/* Routines for discovering and unpropagating edge equivalences. Copyright (C) 2005 Free Software Foundation, Inc. This file is part of GCC. GCC is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version. GCC is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with GCC; see the file COPYING. If not, write to the Free Software Foundation, 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */ #include "config.h" #include "system.h" #include "coretypes.h" #include "tm.h" #include "tree.h" #include "flags.h" #include "rtl.h" #include "tm_p.h" #include "ggc.h" #include "basic-block.h" #include "output.h" #include "expr.h" #include "function.h" #include "diagnostic.h" #include "timevar.h" #include "tree-dump.h" #include "tree-flow.h" #include "domwalk.h" #include "real.h" #include "tree-pass.h" #include "tree-ssa-propagate.h" #include "langhooks.h" /* The basic structure describing an equivalency created by traversing an edge. Traversing the edge effectively means that we can assume that we've seen an assignment LHS = RHS. */ struct edge_equivalency { tree rhs; tree lhs; }; /* This routine finds and records edge equivalences for every edge in the CFG. When complete, each edge that creates an equivalency will have an EDGE_EQUIVALENCY structure hanging off the edge's AUX field. The caller is responsible for freeing the AUX fields. */ static void associate_equivalences_with_edges (void) { basic_block bb; /* Walk over each block. If the block ends with a control statement, then it might create a useful equivalence. */ FOR_EACH_BB (bb) { block_stmt_iterator bsi = bsi_last (bb); tree stmt; /* If the block does not end with a COND_EXPR or SWITCH_EXPR then there is nothing to do. */ if (bsi_end_p (bsi)) continue; stmt = bsi_stmt (bsi); if (!stmt) continue; /* A COND_EXPR may create an equivalency in a variety of different ways. */ if (TREE_CODE (stmt) == COND_EXPR) { tree cond = COND_EXPR_COND (stmt); edge true_edge; edge false_edge; struct edge_equivalency *equivalency; extract_true_false_edges_from_block (bb, &true_edge, &false_edge); /* If the conditional is a single variable 'X', record 'X = 1' for the true edge and 'X = 0' on the false edge. */ if (TREE_CODE (cond) == SSA_NAME && !SSA_NAME_OCCURS_IN_ABNORMAL_PHI (cond)) { equivalency = XNEW (struct edge_equivalency); equivalency->rhs = constant_boolean_node (1, TREE_TYPE (cond)); equivalency->lhs = cond; true_edge->aux = equivalency; equivalency = XNEW (struct edge_equivalency); equivalency->rhs = constant_boolean_node (0, TREE_TYPE (cond)); equivalency->lhs = cond; false_edge->aux = equivalency; } /* Equality tests may create one or two equivalences. */ else if (TREE_CODE (cond) == EQ_EXPR || TREE_CODE (cond) == NE_EXPR) { tree op0 = TREE_OPERAND (cond, 0); tree op1 = TREE_OPERAND (cond, 1); /* Special case comparing booleans against a constant as we know the value of OP0 on both arms of the branch. i.e., we can record an equivalence for OP0 rather than COND. */ if (TREE_CODE (op0) == SSA_NAME && !SSA_NAME_OCCURS_IN_ABNORMAL_PHI (op0) && TREE_CODE (TREE_TYPE (op0)) == BOOLEAN_TYPE && is_gimple_min_invariant (op1)) { if (TREE_CODE (cond) == EQ_EXPR) { equivalency = XNEW (struct edge_equivalency); equivalency->lhs = op0; equivalency->rhs = (integer_zerop (op1) ? boolean_false_node : boolean_true_node); true_edge->aux = equivalency; equivalency = XNEW (struct edge_equivalency); equivalency->lhs = op0; equivalency->rhs = (integer_zerop (op1) ? boolean_true_node : boolean_false_node); false_edge->aux = equivalency; } else { equivalency = XNEW (struct edge_equivalency); equivalency->lhs = op0; equivalency->rhs = (integer_zerop (op1) ? boolean_true_node : boolean_false_node); true_edge->aux = equivalency; equivalency = XNEW (struct edge_equivalency); equivalency->lhs = op0; equivalency->rhs = (integer_zerop (op1) ? boolean_false_node : boolean_true_node); false_edge->aux = equivalency; } } if (TREE_CODE (op0) == SSA_NAME && !SSA_NAME_OCCURS_IN_ABNORMAL_PHI (op0) && (is_gimple_min_invariant (op1) || (TREE_CODE (op1) == SSA_NAME && !SSA_NAME_OCCURS_IN_ABNORMAL_PHI (op1)))) { /* For IEEE, -0.0 == 0.0, so we don't necessarily know the sign of a variable compared against zero. If we're honoring signed