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POWhy is this code slower even if the function is inlined?
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  1. POWhy is this code slower even if the function is inlined?
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    copied!<p>I have a method like this : </p> <pre><code>bool MyFunction(int&amp; i) { switch(m_step) { case 1: if (AComplexCondition) { i = m_i; return true; } case 2: // some code case 3: // some code } } </code></pre> <p>Since there are lots of case statements (more than 3) and the function is becoming large, I tried to extract the code in case 1 and put it in an inline function like this:</p> <pre><code>inline bool funct(int&amp; i) { if (AComplexCondition) { i = m_i; return true; } return false; } bool MyFunction(int&amp; i) { switch(m_step) { case 1: if (funct(i)) { return true; } case 2: // some code case 3: // some code } } </code></pre> <p>It seems this code is significantly slower than the original. <strong>I checked with -Winline and the function is inlined.</strong> Why is this code slower? I thought it would be equivalent. The only difference I see is there is one more conditional check in the second version, but I thought the compiler should be able to optimize it away. Right?</p> <p><strong>edit:</strong> Some peoples suggested that I should use gdb to stop over every assembly instructions in both versions to see the differences. I did this.</p> <p>The first version look like this :</p> <pre><code>mov callq (Call to AComplexCondition()) test je (doesn't jump) mov (i = m_i) movl (m_step = 1) </code></pre> <p>The second version, that is a bit slower seems simpler.</p> <pre><code>movl (m_step = 1) callq (Call to AComplexCondition()) test je (doesn't jump) mov (i = m_i) xchg %ax,%ax (This is a nop I think) </code></pre> <p>These two version seems to do the same thing, so I still don't know why the second version is still slower.</p>
 

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