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POn is negative, positive or zero? return 1, 2, or 4
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  1. POn is negative, positive or zero? return 1, 2, or 4
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    copied!<p>I'm building a PowerPC interpreter, and it works quite well. In the Power architecture the condition register CR0 (EFLAGS on x86) is updated on almost any instruction. It is set like this. The value of CR0 is 1, if the last result was negative, 2 if the last result was positive, 4 otherwise.</p> <p>My first naive method to interpret this is:</p> <pre><code>if (n &lt; 0) cr0 = 1 else if (n &gt; 0) cr0 = 2; else cr0 = 4; </code></pre> <p>However I understand that all those branches won't be optimal, being run millions of times per second. I've seen some bit hacking on SO, but none seemed adeguate. For example I found many examples to convert a number to -1, 0, or 1 accordingly to the sign or 0. But how to make -1 = 1, 1 = 2, 0 = 4? I'm asking for the help of the Bit Hackers...</p> <p>Thanks in advance</p> <p><strong>Update:</strong> First of all: thanks guys, you've been great. I'll test all of your codes carefully for speed and you'll be the first to know who's the winner.</p> <p>@jalf: About your first advice, I wasn't actually calculating CR0 on every instruction. I was rather keeping a lastResult variable, and when (and if) a following instruction asked for a flag, do the comparison. Three main motivations took me back to "everytime" update: </p> <ol> <li>On PPC you're not forced to update CR0 like on x86 (where ADD always change EFLAGS, even if not needed), you have two flavours of ADD, one updating. If the compiler chooses to use the updating one, it means that it's going to use the CR0 at some point, so there no point at delaying...</li> <li>There's a particularly painful instruction called mtcrf, that enables you to change the CR0 arbitrarly. You can even set it to 7, with no arithmetic meaning... This just destroys the possibility of keeping a "lastResult" variable.</li> </ol>
 

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