Re: [PATCH 3/6] C-language equivalents of include/asm-*/bitops.h

Russell King
2006-01-26T08:55:41Z

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