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POEfficient integer compare function
 

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    <p>The <code>compare</code> function is a function that takes two arguments <code>a</code> and <code>b</code> and returns an integer describing their order. If <code>a</code> is smaller than <code>b</code>, the result is some negative integer. If <code>a</code> is bigger than <code>b</code>, the result is some positive integer. Otherwise, <code>a</code> and <code>b</code> are equal, and the result is zero.</p> <p>This function is often used to parametrize sorting and searching algorithms from standard libraries.</p> <p>Implementing the <code>compare</code> function for characters is quite easy; you simply subtract the arguments:</p> <pre><code>int compare_char(char a, char b) { return a - b; } </code></pre> <p>This works because the difference between two characters is generally assumed to fit into an integer. (Note that this assumption does not hold for systems where <code>sizeof(char) == sizeof(int)</code>.)</p> <p>This trick cannot work to compare integers, because the difference between two integers generally does not fit into an integer. For example, <code>INT_MAX - (-1) = INT_MIN</code> suggests that <code>INT_MAX</code> is smaller than <code>-1</code> (technically, the overflow leads to undefined behavior, but let's assume modulo arithmetic).</p> <p>So how can we implement the compare function efficiently for integers? Here is my first attempt:</p> <pre><code>int compare_int(int a, int b) { int temp; int result; __asm__ __volatile__ ( "cmp %3, %2 \n\t" "mov $0, %1 \n\t" "mov $1, %0 \n\t" "cmovg %0, %1 \n\t" "mov $-1, %0 \n\t" "cmovl %0, %1 \n\t" : "=r"(temp), "=r"(result) : "r"(a), "r"(b) : "cc"); return result; } </code></pre> <p>Can it be done in less than 6 instructions? Is there a less straightforward way that is more efficient?</p>
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    1. COHave you disassembled `return (a<b)?-1:(a>b);`?
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    2. CO@user315052 Only five instructions, nice! Now I feel stupid :)
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    3. COOn side note isn't `bool a_less_than_b(int a, int b) { return a < b; }` look better than `int compare_int(int a, int b);` ?
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