Re: [PATCH 3/6] C-language equivalents of include/asm-*/bitops.h
Russell King <[email protected]> Thu, 26 Jan 2006 08:55:41 +0000
| Newsgroups | org.kernel.vger.ultralinux,org.kernel.vger.linux-ia64,org.kernel.vger.linux-kernel,org.kernel.vger.linux-mips,org.kernel.vger.sparclinux,org.ozlabs.lists.linuxppc-dev |
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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