stat sb = {0}; /* fstat result */
      size_t length = 0; /* file size */

      fd = open(argv[1], O_RDONLY);
      if (fd == -1)
        report_error_and_exit("open");

      if (fstat(fd, &sb) == -1) /* obtain file size */
        report_error_and_exit("fstat");
      length = sb.st_size;

      if (length > 0) {
	int32_t partial = 0; /* hold part of the current integer that is parsed */
	char *addr = mmap(NULL, length, PROT_READ, MAP_PRIVATE, fd, 0);
	char *p = addr;
	if (addr == MAP_FAILED)
	  report_error_and_exit("mmap");

	for (; p != &addr[length]; ++p) {
	  if (isdigit(*p)) {
	    partial = 10*partial + *p - '0';
	  }
	  else if (partial) { /* non-digit found */
	    total_sum += partial; /* add the parsed number */
	    partial = 0; /* start new number */
	  }
	}
      }
      printf("%" PRIu64 "\n", total_sum); /* print result */
      exit(EXIT_SUCCESS);
    }
    else { /* argc != 2 */
      fprintf(stderr, "Usage: %s filename\n", argv[0]);
      exit(2);
    }
}