[SOLVED] why program built by CMake run slower than program built by command

Issue

this is code below

sieve.c

/* -*- mode: c -*-
 * $Id$
 * http://www.bagley.org/~doug/shootout/
 */

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

int
main(int argc, char *argv[]) {
#ifdef SMALL_PROBLEM_SIZE
#define LENGTH 17000
#else
#define LENGTH 170000
#endif
    int NUM = ((argc == 2) ? atoi(argv[1]) : LENGTH);
    static char flags[8192 + 1];
    long i, k;
    int count = 0;

    while (NUM--) {
    count = 0; 
    for (i=2; i <= 8192; i++) {
        flags[i] = 1;
    }
    for (i=2; i <= 8192; i++) {
        if (flags[i]) {
                /* remove all multiples of prime: i */
        for (k=i+i; k <= 8192; k+=i) {
            flags[k] = 0;
        }
        count++;
        }
    }
    }
    printf("Count: %d\n", count);
    return(0);
}

this is my CMakeLists.txt

cmake_minimum_required(VERSION 3.16)
project(sieve)
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_C_COMPILER "gcc")
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O3")
add_executable(sieve   "sieve.c")

built by CMakeLists.txt result:

time ./sieve
Count: 1028

real    0m15.436s
user    0m15.431s
sys     0m0.000s

command: gcc sieve.c -O3 -o sieve

result:

time ./sieve
Count: 1028

real    0m3.087s
user    0m3.086s
sys     0m0.000s

Why is there such a significant difference? I would greatly appreciate it if someone could provide me with an answer.

cpu:Intel(R) Xeon(R) Platinum 8255C CPU @ 2.50GHz

OS: ubuntu22.04 LTS

Solution

You're not enabling optimizations correctly in you CMakeLists.txt.

The line

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O3")

adds -O3 to the arguments that are passed to g++ when compiling C++ files. But sieve.c is a C file, not a C++ file.

You need to set CMAKE_C_FLAGS (note: C, not CXX) to control the arguments passed to gcc when C files get compiled:

set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -O3")

Answered By - Brian61354270

Answer Checked By - Willingham (WPSolving Volunteer)