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POstack frame align to different size?
 

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  1. POstack frame align to different size?
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    <p>It's a simple test. A similar topic has been discussed in <a href="https://stackoverflow.com/questions/1061818/stack-allocation-padding-and-alignment">Stack allocation, padding, and alignment</a> but when i compile the following two source code i have conflicting results.</p> <pre><code> #include &lt;stdio.h&gt; //first source code void haha() { printf("HI"); } int main() { int b = 2; b = b &lt;&lt; 1; haha(); //printf("n = %d",b); return 0; } </code></pre> <hr> <pre><code> #include &lt;stdio.h&gt; //second source code void haha3() { int c,a,b; c=a+b; } void haha2() { int c,a,b; c=a+b; haha3(); } int main() { int b = 2; b = b &lt;&lt; 1; haha2(); //printf("n = %d",b); return 0; } </code></pre> <p>For the first source code i got the assembly code:</p> <pre><code>haha: pushl %ebp movl %esp, %ebp subl $24, %esp movl $.LC0, %eax movl %eax, (%esp) call printf leave ret .size haha, .-haha .globl main .type main, @function main: pushl %ebp movl %esp, %ebp andl $-16, %esp subl $16, %esp movl $2, 12(%esp) sall 12(%esp) call haha movl $0, %eax leave ret </code></pre> <p>For the second source code i got the assembly code:</p> <pre><code>haha3: pushl %ebp movl %esp, %ebp subl $16, %esp movl -4(%ebp), %eax movl -8(%ebp), %edx leal (%edx,%eax), %eax movl %eax, -12(%ebp) leave ret .size haha3, .-haha3 .globl haha2 .type haha2, @function haha2: pushl %ebp movl %esp, %ebp subl $16, %esp movl -4(%ebp), %eax movl -8(%ebp), %edx leal (%edx,%eax), %eax movl %eax, -12(%ebp) call haha3 leave ret .size haha2, .-haha2 .globl main .type main, @function main: pushl %ebp movl %esp, %ebp subl $16, %esp movl $2, -4(%ebp) sall -4(%ebp) call haha2 movl $0, %eax leave ret </code></pre> <p>My question is why the stack frame of the first code aligns to 24(subl $24, %esp) and the second aligns to 16(subl $16, %esp)? It's said in <a href="https://stackoverflow.com/questions/1061818/stack-allocation-padding-and-alignment">Stack allocation, padding, and alignment</a> the SSEx family of instructions REQUIRES packed 128-bit vectors to be aligned to 16 bytes. So, I expect the stack frame of the first code has a subl $32, %esp and subl $16, %esp for the second one, or subl $24, %esp for the first and subl $8, %esp for the second because of the leave and ret reserved 8 bytes as it said. However, the fact is the first code use 'subl $24, %esp' and the second use 'subl $16, %esp' Thank you for your interest.</p>
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    1. COIf you have extra local variables in the second example, why would you expect the stack frame to have the same size?
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      1. POstack frame align to different size?
      1. USBrett Hale
    2. COIt's said in Stack allocation, padding, and alignment (http://stackoverflow.com/questions/1061818/stack-allocation-padding-and-alignment) the SSEx family of instructions REQUIRES packed 128-bit vectors to be aligned to 16 bytes. So, I expect the stack frame of the first code has a `subl $32, %esp` and `subl $16, %esp` for the second one, or `subl $24, %esp` for the first and `subl $8, %esp` for the second because of the leave and ret reserved 8 bytes as it said. However, the fact is the first code use 'subl $24, %esp' and the second use 'subl $16, %esp'. How to explain this result? thank you:)
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