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    <p>At my machine <code>kroundup32</code> gives 6.000m rounds/sec<br> And next function gives 7.693m rounds/sec</p> <pre><code>inline int scan_msb(int x) { #if defined(__i386__) || defined(__x86_64__) int y; __asm__("bsr %1, %0" : "=r" (y) : "r" (x) : "flags"); /* ZF */ return y; #else #error "Implement me for your platform" #endif } inline int roundup32(int x) { if (x == 0) return x; else { const int bit = scan_msb(x); const int mask = ~((~0) &lt;&lt; bit); if (x &amp; mask) return (1 &lt;&lt; (bit+1)); else return (1 &lt;&lt; bit); } } </code></pre> <p>So @thomasrutter I woudn't say that it is "highly optimized".</p> <p>And appropriate (only meaningful part) assembly (for GCC 4.4.4):</p> <pre><code>kroundup32: subl $1, %edi movl %edi, %eax sarl %eax orl %edi, %eax movl %eax, %edx sarl $2, %edx orl %eax, %edx movl %edx, %eax sarl $4, %eax orl %edx, %eax movl %eax, %edx sarl $8, %edx orl %eax, %edx movl %edx, %eax sarl $16, %eax orl %edx, %eax addl $1, %eax ret roundup32: testl %edi, %edi movl %edi, %eax je .L6 movl $-1, %edx bsr %edi, %ecx sall %cl, %edx notl %edx testl %edi, %edx jne .L10 movl $1, %eax sall %cl, %eax .L6: rep ret .L10: addl $1, %ecx movl $1, %eax sall %cl, %eax ret </code></pre> <p>By some reason I haven't found appropriate implementation of <code>scan_msb</code> (like <code>#define scan_msb(x) if (__builtin_constant_p (x)) ...</code>) within standart headers of GCC (only <a href="http://lists.freebsd.org/pipermail/freebsd-hackers/2007-September/021846.html" rel="nofollow noreferrer"><code>__TBB_machine_lg</code>/<code>__TBB_Log2</code></a>).</p>
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    1. POCould someone help explain what this C one liner does?
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    1. COfair enough. I guess you could say that whoever wrote it was *attempting* to make something highly optimised, even though that attempt may not have been hugely successful ;)
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