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POWhy does GCC insert seemingly non-essential instructions before a printf call?
 

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  1. POWhy does GCC insert seemingly non-essential instructions before a printf call?
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    <p>I'm trying to learn x86 on my own and I decided to dissect a simple c program and see what GCC outputs. The program is this:</p> <pre><code>#include &lt;stdio.h&gt; int main() { printf("%s","Hello World"); return 0; } </code></pre> <p>I compiled the code with -S and then stripped out things that I found unnecessary and reduced the assembly code to this.</p> <pre><code>.pfArg: .string "%s" .text .Hello: .string "Hello World" .text .globl main .type main, @function main: pushq %rbp # push what was in base pointer onto stack movq %rsp, %rbp # move stack pointer to base pointer subq $16, %rsp # subtract 16 from sp and store in stack pointer # prepare arguments for printf movl $.Hello, %esi # put &amp; of "Hello World" into %esi movq $.pfArg, %rdi # put &amp; of "%d" into %eax call printf leave ret </code></pre> <p>Now almost everything in the code above makes sense to me except the first two under main. Although this is what I get without stripping things out.</p> <pre><code>.LC0: .string "%s" .LC1: .string "Hello World" .text .globl main .type main, @function main: .LFB0: pushq %rbp # push what was in base pointer onto stack movq %rsp, %rbp # move stack pointer to base pointer # prepare arguments for printf movl $.LC0, %eax # put arg into %eax movl $.LC1, %esi # put second arg into %esi movq %rax, %rdi # move value in %rax to %rdi ???? ( why not just put $.LCO into %rax directly ) movl $0, %eax # clear out %eax ???? ( why do we need to clear it out ) call printf movl $0, %eax # return 0 leave ret .LFE0: .size main, .-main .ident "GCC: (Ubuntu/Linaro 4.5.2-8ubuntu4) 4.5.2" .section .note.GNU-stack,"",@progbits </code></pre> <p>There are 2 instructions that I've marked with ???? that I don't understand. </p> <p>The first instruction is moving what is in %rax into %rdi to prepare for the printf call. Thats all fine except we just moved $.LC0 (which is the string "%s") into %eax. This seems unnecessary why didn't we just move $.LC0 into %rdi in the first place instead of moving it into %eax and then into %rdi?</p> <p>The second instruction is clearing out %eax which I understand to be the return value of a function. But if the function is going to just clobber it anyways why do GCC care to clear it out?</p>
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    1. USCiro Santilli 新疆改造中心 六四事件 法轮功
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    1. COLooks like you are using a x86-64 compiler rather than x86.
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    2. COYeah you are right I forgot to say that.
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