Messages in this thread | | | Date | Thu, 26 Jan 2006 05:34:12 +0100 | From | Edgar Toernig <> | Subject | Re: [PATCH 3/6] C-language equivalents of include/asm-*/bitops.h |
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Richard Henderson wrote: > > On Wed, Jan 25, 2006 at 08:02:50PM +0000, Russell King wrote: > > > + s = 16; if (word << 16 != 0) s = 0; b += s; word >>= s; > > > + s = 8; if (word << 24 != 0) s = 0; b += s; word >>= s; > > > + s = 4; if (word << 28 != 0) s = 0; b += s; word >>= s; > ... > > Basically, shifts which depend on a variable are more expensive than > > constant-based shifts. > > Actually, they're all constant shifts. Just written stupidly.
Why shift at all?
int ffs(u32 word) { int bit = 0;
word &= -word; // only keep the lsb.
if (word & 0xffff0000) bit |= 16; if (word & 0xff00ff00) bit |= 8; if (word & 0xf0f0f0f0) bit |= 4; if (word & 0xcccccccc) bit |= 2; if (word & 0xaaaaaaaa) bit |= 1;
return bit; }
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