On Sun, Oct 01, 2006 at 09:21:21AM -0700, Linus Torvalds wrote: > The only reason I think you're doing it wrong is that you're using this > totally new separate mechanism for it (which is not a bad mechanism, but > it's very much a special case), and you should not need to! > The fact is, regardless of any bitwise issues, the > > (cast) narrower-type <op> (wider-type "&" (cast) narrower-type) > > should _always_ be simplified to a small-type. Sure. Now try to carry that to more complex expressions. > And I _think_ we should be able to do exactly the same for this one, by > simply noticing that doing a > > (widening-cast) a == b & (widening-cast) c > > is by definition throwing away the higher bits on both sides, and we can > just turn it into the simpler > > a == c & (narrowing-cast) b > > instead. Of course, we can only do this with ops that don't have overflow > from the narrower type (but comparisons and bitops are ok). > > No? At which phase would you do that? We assign types pretty much leaves to root - type of node is derived from the types of children. Now, you suggest basically going *other* way. I.e. noticing that we have <complex expression in narrow> & <complex expression in wide> and trying to propagate narrowing down another subexpression. Note that we need *both* arguments typed - we don't know which one might be narrow here. So it's really re-evaluation of subexpression. OK, so we have expand phase that could do that. _However_, we'd have to postpone the bitwise checks until that phase, or we'd get them anyway. And that is not to mention the fun with evaluate_expression() mangling the tree, so re-walking it won't be easy. IOW, I very much prefer to deal with that by _existing_ mechanism; namely, the type assignment. It's easier to propagate the information along with the rest of type information, in the direction we are already doing. - To unsubscribe from this list: send the line "unsubscribe linux-sparse" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
