| /* Language independent return value optimizations |
| Copyright (C) 2004, 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, 59 Temple Place - Suite 330, |
| Boston, MA 02111-1307, USA. */ |
| |
| #include "config.h" |
| #include "system.h" |
| #include "coretypes.h" |
| #include "tm.h" |
| #include "tree.h" |
| #include "rtl.h" |
| #include "function.h" |
| #include "basic-block.h" |
| #include "expr.h" |
| #include "diagnostic.h" |
| #include "tree-flow.h" |
| #include "timevar.h" |
| #include "tree-dump.h" |
| #include "tree-pass.h" |
| #include "langhooks.h" |
| |
| /* This file implements return value optimizations for functions which |
| return aggregate types. |
| |
| Basically this pass searches the function for return statements which |
| return a local aggregate. When converted to RTL such statements will |
| generate a copy from the local aggregate to final return value destination |
| mandated by the target's ABI. |
| |
| That copy can often be avoided by directly constructing the return value |
| into the final destination mandated by the target's ABI. |
| |
| This is basically a generic equivalent to the C++ front-end's |
| Named Return Value optimization. */ |
| |
| struct nrv_data |
| { |
| /* This is the temporary (a VAR_DECL) which appears in all of |
| this function's RETURN_EXPR statements. */ |
| tree var; |
| |
| /* This is the function's RESULT_DECL. We will replace all occurrences |
| of VAR with RESULT_DECL when we apply this optimization. */ |
| tree result; |
| }; |
| |
| static tree finalize_nrv_r (tree *, int *, void *); |
| |
| /* Callback for the tree walker. |
| |
| If TP refers to a RETURN_EXPR, then set the expression being returned |
| to nrv_data->result. |
| |
| If TP refers to nrv_data->var, then replace nrv_data->var with |
| nrv_data->result. |
| |
| If we reach a node where we know all the subtrees are uninteresting, |
| then set *WALK_SUBTREES to zero. */ |
| |
| static tree |
| finalize_nrv_r (tree *tp, int *walk_subtrees, void *data) |
| { |
| struct nrv_data *dp = (struct nrv_data *)data; |
| |
| /* No need to walk into types. */ |
| if (TYPE_P (*tp)) |
| *walk_subtrees = 0; |
| |
| /* If this is a RETURN_EXPR, set the expression being returned to RESULT. */ |
| else if (TREE_CODE (*tp) == RETURN_EXPR) |
| TREE_OPERAND (*tp, 0) = dp->result; |
| |
| /* Otherwise replace all occurrences of VAR with RESULT. */ |
| else if (*tp == dp->var) |
| *tp = dp->result; |
| |
| /* Keep iterating. */ |
| return NULL_TREE; |
| } |
| |
| /* Main entry point for return value optimizations. |
| |
| If this function always returns the same local variable, and that |
| local variable is an aggregate type, then replace the variable with |
| the function's DECL_RESULT. |
| |
| This is the equivalent of the C++ named return value optimization |
| applied to optimized trees in a language independent form. If we |
| ever encounter languages which prevent this kind of optimization, |
| then we could either have the languages register the optimization or |
| we could change the gating function to check the current language. */ |
| |
| static void |
| tree_nrv (void) |
| { |
| tree result = DECL_RESULT (current_function_decl); |
| tree result_type = TREE_TYPE (result); |
| tree found = NULL; |
| basic_block bb; |
| struct nrv_data data; |
| |
| /* If this function does not return an aggregate type in memory, then |
| there is nothing to do. */ |
| if (!aggregate_value_p (result, current_function_decl)) |
| return; |
| |
| /* Look through each block for suitable return expressions. RETURN_EXPRs |
| end basic blocks, so we only have to look at the last statement in |
| each block. That makes this very fast. */ |
| FOR_EACH_BB (bb) |
| { |
| tree stmt = last_stmt (bb); |
| |
| if (stmt && TREE_CODE (stmt) == RETURN_EXPR) |
| { |
| tree ret_expr = TREE_OPERAND (stmt, 0); |
| |
| /* This probably should not happen, but just to be safe do |
| not perform NRV optimizations if only some of the return |
| statement return a value. */ |
| if (!ret_expr |
| || TREE_CODE (ret_expr) != MODIFY_EXPR |
| || TREE_CODE (TREE_OPERAND (ret_expr, 0)) != RESULT_DECL) |
| return; |
| |
| /* Now verify that this return statement uses the same value |
| as any previously encountered return statement. */ |
| if (found != NULL) |
| { |
| /* If we found a return statement using a different variable |
| than previous return statements, then we can not perform |
| NRV optimizations. */ |
| if (found != TREE_OPERAND (ret_expr, 1)) |
| return; |
| } |
| else |
| found = TREE_OPERAND (ret_expr, 1); |
| |
| /* The returned value must be a local automatic variable of the |
| same type and alignment as the function's result. */ |
| if (TREE_CODE (found) != VAR_DECL |
| || TREE_THIS_VOLATILE (found) |
| || DECL_CONTEXT (found) != current_function_decl |
| || TREE_STATIC (found) |
| || TREE_ADDRESSABLE (found) |
| || DECL_ALIGN (found) > DECL_ALIGN (result) |
| || !lang_hooks.types_compatible_p (TREE_TYPE (found), |
| result_type)) |
| return; |
| } |
| } |
| |
| if (!found) |
| return; |
| |
| /* If dumping details, then note once and only the NRV replacement. */ |
| if (dump_file && (dump_flags & TDF_DETAILS)) |
| { |
| fprintf (dump_file, "NRV Replaced: "); |
| print_generic_expr (dump_file, found, dump_flags); |
| fprintf (dump_file, " with: "); |
| print_generic_expr (dump_file, result, dump_flags); |
| fprintf (dump_file, "\n"); |
| } |
| |
| /* At this point we know that all the return statements return the |
| same local which has suitable attributes for NRV. Copy debugging |
| information from FOUND to RESULT. */ |
| DECL_NAME (result) = DECL_NAME (found); |
| DECL_SOURCE_LOCATION (result) = DECL_SOURCE_LOCATION (found); |
| DECL_ABSTRACT_ORIGIN (result) = DECL_ABSTRACT_ORIGIN (found); |
| TREE_ADDRESSABLE (result) = TREE_ADDRESSABLE (found); |
| |
| /* Now walk through the function changing all references to VAR to be |
| RESULT. */ |
| data.var = found; |
| data.result = result; |
| FOR_EACH_BB (bb) |
| { |
| block_stmt_iterator bsi; |
| |
| for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi)) |
| walk_tree (bsi_stmt_ptr (bsi), finalize_nrv_r, &data, 0); |
| } |
| |
| /* FOUND is no longer used. Ensure it gets removed. */ |
| var_ann (found)->used = 0; |
| } |
| |
| struct tree_opt_pass pass_nrv = |
| { |
| "nrv", /* name */ |
| NULL, /* gate */ |
| tree_nrv, /* execute */ |
| NULL, /* sub */ |
| NULL, /* next */ |
| 0, /* static_pass_number */ |
| TV_TREE_NRV, /* tv_id */ |
| PROP_cfg, /* properties_required */ |
| 0, /* properties_provided */ |
| 0, /* properties_destroyed */ |
| 0, /* todo_flags_start */ |
| TODO_dump_func | TODO_ggc_collect, /* todo_flags_finish */ |
| 0 /* letter */ |
| }; |