On Wed, 24 Nov 2021, Jan Hubicka wrote: > > This improves value-numbering of calls that read memory, calls > > to const functions with aggregate arguments and calls to > > pure functions where the latter include const functions we > > demoted to pure for the fear of interposing with a less > > optimized version. Note that for pure functions we do not > > handle functions that access global memory. > > Thank you! I am happy we finally undid some of the pessimization caused > by the interposition panic. I was wondering if I should try next stage1 > start tracking eliminated reads in functions, but that is tricky to do > since things like if (*global_var == *globa_var) is folded already in > frontend. > > I was thinking a bit what to do abou global accesses and I think we > still can do something (also next stage1). > > Currently we disambiguate using > if (stmt_may_clobber_ref_p_1 (def, &ref, true)) > where ref is the REF is a ao_ref we built from the summary of STMT and > DEF is another statement. This is fine but we ignore info we have from > PTA on STMT (the statement we try to optimize). > > I we could look at DEF, and try to disambiguate all memory it > writes against STMT using PTA oracle and that would let us to handle > global memory well (we don't need REF for that) because we will work out > that some accesses are not escaping to STMT becaue they are not in > CALLUSED. Somewhat anoying is that we don't have predicate in > tree-ssa-alias for that (stmt_clobber_stmt_p :)
Yes, note that we don't have callused unless IPA PTA is enabled, but it might be salveagable from IPA reference info? What we're missing is a stmt_clobbers_pt_solution_p, or rather a reasonably cheap way to construct an ao_ref covering all of a points-to solution. The not-so-cheap way to do that is tree tem = make_ssa_name (ptr_type_node); ptr_info_def *pi = get_ptr_info (p); pt->pt = *gimple_call_use_set (call_stmt); tree ref = build2 (MEM_REF, void_type_node /* ?? */, tem, build_zero_cst (ptr_type_node /* that effectively is ref-all */)); ao_ref_init (&r, ref); r->base = ref; r->ref = NULL_TREE; r->offset = 0; r->alias_set = 0; r->base_alias_set = 0; and if we come from IPA reference we first have to build a points-to bitmap as well. What would be a bit more convenient is probably adding a pt_solution * member to ao_ref. Maybe also avoiding the MEM_REF build we already do in some cases and overload the base field using a union and a designator ... But yes, sth for next stage1. Richard.
