On Mon, 2002-08-05 at 02:49, Ian Jeffray wrote: > > how does one teach the optimiser that this is equivalent and 33% faster: > > > > rsbs ip, r4, #1 > > movls ip, #0 > > > Hang on a minute, that's not the same algorithm though. For negative > values of r4, you'll get bogus output... 1 - -2 is 3 wheras you need > zero. Surely? (ie, !-2 is 0, not 3)
No, I think it's okay. In terms of twos complement, your example is actually (1 - 0xfffffffe). That will give you three but with C clear, and the LS conditional will match. p.
