/* 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 */ };