On Tue, Mar 28, 2017 at 11:02:32AM -0700, Linus Torvalds wrote: > On Tue, Mar 28, 2017 at 10:00 AM, Luc Van Oostenryck > <luc.vanoostenryck@xxxxxxxxx> wrote: > > > > I agree, of course, but I think there is a problem anyway. > > Yes. I just wanted to point out that it's not the simplification phase > that is the problem, and that sparse is technically correct (for some > definition of "technically" ;) Ok :) > > The problem here with this %r4 is created by the single-store > > shortcut which replace all loads with the same value as the one > > used for the unique store (even loads that are not dominated > > by the store). Fair enough, don't use uninitialized vars. > > Yeah. > > Personally, I absolutely detest a lot of the undefined C corner cases. > I'm a huge fan of the language, but almost all of the the "undefined > behavior" in C is a huge mistake. And things become even more fun once you try to understand the standard. > ... > > > But what annoys me really is that: > > -) once you remove the single-store shortcut, it seems there is > > other problems (I just saw it by doing some code comparison, > > I need to create a few clear examples). > > The single-store shortcut is actually pretty important, because it is > what turns a *lot* of local variables from stack memory things into > pseudos. That's partly because the sparse optimizations aren't all > that smart. Everytime I look at this part of the code, I think the same: why turn all vars into stack memory to have to turn them later into pseudos? > Sparse code gen sometimes looks really good, but it's really largely > due to SSA really being a wonderful model. It makes some "complex" > optimizations magically trivial. But outside of those things, sparse > really is pretty damn stupid. > > I actually would whole-heartedly support a "all local variables are > initialized to zero" mode, which would get rid of a lot of these > undefined cases. It shouldn't be hard but then things like single-store shortcut then become useless. > But it would also be good to just warn about it I have a trivial patch for that (just look at ep->entry->bb->needs) but for the moment it's not yet useful because there is too much problems there (but maybe I should look at it again). > - the problem is that > sparse doesn't even necessarily *understand* when it generates garbage > from garbage input, because the transformations are so mindless, and > the SSA code works without ever understanding the notion of truly > "dominating" operations. > > Linus -- 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
