https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80155
--- Comment #13 from rguenther at suse dot de <rguenther at suse dot de> --- On Thu, 23 Mar 2017, thopre01 at gcc dot gnu.org wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80155 > > --- Comment #12 from Thomas Preud'homme <thopre01 at gcc dot gnu.org> --- > (In reply to Richard Biener from comment #9) > > Ah, the patches do not fix the testcase because the testcase is _not_ the > > PRE-creates-IV case. It's indeed simply hoisting/PRE at work transforming > > > > # a_14 = PHI <a_10(, a_5(D)> > > if (!b) > > a_8 = a_14 + 1; > > > > # a_2 = PHI <a_14(10), a_8(4)> > > a_10 = a_2 + 1; > > ... = *(a_2 + 1); > > > > to > > > > # a_14 = PHI <prephimp_12, a_5(D)> > > _4 = a_14 + 1; > > if (b) > > _3 = _4 + 1; > > > > # a_2 = PHI <a_14, _4> > > # prephitmp_12 = PHI <_4, _3> > > ... = *(a_2 + 1); > > > > increasing register pressure mainly because nothing figures that a_2 + 1 > > in the dereference can be replaced by prephitmp_12 ... > > > > So this is a missed SLSR opportunity or, in this simple form, a missed > > PRE/CSE opportunity. Fixing that with the following (otherwise untested) > > restores good code generation for the testcase: > > > > Index: gcc/tree-ssa-pre.c > > =================================================================== > > --- gcc/tree-ssa-pre.c (revision 246414) > > +++ gcc/tree-ssa-pre.c (working copy) > > @@ -4636,6 +4610,35 @@ eliminate_dom_walker::before_dom_childre > > } > > } > > > > + if (gimple_has_ops (stmt)) > > + for (unsigned i = 0; i < gimple_num_ops (stmt); ++i) > > + { > > + tree op = gimple_op (stmt, i); > > + if (op) > > + op = get_base_address (op); > > + if (op > > + && TREE_CODE (op) == MEM_REF > > + && ! integer_zerop (TREE_OPERAND (op, 1))) > > + { > > + tree ops[2]; > > + vn_nary_op_t nary; > > + ops[0] = TREE_OPERAND (op, 0); > > + ops[1] = TREE_OPERAND (op, 1); > > + tree res = vn_nary_op_lookup_pieces (2, POINTER_PLUS_EXPR, > > + TREE_TYPE (ops[0]), > > + ops, &nary); > > + if (res && TREE_CODE (res) == SSA_NAME) > > + res = eliminate_avail (res); > > + if (res) > > + { > > + TREE_OPERAND (op, 0) = res; > > + TREE_OPERAND (op, 1) > > + = build_int_cst (TREE_TYPE (TREE_OPERAND (op, 1)), 0); > > + gimple_set_modified (stmt, true); Add if (TREE_CODE (res) == SSA_NAME && !is_gimple_debug (stmt)) gimple_set_plf (SSA_NAME_DEF_STMT (res), NECESSARY, true); here. > > + } > > + } > > + } > > + > > if (gimple_modified_p (stmt)) > > { > > /* If a formerly non-invariant ADDR_EXPR is turned into an > > > > note that in general optimzing > > > > q = p + 1; > > = ...*(p + 1); > > > > "back" to *q will be undone by forwprop/stmt folding later but in this > > case the feeding stmt is a PHI node and not a pointer-plus. It still > > means that the change might be a bit too disruptive at this point > > (we could restricit it a bit by only handling the case where we don't > > replace with a pointer-plus result). > > Thanks for your work on this! Sadly GCC ICEs with this patch: > > 0xd36f53 update_dep_bb > gcc/tree-ssa-tail-merge.c:411 > 0xd38f54 stmt_update_dep_bb > gcc/tree-ssa-tail-merge.c:429 > 0xd38f54 same_succ_hash > gcc/tree-ssa-tail-merge.c:452 > 0xd38f54 find_same_succ_bb > gcc/tree-ssa-tail-merge.c:717 > 0xd39927 find_same_succ > gcc/tree-ssa-tail-merge.c:748 > 0xd39927 init_worklist > gcc/tree-ssa-tail-merge.c:767 > 0xd39927 tail_merge_optimize(unsigned int) > gcc/tree-ssa-tail-merge.c:1726 > 0xce2d6a execute > gcc/tree-ssa-pre.c:5164 > >
