On Wed, Jan 20, 2016 at 3:13 AM, Richard Biener <richard.guent...@gmail.com> wrote: > On Wed, Jan 20, 2016 at 6:48 AM, Ian Lance Taylor <i...@golang.org> wrote: >> As discussed at https://gcc.gnu.org/ml/gcc/2016-01/msg00023.html , the >> Go frontend needs some way to prevent ivopts from temporarily removing >> all pointers into a memory block. This patch adds a new option >> -fcollectible-pointers which makes that happen. This is not the best >> way to solve the problem, but it seems safe for GCC 6. >> >> I made the option -fcollectible-pointers intending that any similar >> optimizations (or, rather, non-optimizations) can be captured in the >> same option. And if we develop a better approach for ivopts, it can >> still be covered under this option name. >> >> Bootstrapped and tested on x86_64-pc-linux-gnu. OK for mainline? > > + // -fcollectible-pointers means that we have to keep a real pointer > + // live, but the ivopts code may replace a real pointer with one > + // pointing before or after the memory block that is then adjusted > + // into the memory block during the loop. > + // FIXME: It would likely be better to actually force the pointer > + // live and still use ivopts; for example, it would be enough to > + // write the pointer into memory and keep it there until after the > + // loop. > + if (flag_collectible_pointers && POINTER_TYPE_P (TREE_TYPE (base))) > + return; > > please use C-style comments.
Whoops, sorry, too much Go coding. > The above is to add_autoinc_candidates > which I find weird - we certainly produce out-of-bound pointers even on > x86 which isn't auto-inc. Despite the name, this is used on all systems. That is the function where we consider using BASE + STEP * i in a loop. > So this can't be a complete fix. I guess you > are correct in disabling some offsetted address IV candidates but > I think there's some other issues regarding to exit test replacement > maybe (replace ptr <= ptr2 with ptr != ptr2 or so). I'll look into that. > While the docs of the option look fine I find > > +fcollectible-pointers > +Common Report Var(flag_collectible_pointers) Optimization > +Ensure that pointers are always collectible by a garbage collector > > somewhat confusing (we don't collect pointers but pointed-to memory). > Maybe "Ensure that derived pointers always point to the original object"? > In that light -fcollectible-pointers is a bad option name as well. Maybe > -fall-pointers-are-gc-roots or sth like that. I'm OK with that, or how about -fkeep-gc-roots-live? Ian
