GCC Optimizations

Introduction#

The GNU compiler offers various levels of optimizations for the compilation process. These optimizations are used to improve the code performance and/or code size. Compiling a code with optimizations on, typically takes longer to complete.

This command tells you what optimizations are available on your system : $gcc -Q —help=optimizations

Here is a detailed documentation of options to control optimizations:

https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

Differnce between codes compiled with O0 and O3

I wrote a simple C code foo.c

int main()
{
    int i = 0;
    int j = 0;
    for (i = 0; i < 5; i++) {
            j = i + 1;
    }
    return 0;
}

When compiled with -O0 i.e. by disabling all compiler optimizations

$ gcc -o foo.S foo.c -O0 -S

I got this:

    .file   "foo.c"
    .text
    .globl  main
    .type   main, @function
main:
.LFB0:
    .cfi_startproc
    pushq   %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset 6, -16 
    movq    %rsp, %rbp
    .cfi_def_cfa_register 6
    movl    $0, -4(%rbp)
    movl    $0, -8(%rbp)
    movl    $0, -4(%rbp)
    jmp     .L2
.L3:
    movl    -4(%rbp), %eax
    addl    $1, %eax
    movl    %eax, -8(%rbp)
    addl    $1, -4(%rbp)
.L2:
    cmpl    $4, -4(%rbp)
    jle     .L3 
    movl    $0, %eax
    popq    %rbp
    .cfi_def_cfa 7, 8
    ret
    .cfi_endproc
.LFE0:
    .size   main, .-main
    .ident  "GCC: (GNU) 6.2.0"
    .section        .note.GNU-stack,"",@progbits

GCC took all the pain to convert my code into assembly language verbatim.

But when I compiled my code with O3 i.e. with the highest level of optimizations

I got this:

.file   "foo.c"
.section        .text.startup,"ax",@progbits
.p2align 4,,15
.globl  main
.type   main, @function
main:
.LFB11:
    .cfi_startproc
    xorl    %eax, %eax
    ret
    .cfi_endproc
.LFE11:
    .size   main, .-main
    .ident  "GCC: (GNU) 6.2.0"
    .section        .note.GNU-stack,"",@progbits

GCC understood that I was just doodling and doing nothing important with the variables and the loop. So it left me a blank stub with no code.

DAYUM!