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);
> + }
> + }
> + }
> +
> 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
