| /* 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, 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 "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; |
| |
| /* 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 unsigned int |
| tree_nrv (void) |
| { |
| tree result = DECL_RESULT (current_function_decl); |
| tree result_type = TREE_TYPE (result); |
| tree found = NULL; |
| basic_block bb; |
| block_stmt_iterator bsi; |
| 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 0; |
| |
| /* Look through each block for assignments to the RESULT_DECL. */ |
| FOR_EACH_BB (bb) |
| { |
| for (bsi = bsi_start (bb); !bsi_end_p (bsi); bsi_next (&bsi)) |
| { |
| tree stmt = bsi_stmt (bsi); |
| tree ret_expr; |
| |
| if (TREE_CODE (stmt) == RETURN_EXPR) |
| { |
| /* In a function with an aggregate return value, the |
| gimplifier has changed all non-empty RETURN_EXPRs to |
| return the RESULT_DECL. */ |
| ret_expr = TREE_OPERAND (stmt, 0); |
| if (ret_expr) |
| gcc_assert (ret_expr == result); |
| } |
| else if (TREE_CODE (stmt) == MODIFY_EXPR |
| && TREE_OPERAND (stmt, 0) == result) |
| { |
| ret_expr = TREE_OPERAND (stmt, 1); |
| |
| /* 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 != ret_expr) |
| return 0; |
| } |
| else |
| found = ret_expr; |
| |
| /* 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 0; |
| } |
| else if (TREE_CODE (stmt) == MODIFY_EXPR) |
| { |
| tree addr = get_base_address (TREE_OPERAND (stmt, 0)); |
| /* If there's any MODIFY of component of RESULT, |
| then bail out. */ |
| if (addr && addr == result) |
| return 0; |
| } |
| } |
| } |
| |
| if (!found) |
| return 0; |
| |
| /* 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) |
| { |
| for (bsi = bsi_start (bb); !bsi_end_p (bsi); ) |
| { |
| tree *tp = bsi_stmt_ptr (bsi); |
| /* If this is a copy from VAR to RESULT, remove it. */ |
| if (TREE_CODE (*tp) == MODIFY_EXPR |
| && TREE_OPERAND (*tp, 0) == result |
| && TREE_OPERAND (*tp, 1) == found) |
| bsi_remove (&bsi, true); |
| else |
| { |
| walk_tree (tp, finalize_nrv_r, &data, 0); |
| bsi_next (&bsi); |
| } |
| } |
| } |
| |
| /* FOUND is no longer used. Ensure it gets removed. */ |
| var_ann (found)->used = 0; |
| return 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 */ |
| }; |
| |
| /* Determine (pessimistically) whether DEST is available for NRV |
| optimization, where DEST is expected to be the LHS of a modify |
| expression where the RHS is a function returning an aggregate. |
| |
| We search for a base VAR_DECL and look to see if it, or any of its |
| subvars are clobbered. Note that we could do better, for example, by |
| attempting to doing points-to analysis on INDIRECT_REFs. */ |
| |
| static bool |
| dest_safe_for_nrv_p (tree dest) |
| { |
| switch (TREE_CODE (dest)) |
| { |
| case VAR_DECL: |
| { |
| subvar_t subvar; |
| if (is_call_clobbered (dest)) |
| return false; |
| for (subvar = get_subvars_for_var (dest); |
| subvar; |
| subvar = subvar->next) |
| if (is_call_clobbered (subvar->var)) |
| return false; |
| return true; |
| } |
| case ARRAY_REF: |
| case COMPONENT_REF: |
| return dest_safe_for_nrv_p (TREE_OPERAND (dest, 0)); |
| default: |
| return false; |
| } |
| } |
| |
| /* Walk through the function looking for MODIFY_EXPRs with calls that |
| return in memory on the RHS. For each of these, determine whether it is |
| safe to pass the address of the LHS as the return slot, and mark the |
| call appropriately if so. |
| |
| The NRV shares the return slot with a local variable in the callee; this |
| optimization shares the return slot with the target of the call within |
| the caller. If the NRV is performed (which we can't know in general), |
| this optimization is safe if the address of the target has not |
| escaped prior to the call. If it has, modifications to the local |
| variable will produce visible changes elsewhere, as in PR c++/19317. */ |
| |
| static unsigned int |
| execute_return_slot_opt (void) |
| { |
| basic_block bb; |
| |
| FOR_EACH_BB (bb) |
| { |
| block_stmt_iterator i; |
| for (i = bsi_start (bb); !bsi_end_p (i); bsi_next (&i)) |
| { |
| tree stmt = bsi_stmt (i); |
| tree call; |
| |
| if (TREE_CODE (stmt) == MODIFY_EXPR |
| && (call = TREE_OPERAND (stmt, 1), |
| TREE_CODE (call) == CALL_EXPR) |
| && !CALL_EXPR_RETURN_SLOT_OPT (call) |
| && aggregate_value_p (call, call)) |
| /* Check if the location being assigned to is |
| call-clobbered. */ |
| CALL_EXPR_RETURN_SLOT_OPT (call) = |
| dest_safe_for_nrv_p (TREE_OPERAND (stmt, 0)) ? 1 : 0; |
| } |
| } |
| return 0; |
| } |
| |
| struct tree_opt_pass pass_return_slot = |
| { |
| "retslot", /* name */ |
| NULL, /* gate */ |
| execute_return_slot_opt, /* execute */ |
| NULL, /* sub */ |
| NULL, /* next */ |
| 0, /* static_pass_number */ |
| 0, /* tv_id */ |
| PROP_ssa | PROP_alias, /* properties_required */ |
| 0, /* properties_provided */ |
| 0, /* properties_destroyed */ |
| 0, /* todo_flags_start */ |
| 0, /* todo_flags_finish */ |
| 0 /* letter */ |
| }; |