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. 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. 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). 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. Richard. > Ian > > gcc/ChangeLog: > > 2016-01-19 Ian Lance Taylor <i...@google.com> > > * common.opt (fcollectible-pointers): New option. > * tree-ssa-loop-ivopts.c (add_autoinc_candidates): If > -fcollectible-pointers, skip pointers. > * doc/invoke.texi (Optimize Options): Document > -fcollectible-pointers > > gcc/testsuite/ChangeLog: > > 2016-01-19 Ian Lance Taylor <i...@google.com> > > * gcc.dg/tree-ssa/ivopt_5.c: New test.
