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    <p>Not replying the question but giving an example of compile time optimization. gcc optimizes the code when asked to do so. The -O (Optimize) option enables optimization in different leves. It can be used as -O1, -O2 and -O3. The gcc man page describes precisely the meaning of each level.</p> <p>The -S option translates C into assembly and saves on .s file.</p> <p>test.c</p> <pre><code>#include &lt;stdio.h&gt; int abc;//Global variable void main() { abc = 3; if(abc == 3) printf("abc will be always 3"); else printf("This will never executed"); } </code></pre> <p><strong>Whitout gcc optimization the two strings appear on the assembly code.</strong></p> <p>$ gcc -S test.c;cat test.s</p> <pre><code> .file "test.c" .comm abc,4,4 .section .rodata .LC0: .string "abc will be always 3" .LC1: .string "This will never executed" .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 $3, abc(%rip) movl abc(%rip), %eax cmpl $3, %eax jne .L2 movl $.LC0, %eax movq %rax, %rdi movl $0, %eax call printf jmp .L1 .L2: movl $.LC1, %eax movq %rax, %rdi movl $0, %eax call printf .L1: popq %rbp .cfi_def_cfa 7, 8 ret .cfi_endproc .LFE0: .size main, .-main .ident "GCC: (GNU) 4.6.1 20110908 (Red Hat 4.6.1-9)" .section .note.GNU-stack,"",@progbits </code></pre> <p><strong>Whit gcc level 1 optimization only one string is translated into assembly</strong></p> <p>$ gcc -O1 -S test.c;cat test.s</p> <pre><code> .file "test.c" .section .rodata.str1.1,"aMS",@progbits,1 .LC0: .string "abc will be always 3" .text .globl main .type main, @function main: .LFB11: .cfi_startproc subq $8, %rsp .cfi_def_cfa_offset 16 movl $3, abc(%rip) movl $.LC0, %edi movl $0, %eax call printf addq $8, %rsp .cfi_def_cfa_offset 8 ret .cfi_endproc .LFE11: .size main, .-main .comm abc,4,4 .ident "GCC: (GNU) 4.6.1 20110908 (Red Hat 4.6.1-9)" .section .note.GNU-stack,"",@progbits </code></pre>
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    1. POWhen code optimization happens?
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    1. USPeter Senna
    1. USPeter Senna
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    1. COExcellent.... explanation
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      1. USRasmi Ranjan Nayak
 

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