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Diffstat (limited to 'gcc/tree-nrv.c')
| -rw-r--r-- | gcc/tree-nrv.c | 217 |
1 files changed, 217 insertions, 0 deletions
diff --git a/gcc/tree-nrv.c b/gcc/tree-nrv.c new file mode 100644 index 00000000000..2c344967d66 --- /dev/null +++ b/gcc/tree-nrv.c @@ -0,0 +1,217 @@ +/* Language independent return value optimizations + Copyright (C) 2004 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 occurences + 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, then set the expression being returned + to RESULT. */ + else if (TREE_CODE (*tp) == RETURN_EXPR) + TREE_OPERAND (*tp, 0) = dp->result; + /* Replace all occurences 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 + || 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 */ +}; |