/* Language independent return value optimizations Copyright (C) 2004, 2005, 2007, 2008, 2009, 2010 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 . */ #include "config.h" #include "system.h" #include "coretypes.h" #include "tm.h" #include "tree.h" #include "function.h" #include "basic-block.h" #include "tree-pretty-print.h" #include "tree-flow.h" #include "timevar.h" #include "tree-dump.h" #include "tree-pass.h" #include "langhooks.h" #include "flags.h" /* For "optimize" in gate_pass_return_slot. FIXME: That should be up to the pass manager, but pass_nrv is not in pass_all_optimizations. */ /* 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; 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 *dp = (struct nrv_data *) 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. */ 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; gimple_stmt_iterator gsi; 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; /* 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 (bb) { for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi)) { gimple stmt = gsi_stmt (gsi); tree ret_val; if (gimple_code (stmt) == GIMPLE_RETURN) { /* 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 (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 can not 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 (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) || !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"); } /* 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 if it will be useful. But don't set DECL_ABSTRACT_ORIGIN to point at another function. */ if (!DECL_IGNORED_P (found) && !(DECL_ABSTRACT_ORIGIN (found) && DECL_CONTEXT (DECL_ABSTRACT_ORIGIN (found)) != current_function_decl)) { 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 (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; /* FOUND is no longer used. Ensure it gets removed. */ clear_is_used (found); return 0; } static bool gate_pass_return_slot (void) { return optimize > 0; } struct gimple_opt_pass pass_nrv = { { GIMPLE_PASS, "nrv", /* name */ gate_pass_return_slot, /* gate */ tree_nrv, /* execute */ NULL, /* sub */ NULL, /* next */ 0, /* static_pass_number */ TV_TREE_NRV, /* tv_id */ PROP_ssa | PROP_cfg, /* properties_required */ 0, /* properties_provided */ 0, /* properties_destroyed */ 0, /* todo_flags_start */ TODO_ggc_collect /* todo_flags_finish */ } }; /* 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 (gimple 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) || ref_maybe_used_by_stmt_p (call, dest)) 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. */ static unsigned int execute_return_slot_opt (void) { basic_block bb; FOR_EACH_BB (bb) { gimple_stmt_iterator gsi; for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi)) { gimple stmt = gsi_stmt (gsi); bool slot_opt_p; if (is_gimple_call (stmt) && gimple_call_lhs (stmt) && !gimple_call_return_slot_opt_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; } struct gimple_opt_pass pass_return_slot = { { GIMPLE_PASS, "retslot", /* name */ NULL, /* gate */ execute_return_slot_opt, /* execute */ NULL, /* sub */ NULL, /* next */ 0, /* static_pass_number */ TV_NONE, /* tv_id */ PROP_ssa, /* properties_required */ 0, /* properties_provided */ 0, /* properties_destroyed */ 0, /* todo_flags_start */ 0 /* todo_flags_finish */ } };