On 15/05/17 17:04, David Miller wrote: > If we use 1<<31, then sequences like: > > R1 = 0 > R1 <<= 2 > > do silly things. Hmm. It might be a bit late for this, but I wonder if, instead of handling alignments as (1 << align), you could store them as -(1 << align), i.e. leading 1s followed by 'align' 0s. Now the alignment of 0 is 0 (really 1 << 32), which doesn't change when left-shifted some more. Shifts of other numbers' alignments also do the right thing, e.g. align(6) << 2 = (-2) << 2 = -8 = align(6 << 2). Of course you do all this in unsigned, to make sure right shifts work. This also makes other arithmetic simple to track; for instance, align(a + b) is at worst align(a) | align(b). (Of course, this bound isn't tight.) A number is 2^(n+1)-aligned if the 2^n bit of its alignment is cleared. Considered as unsigned numbers, smaller values are stricter alignments.
-Ed