zeros, then we cannot record this value unless we know that the value is nonzero. */ if (HONOR_SIGNED_ZEROS (TYPE_MODE (TREE_TYPE (op0))) && (TREE_CODE (op1) != REAL_CST || REAL_VALUES_EQUAL (dconst0, TREE_REAL_CST (op1)))) continue; equivalency = XNEW (struct edge_equivalency); equivalency->lhs = op0; equivalency->rhs = op1; if (TREE_CODE (cond) == EQ_EXPR) true_edge->aux = equivalency; else false_edge->aux = equivalency; } } /* ??? TRUTH_NOT_EXPR can create an equivalence too. */ } /* For a SWITCH_EXPR, a case label which represents a single value and which is the only case label which reaches the target block creates an equivalence. */ if (TREE_CODE (stmt) == SWITCH_EXPR) { tree cond = SWITCH_COND (stmt); if (TREE_CODE (cond) == SSA_NAME && !SSA_NAME_OCCURS_IN_ABNORMAL_PHI (cond)) { tree labels = SWITCH_LABELS (stmt); int i, n_labels = TREE_VEC_LENGTH (labels); tree *info = XCNEWVEC (tree, n_basic_blocks); /* Walk over the case label vector. Record blocks which are reached by a single case label which represents a single value. */ for (i = 0; i < n_labels; i++) { tree label = TREE_VEC_ELT (labels, i); basic_block bb = label_to_block (CASE_LABEL (label)); if (CASE_HIGH (label) || !CASE_LOW (label) || info[bb->index]) info[bb->index] = error_mark_node; else info[bb->index] = label; } /* Now walk over the blocks to determine which ones were marked as being reached by a useful case label. */ for (i = 0; i < n_basic_blocks; i++) { tree node = info[i]; if (node != NULL && node != error_mark_node) { tree x = fold_convert (TREE_TYPE (cond), CASE_LOW (node)); struct edge_equivalency *equivalency; /* Record an equivalency on the edge from BB to basic block I. */ equivalency = XNEW (struct edge_equivalency); equivalency->rhs = x; equivalency->lhs = cond; find_edge (bb, BASIC_BLOCK (i))->aux = equivalency; } } free (info); } } } } /* Translating out of SSA sometimes requires inserting copies and constant initializations on edges to eliminate PHI nodes. In some cases those copies and constant initializations are redundant because the target already has the value on the RHS of the assignment. We previously tried to catch these cases after translating out of SSA form. However, that code often missed cases. Worse yet, the cases it missed were also often missed by the RTL optimizers. Thus the resulting code had redundant instructions. This pass attempts to detect these situations before translating out of SSA form. The key concept that this pass is built upon is that these redundant copies and constant initializations often occur due to constant/copy propagating equivalences resulting from COND_EXPRs and SWITCH_EXPRs. We want to do those propagations as they can sometimes allow the SSA optimizers to do a better job. However, in the cases where such propagations do not result in further optimization, we would like to "undo" the propagation to avoid the redundant copies and constant initializations. This pass works by first associating equivalences with edges in the CFG. For example, the edge leading from a SWITCH_EXPR to its associated CASE_LABEL will have an equivalency between SWITCH_COND and the value in the case label. Once we have found the edge equivalences, we proceed to walk the CFG in dominator order. As we traverse edges we record equivalences associated with those edges we traverse. When we encounter a PHI node, we walk its arguments to see if we have an equivalence for the PHI argument. If so, then we replace the argument. Equivalences are looked up based on their value (think of it as the RHS of an assignment). A value may be an SSA_NAME or an invariant. We may have several SSA_NAMEs with the same value, so with each value we have a list of SSA_NAMEs that have the same value. */ /* As we enter each block we record the value for any edge equivalency leading to this block. If no such edge equivalency exists, then we record NULL. These equivalences are live until we leave the dominator subtree rooted at the block where we record the equivalency. */ static VEC(tree,heap) *equiv_stack; /* Global hash table implementing a mapping from invariant values to a list of SSA_NAMEs which have the same value. We might be able to reuse tree-vn for this code. */ static htab_t equiv; /* Main structure for recording equivalences into our hash table. */ struct equiv_hash_elt { /* The value/key of this entry. */ tree value; /* List of SSA_NAMEs which have the same value/key. */ VEC(tree,heap) *equivalences; }; static void uncprop_initialize_block (struct dom_walk_data *, basic_block); static void uncprop_finalize_block (struct dom_walk_data *, basic_block); static void uncprop_into_successor_phis (struct dom_walk_data *, basic_block); /* Hashing and equality routines for the hash table. */ static hashval_t equiv_hash (const void *p) { tree value = ((struct equiv_hash_elt *)p)->value; return iterative_hash_expr (value, 0); } static int equiv_eq (const void *p1, const void *p2) { tree value1 = ((struct equiv_hash_elt *)p1)->value; tree value2 = ((struct equiv_hash_elt *)p2)->value; return operand_equal_p (value1, value2, 0); } /* Free an instance of equiv_hash_elt. */ static void equiv_free (void *p) { struct equiv_hash_elt *elt = (struct equiv_hash_elt *) p; VEC_free (tree, heap, elt->equivalences); free (elt); } /* Remove the most recently recorded equivalency for VALUE. */ static void remove_equivalence (tree value) { struct equiv_hash_elt equiv_hash_elt, *equiv_hash_elt_p; void **slot; equiv_hash_elt.value = value; equiv_hash_elt.equivalences = NULL; slot = htab_find_slot (equiv, &equiv_hash_elt, NO_INSERT); equiv_hash_elt_p = (struct equiv_hash_elt *) *slot; VEC_pop (tree, equiv_hash_elt_p->equivalences); } /* Record EQUIVALENCE = VALUE into our hash table. */ static void record_equiv (tree value, tree equivalence) { struct equiv_hash_elt *equiv_hash_elt; void **slot; equiv_hash_elt = XNEW (struct equiv_hash_elt); equiv_hash_elt->value = value; equiv_hash_elt->equivalences = NULL; slot = htab_find_slot (equiv, equiv_hash_elt, INSERT); if (*slot == NULL) *slot = (void *) equiv_hash_elt; else free (equiv_hash_elt); equiv_hash_elt = (struct equiv_hash_elt *) *slot; VEC_safe_push (tree, heap, equiv_hash_elt->equivalences, equivalence); } /* Main driver for un-cprop. */ static unsigned int tree_ssa_uncprop (void) { struct dom_walk_data walk_data; basic_block bb; associate_equivalences_with_edges (); /* Create our global data structures. */ equiv = htab_create (1024, equiv_hash, equiv_eq, equiv_free); equiv_stack = VEC_alloc (tree, heap, 2); /* We're going to do a dominator walk, so ensure that we have dominance information. */ calculate_dominance_info (CDI_DOMINATORS); /* Setup callbacks for the generic dominator tree walker. */ walk_data.walk_stmts_backward = false; walk_data.dom_direction = CDI_DOMINATORS; walk_data.initialize_block_local_data = NULL; walk_data.before_dom_children_before_stmts = uncprop_initialize_block; walk_data.before_dom_children_walk_stmts = NULL; walk_data.before_dom_children_after_stmts = uncprop_into_successor_phis; walk_data.after_dom_children_before_stmts = NULL; walk_data.after_dom_children_walk_stmts = NULL; walk_data.after_dom_children_after_stmts = uncprop_finalize_block; walk_data.global_data = NULL; walk_data.block_local_data_size = 0; walk_data.interesting_blocks = NULL; /* Now initialize the dominator walker. */ init_walk_dominator_tree (&walk_data); /* Recursively walk the dominator tree undoing unprofitable constant/copy propagations. */ walk_dominator_tree (&walk_data, ENTRY_BLOCK_PTR); /* Finalize and clean up. */ fini_walk_dominator_tree (&walk_data); /* EQUIV_STACK should already be empty at this point, so we just need to empty elements out of the hash table, free EQUIV_STACK, and cleanup the AUX field on the edges. */ htab_delete (equiv); VEC_free (tree, heap, equiv_stack); FOR_EACH_BB (bb) { edge e; edge_iterator ei; FOR_EACH_EDGE (e, ei, bb->succs) { if (e->aux) { free (e->aux); e->aux = NULL; } } } return 0; } /* We have finished processing the dominator children of BB, perform any finalization actions in preparation for leaving this node in the dominator tree. */ static void uncprop_finalize_block (struct dom_walk_data *walk_data ATTRIBUTE_UNUSED, basic_block bb ATTRIBUTE_UNUSED) { /* Pop the topmost value off the equiv stack. */ tree value = VEC_pop (tree, equiv_stack); /* If that value was non-null, then pop the topmost equivalency off its equivalency stack. */ if (value != NULL) remove_equivalence (value); } /* Unpropagate values from PHI nodes in successor blocks of BB. */ static void uncprop_into_successor_phis (struct dom_walk_data *walk_data ATTRIBUTE_UNUSED, basic_block bb) { edge e; edge_iterator ei; /* For each successor edge, first temporarily record any equivalence on that edge. Then unpropagate values in any PHI nodes at the destination of the edge. Then remove the temporary equivalence. */ FOR_EACH_EDGE (e, ei, bb->succs) { tree phi = phi_nodes (e->dest); /* If there are no PHI nodes in this destination, then there is no sense in recording any equivalences. */ if (!phi) continue; /* Record any equivalency associated with E. */ if (e->aux) { struct edge_equivalency *equiv = (struct edge_equivalency *) e->aux; record_equiv (equiv->rhs, equiv->lhs); } /* Walk over the PHI nodes, unpropagating values. */ for ( ; phi; phi = PHI_CHAIN (phi)) { /* Sigh. We'll have more efficient access to this one day. */ tree arg = PHI_ARG_DEF (phi, e->dest_idx); struct equiv_hash_elt equiv_hash_elt; void **slot; /* If the argument is not an invariant, or refers to the same underlying variable as the PHI result, then there's no point in un-propagating the argument. */ if (!is_gimple_min_invariant (arg) && SSA_NAME_VAR (arg) != SSA_NAME_VAR (PHI_RESULT (phi))) continue; /* Lookup this argument's value in the hash table. */ equiv_hash_elt.value = arg; equiv_hash_elt.equivalences = NULL; slot = htab_find_slot (equiv, &equiv_hash_elt, NO_INSERT); if (slot) { struct equiv_hash_elt *elt = (struct equiv_hash_elt *) *slot; int j; /* Walk every equivalence with the same value. If we find one with the same underlying variable as the PHI result, then replace the value in the argument with its equivalent SSA_NAME. Use the most recent equivalence as hopefully that results in shortest lifetimes. */ for (j = VEC_length (tree, elt->equivalences) - 1; j >= 0; j--) { tree equiv = VEC_index (tree, elt->equivalences, j); if (SSA_NAME_VAR (equiv) == SSA_NAME_VAR (PHI_RESULT (phi))) { SET_PHI_ARG_DEF (phi, e->dest_idx, equiv); break; } } } } /* If we had an equivalence associated with this edge, remove it. */ if (e->aux) { struct edge_equivalency *equiv = (struct edge_equivalency *) e->aux; remove_equivalence (equiv->rhs); } } } /* Ignoring loop backedges, if BB has precisely one incoming edge then return that edge. Otherwise return NULL. */ static edge single_incoming_edge_ignoring_loop_edges (basic_block bb) { edge retval = NULL; edge e; edge_iterator ei; FOR_EACH_EDGE (e, ei, bb->preds) { /* A loop back edge can be identified by the destination of the edge dominating the source of the edge. */ if (dominated_by_p (CDI_DOMINATORS, e->src, e->dest)) continue; /* If we have already seen a non-loop edge, then we must have multiple incoming non-loop edges and thus we return NULL. */ if (retval) return NULL; /* This is the first non-loop incoming edge we have found. Record it. */ retval = e; } return retval; } static void uncprop_initialize_block (struct dom_walk_data *walk_data ATTRIBUTE_UNUSED, basic_block bb) { basic_block parent; edge e; bool recorded = false; /* If this block is dominated by a single incoming edge and that edge has an equivalency, then record the equivalency and push the VALUE onto EQUIV_STACK. Else push a NULL entry on EQUIV_STACK. */ parent = get_immediate_dominator (CDI_DOMINATORS, bb); if (parent) { e = single_incoming_edge_ignoring_loop_edges (bb); if (e && e->src == parent && e->aux) { struct edge_equivalency *equiv = (struct edge_equivalency *) e->aux; record_equiv (equiv->rhs, equiv->lhs); VEC_safe_push (tree, heap, equiv_stack, equiv->rhs); recorded = true; } } if (!recorded) VEC_safe_push (tree, heap, equiv_stack, NULL_TREE); } static bool gate_uncprop (void) { return flag_tree_dom != 0; } struct tree_opt_pass pass_uncprop = { "uncprop", /* name */ gate_uncprop, /* gate */ tree_ssa_uncprop, /* execute */ NULL, /* sub */ NULL, /* next */ 0, /* static_pass_number */ TV_TREE_SSA_UNCPROP, /* tv_id */ PROP_cfg | PROP_ssa, /* properties_required */ 0, /* properties_provided */ 0, /* properties_destroyed */ 0, /* todo_flags_start */ TODO_dump_func | TODO_verify_ssa, /* todo_flags_finish */ 0 /* letter */ };