| /* Language independent return value optimizations |
| Copyright (C) 2004-2022 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 3, 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 COPYING3. If not see |
| <http://www.gnu.org/licenses/>. */ |
| |
| #include "config.h" |
| #include "system.h" |
| #include "coretypes.h" |
| #include "backend.h" |
| #include "tree.h" |
| #include "gimple.h" |
| #include "tree-pass.h" |
| #include "ssa.h" |
| #include "tree-pretty-print.h" |
| #include "gimple-iterator.h" |
| #include "gimple-walk.h" |
| #include "internal-fn.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_t |
| { |
| /* 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; |
| int modified; |
| }; |
| |
| 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 walk_stmt_info *wi = (struct walk_stmt_info *) data; |
| struct nrv_data_t *dp = (struct nrv_data_t *) wi->info; |
| |
| /* 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; |
| dp->modified = 1; |
| } |
| |
| /* 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. */ |
| |
| namespace { |
| |
| const pass_data pass_data_nrv = |
| { |
| GIMPLE_PASS, /* type */ |
| "nrv", /* name */ |
| OPTGROUP_NONE, /* optinfo_flags */ |
| TV_TREE_NRV, /* tv_id */ |
| ( PROP_ssa | PROP_cfg ), /* properties_required */ |
| 0, /* properties_provided */ |
| 0, /* properties_destroyed */ |
| 0, /* todo_flags_start */ |
| 0, /* todo_flags_finish */ |
| }; |
| |
| class pass_nrv : public gimple_opt_pass |
| { |
| public: |
| pass_nrv (gcc::context *ctxt) |
| : gimple_opt_pass (pass_data_nrv, ctxt) |
| {} |
| |
| /* opt_pass methods: */ |
| bool gate (function *) final override { return optimize > 0; } |
| |
| unsigned int execute (function *) final override; |
| |
| }; // class pass_nrv |
| |
| unsigned int |
| pass_nrv::execute (function *fun) |
| { |
| tree result = DECL_RESULT (current_function_decl); |
| tree result_type = TREE_TYPE (result); |
| tree found = NULL; |
| basic_block bb; |
| gimple_stmt_iterator gsi; |
| struct nrv_data_t 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; |
| |
| /* If a GIMPLE type is returned in memory, finalize_nrv_r might create |
| non-GIMPLE. */ |
| if (is_gimple_reg_type (result_type)) |
| return 0; |
| |
| /* If the front end already did something like this, don't do it here. */ |
| if (DECL_NAME (result)) |
| return 0; |
| |
| /* If the result has its address taken then it might be modified |
| by means not detected in the following loop. Bail out in this |
| case. */ |
| if (TREE_ADDRESSABLE (result)) |
| return 0; |
| |
| /* Look through each block for assignments to the RESULT_DECL. */ |
| FOR_EACH_BB_FN (bb, fun) |
| { |
| for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi)) |
| { |
| gimple *stmt = gsi_stmt (gsi); |
| tree ret_val; |
| |
| if (greturn *return_stmt = dyn_cast <greturn *> (stmt)) |
| { |
| /* In a function with an aggregate return value, the |
| gimplifier has changed all non-empty RETURN_EXPRs to |
| return the RESULT_DECL. */ |
| ret_val = gimple_return_retval (return_stmt); |
| if (ret_val) |
| gcc_assert (ret_val == result); |
| } |
| else if (gimple_has_lhs (stmt) |
| && gimple_get_lhs (stmt) == result) |
| { |
| tree rhs; |
| |
| if (!gimple_assign_copy_p (stmt)) |
| return 0; |
| |
| rhs = gimple_assign_rhs1 (stmt); |
| |
| /* 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 cannot perform |
| NRV optimizations. */ |
| if (found != rhs) |
| return 0; |
| } |
| else |
| found = rhs; |
| |
| /* The returned value must be a local automatic variable of the |
| same type and alignment as the function's result. */ |
| if (!VAR_P (found) |
| || TREE_THIS_VOLATILE (found) |
| || !auto_var_in_fn_p (found, current_function_decl) |
