On Nov 26, 2007, "Richard Guenther" <[EMAIL PROTECTED]> wrote:
> On Nov 26, 2007 7:57 AM, Alexandre Oliva <[EMAIL PROTECTED]> wrote: >> On Nov 24, 2007, "Richard Guenther" <[EMAIL PROTECTED]> wrote: >> >> > No, hashing is fine, but doing walks over a hashtable when your algorithm >> > depends on ordering is not. >> >> Point. >> >> > I have patches to fix the instance of walking over all referenced >> > vars. Which is in the case of UIDs using bitmaps and a walk over a >> > bitmap (which ensures walks in UID order). >> >> Why is such memory and CPU overhead better than avoiding the >> divergence of UIDs in the first place? > Actually my patches should be an overall memory savings. Err... I don't see how using a bitmap in addition to a hashtable can save memory over using only a hashtable. Or are you saying you do away with the hashtables? I can see that this is possible and desirable. > But, as you (and me and others) look at bugs that happen because of > UID divergence, it is easier to use UIDs in a way that guarantees > that generated code does not change in such cases. Agreed, this property is desirable. But I wouldn't say it is enough. Ensuring UIDs remain constant across compilations has helped tremendously in locating other compilation divergences, for comparing debug dumps becomes much easier. So, even if we use algorithms that don't depend on UIDs remaining constant across compilations, I believe it is highly desirable that we keep them constant across compilations. > Otherwise what's the point in using UIDs? There are several different reasons for having UIDs, some of which could be having some unique identifier for an object, even in the presence of a moving garbage collector; being able to create fully-ordered sets of objects; being able to easily identify objects across a single compilation; being able to easily identify objects even across multiple compilations; and I'm sure it's possible to come up with other reasons that would justify the idea of UIDs on their own. -- Alexandre Oliva http://www.lsd.ic.unicamp.br/~oliva/ FSF Latin America Board Member http://www.fsfla.org/ Red Hat Compiler Engineer [EMAIL PROTECTED], gcc.gnu.org} Free Software Evangelist [EMAIL PROTECTED], gnu.org}
