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Bit Twiddling Hacks (2009)

v; // we want to find the sign of v int sign; // the result goes here // CHAR_BIT is the number of bits per byte (normally 8). sign = -(v < 0); // if v < 0 then -1, else 0.

More information can be found by reading the paper Using de Bruijn Sequences to Index 1 in a Computer Word by Charles E. Leiserson, Harald Prokof, and Keith H. Randall. (On a Athlon™ XP 2100+ I've found the above shift-left and then OR code is as fast as using a single BSR assembly language instruction, which scans in reverse to find the highest set bit.) The test also returns true if the high byte is 0x80, so there are occasional false positives, but the slower and more reliable version above may then be used on candidates for an overall increase in speed with correct output.

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