| || TREE_ADDRESSABLE (found) |
| || DECL_ALIGN (found) > DECL_ALIGN (result) |
| || !useless_type_conversion_p (result_type, |
| TREE_TYPE (found))) |
| return 0; |
| } |
| else if (gimple_has_lhs (stmt)) |
| { |
| tree addr = get_base_address (gimple_get_lhs (stmt)); |
| /* 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"); |
| } |
| |
| 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_FN (bb, fun) |
| { |
| for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); ) |
| { |
| gimple *stmt = gsi_stmt (gsi); |
| /* If this is a copy from VAR to RESULT, remove it. */ |
| if (gimple_assign_copy_p (stmt) |
| && gimple_assign_lhs (stmt) == result |
| && gimple_assign_rhs1 (stmt) == found) |
| { |
| unlink_stmt_vdef (stmt); |
| gsi_remove (&gsi, true); |
| release_defs (stmt); |
| } |
| else |
| { |
| struct walk_stmt_info wi; |
| memset (&wi, 0, sizeof (wi)); |
| wi.info = &data; |
| data.modified = 0; |
| walk_gimple_op (stmt, finalize_nrv_r, &wi); |
| if (data.modified) |
| update_stmt (stmt); |
| gsi_next (&gsi); |
| } |
| } |
| } |
| |
| SET_DECL_VALUE_EXPR (found, result); |
| DECL_HAS_VALUE_EXPR_P (found) = 1; |
| |
| return 0; |
| } |
| |
| } // anon namespace |
| |
| gimple_opt_pass * |
| make_pass_nrv (gcc::context *ctxt) |
| { |
| return new pass_nrv (ctxt); |
| } |
| |
| /* 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. |
| |
| DEST is available if it is not clobbered or used by the call. */ |
| |
| static bool |
| dest_safe_for_nrv_p (gcall *call) |
| { |
| tree dest = gimple_call_lhs (call); |
| |
| dest = get_base_address (dest); |
| if (! dest) |
| return false; |
| |
| if (TREE_CODE (dest) == SSA_NAME) |
| return true; |
| |
| if (call_may_clobber_ref_p (call, dest, false) |
| || ref_maybe_used_by_stmt_p (call, dest, false)) |
| return false; |
| |
| return true; |
| } |
| |
| /* Walk through the function looking for GIMPLE_ASSIGNs 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. */ |
| |
| namespace { |
| |
| const pass_data pass_data_return_slot = |
| { |
| GIMPLE_PASS, /* type */ |
| "retslot", /* name */ |
| OPTGROUP_NONE, /* optinfo_flags */ |
| TV_NONE, /* tv_id */ |
| PROP_ssa, /* properties_required */ |
| 0, /* properties_provided */ |
| 0, /* properties_destroyed */ |
| 0, /* todo_flags_start */ |
| 0, /* todo_flags_finish */ |
| }; |
| |
| class pass_return_slot : public gimple_opt_pass |
| { |
| public: |
| pass_return_slot (gcc::context *ctxt) |
| : gimple_opt_pass (pass_data_return_slot, ctxt) |
| {} |
| |
| /* opt_pass methods: */ |
| unsigned int execute (function *) final override; |
| |
| }; // class pass_return_slot |
| |
| unsigned int |
| pass_return_slot::execute (function *fun) |
| { |
| basic_block bb; |
| |
| FOR_EACH_BB_FN (bb, fun) |
| { |
| gimple_stmt_iterator gsi; |
| for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi)) |
| { |
| gcall *stmt; |
| bool slot_opt_p; |
| |
| stmt = dyn_cast <gcall *> (gsi_stmt (gsi)); |
| if (stmt |
| && gimple_call_lhs (stmt) |
| && !gimple_call_return_slot_opt_p (stmt) |
| /* Ignore internal functions, those are expanded specially |
| and aggregate_value_p on their result might result in |
| undesirable warnings with some backends. */ |
| && !gimple_call_internal_p (stmt) |
| && aggregate_value_p (TREE_TYPE (gimple_call_lhs (stmt)), |
| gimple_call_fndecl (stmt))) |
| { |
| /* Check if the location being assigned to is |
| clobbered by the call. */ |
| slot_opt_p = dest_safe_for_nrv_p (stmt); |
| gimple_call_set_return_slot_opt (stmt, slot_opt_p); |
| } |
| } |
| } |
| return 0; |
| } |
| |
| } // anon namespace |
| |
| gimple_opt_pass * |
| make_pass_return_slot (gcc::context *ctxt) |
| { |
| return new pass_return_slot (ctxt); |
| } |