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    <blockquote> <p>The first instruction is moving what is in <code>%rax</code> into <code>%rdi</code> to prepare for the <code>printf</code> call. Thats all fine except we just moved <code>$.LC0</code> (which is the string <code>"%s"</code>) into <code>%eax</code>. This seems unnecessary why didn't we just move <code>$.LC0</code> into <code>%rdi</code> in the first place instead of moving it into <code>%eax</code> and then into <code>%rdi</code>?</p> </blockquote> <p>That’s probably because you’re compiling with no optimisations. When I compile your example with GCC 4.2.1 on Mac OS X v10.6.8, I get the following output:</p> <pre><code>.globl _main _main: LFB3: pushq %rbp LCFI0: movq %rsp, %rbp LCFI1: leaq LC0(%rip), %rsi leaq LC1(%rip), %rdi movl $0, %eax call _printf movl $0, %eax leave ret </code></pre> <p>As you can see, the arguments were directly stored into <code>%rsi</code> and <code>%rdi</code>.</p> <blockquote> <p>The second instruction is clearing out <code>%eax</code> 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> </blockquote> <p>Because the x86_64 ABI specifies that if a function takes variable arguments then <code>AL</code> (which is part of <code>%eax</code>) is expected to hold the number of vector registers used for the arguments to that function call. Since you’re not specifying floating-point arguments when calling <code>printf()</code>, no vector registers are used, so <code>AL</code> (<code>%eax</code>) is zeroed out. I give more examples in an answer to another question <a href="https://stackoverflow.com/questions/6212665/why-is-eax-zeroed-before-a-call-to-printf/6212835#6212835">here</a>.</p>
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    1. POWhy does GCC insert seemingly non-essential instructions before a printf call?
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    1. COThanks! The second part of your answer was the type of information I was looking for. I'm not trying to make my code faster per say I'm just trying to figure out why it is doing what it is doing. Although as someone already mentioned maybe it isn't worth it.
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    2. CO@spa The first part of your question is related to what’s called register allocation in compiler implementation. When you compile your code with no optimisations, register allocation can be quite dumb. However, if you ask the compiler to optimise your code via `-O` (e.g. `-O3`), then it’ll use smarter register allocation algorithms.
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    3. CO@spartacus, rewriting your code in assembler is an absolute no-no when tuning for performance (any small change in the computer's architecture, back to square one), writing performance-critical snippets in assembler is the very last resort. Take a look at Bentley's "Programming Pearls" (http://www.cs.bell-labs.com/cm/cs/pearls).
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