When PRE discovers a full redundancy during insertion it cannot unite the two value sets. Instead it inserts a copy old-val = new-val where new-val can also be a constant. The following looks through such copies during elimination, providing one extra level of constant and copy propagation. For the PR this helps avoiding a bogus diagnostic that's emitted on unreachable code during loop optimization.
Bootstrapped and tested on x86_64-unknown-linux-gnu, pushed. Richard. PR tree-optimization/110204 * tree-ssa-sccvn.cc (eliminate_dom_walker::eliminate_avail): Look through copies generated by PRE. --- gcc/tree-ssa-sccvn.cc | 17 ++++++++++++++++- 1 file changed, 16 insertions(+), 1 deletion(-) diff --git a/gcc/tree-ssa-sccvn.cc b/gcc/tree-ssa-sccvn.cc index 11061a374a2..a0b98c18ac8 100644 --- a/gcc/tree-ssa-sccvn.cc +++ b/gcc/tree-ssa-sccvn.cc @@ -6594,7 +6594,22 @@ eliminate_dom_walker::eliminate_avail (basic_block, tree op) if (SSA_NAME_IS_DEFAULT_DEF (valnum)) return valnum; if (avail.length () > SSA_NAME_VERSION (valnum)) - return avail[SSA_NAME_VERSION (valnum)]; + { + tree av = avail[SSA_NAME_VERSION (valnum)]; + /* When PRE discovers a new redundancy there's no way to unite + the value classes so it instead inserts a copy old-val = new-val. + Look through such copies here, providing one more level of + simplification at elimination time. */ + gassign *ass; + if (av && (ass = dyn_cast <gassign *> (SSA_NAME_DEF_STMT (av)))) + if (gimple_assign_rhs_class (ass) == GIMPLE_SINGLE_RHS) + { + tree rhs1 = gimple_assign_rhs1 (ass); + if (CONSTANT_CLASS_P (rhs1) || TREE_CODE (rhs1) == SSA_NAME) + av = rhs1; + } + return av; + } } else if (is_gimple_min_invariant (valnum)) return valnum; -- 2.35.3