I created two C programs

 1. Program 1

        int main()
        {
        }

 2. Program 2

        int main()
        {
        //Some Harmless comments
        }

AFAIK, when compiling, the compiler(gcc) should ignore the comments and redundant whitepaces, and hence the output must be similar.

But when I checked the md5sums of the output binaries, they don&#39;t match. I also tried compiling with optimisation `-O3` and `-Ofast` but they still didn&#39;t match.

What is happening here?

EDIT:
 the exact commands and there md5sums are(t1.c is program 1 and t2.c is program 2)

    gcc ./t1.c -o aaa
    gcc ./t2.c -o bbb
    98c1a86e593fd0181383662e68bac22f  aaa
    c10293cbe6031b13dc6244d01b4d2793  bbb

    gcc ./t2.c -Ofast -o bbb
    gcc ./t1.c -Ofast -o aaa
    2f65a6d5bc9bf1351bdd6919a766fa10  aaa
    c0bee139c47183ce62e10c3dbc13c614  bbb


    gcc ./t1.c -O3 -o aaa
    gcc ./t2.c -O3 -o bbb
    564a39d982710b0070bb9349bfc0e2cd  aaa
    ad89b15e73b26e32026fd0f1dc152cd2  bbb

And yes, md5sums match across multiple compilations with same flags.

BTW my system is `gcc (GCC) 5.2.0` and `Linux 4.2.0-1-MANJARO #1 SMP PREEMPT x86_64 GNU/Linux`

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Why don't two binaries of programs with only comments changed exactly match in gcc?