[Bug tree-optimization/113433] [12/13/14 Regression] Missed optimization for redundancy computation elimination

["law at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113433

Jeffrey A. Law <law at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |law at gcc dot gnu.org

--- Comment #2 from Jeffrey A. Law <law at gcc dot gnu.org> ---
So we could attack this as a DOM problem.  Not all the infrastructure is in
there.  But the recognition of negated expressions isn't hard.  Something like
this in tree-ssa-scopedtables will detect the negated expression in the hash
table.  

            /* We might be able to lookup the negated expression.  */
            case PLUS_EXPR:
              {
                tree x = gimple_assign_rhs_to_tree (stmt);
                x = fold_unary (NEGATE_EXPR, TREE_TYPE (x), x);
                struct hashable_expr expr;
                expr.type = TREE_TYPE (x);
                expr.kind = EXPR_BINARY;
                expr.ops.binary.op = MINUS_EXPR;
                expr.ops.binary.opnd0 = TREE_OPERAND (x, 0);
                expr.ops.binary.opnd1 = TREE_OPERAND (x, 1);
                class expr_hash_elt element2 (&expr, NULL_TREE);
                expr_hash_elt **slot
                  = m_avail_exprs->find_slot (&element2, NO_INSERT);
                if (slot && *slot)
                  return fold_build1 (NEGATE_EXPR, TREE_TYPE (x),  (*slot)->lhs
());
                return NULL_TREE;
              }

Right now DOM isn't prepared for avail_expr_stack::simplify_binary_operation to
return anything other than a constant, ssa_name or NULL.  But how hard could it
be to expand further :-)


Not sure if this happens enough to make the extra lookups worth the effort.