> > On 11/15/24 10:19, Jan Hubicka wrote: > > > I have seen this happen when the result of the intersection is an empty > > > set (one of the few comments in value-range.h actually describes an > > > undefined range as an "empty range"). I have only seen this happen when > > > the edge has been redirected to builtin_unreachable because, the has > > > been recognized as impossible. But I am not absolutely convinced we can > > > rely on that always being the case. > > > > > > We can decide to not update jump functions for edges leading to > > > gcc_unreachable, after we have given some thought what to do about > > > reference description counts, but that is a separate effort, I'd say. > > Hmm, I wonder in what scenarios empty range can happen. Probably we > > could redirect such edges to unreachable since such code path can not be > > taken? > > > > It is however probably rare enough to not matter that much in practice. > > > > Honza > > > Empty ranges invariably mean unreachable code. > > if (x > 50) // range is [51, +INF] > if (x < 40) // range is [-INF, 40]
This looks like something reasonably possible. I would expect some of libstdc++ checks which leads to various calls of throw can be eliminated by this. > > When the 2 calculated ranges intersect, we get UNDEFINED or the empty > range. It basically means on this path, X cannot have a range, which makes > it unreachable. You dont see it often because VRP, threading and such often > remove these situations before they are observed. > > The only other time I think it can happen is when we are dealing with > uninitialized values. Sometimes if there is no definition, it can be > UNDEFINED.. this allows us to pick other values for it so we can do > optimizations. I think thats a special case we do however. I *think* > as a general user you will get VARYING for uninitialized values. Looking > through gimple_range_global() (which is used when no value has been or can > be calculated), any undefined values come back VARYING. > > After all that, I think you could redirect those edges to unreachable... Which sounds like a good idea then.. Honza > > Andrew > > >
