Re: [PATCH 3/6] C-language equivalents of include/asm-*/bitops.h
Russell King <[email protected]>
| Newsgroups | gmane.linux.ports.ia64,gmane.linux.kernel,gmane.linux.ports.hppa,gmane.linux.ports.ppc.devel,gmane.linux.ports.sh.devel,gmane.linux.ports.sparc,gmane.linux.ports.ultrapenguin |
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| Message-ID | <[email protected]> |
On Wed, Jan 25, 2006 at 04:06:18PM -0800, 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. Unfortunately that's not correct. You do not appear to have checked the compiler output like I did - this code does _not_ generate constant shifts. -- Russell King Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/ maintainer of: 2.6 Serial core - To unsubscribe from this list: send the line "unsubscribe linux-ia64" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html