tree-nrv.c File Reference

#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"

Include dependency graph for tree-nrv.c:

Go to the source code of this file.

Data Structures
struct	nrv_data
Functions
static tree	finalize_nrv_r (tree *, int *, void *)
static tree	finalize_nrv_r ()
static unsigned int	tree_nrv ()
static bool	dest_safe_for_nrv_p ()
static unsigned int	execute_return_slot_opt ()
Variables
tree_opt_pass	pass_nrv
tree_opt_pass	pass_return_slot

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Function Documentation

static bool dest_safe_for_nrv_p (   )  [static]

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. Definition at line 243 of file tree-nrv.c. References get_subvars_for_var, is_call_clobbered(), subvar::next, TREE_CODE, TREE_OPERAND, and subvar::var. Referenced by execute_return_slot_opt(). { 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; } }

static unsigned int execute_return_slot_opt (   )  [static]

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. Definition at line 280 of file tree-nrv.c. References aggregate_value_p(), bsi_end_p(), bsi_next(), bsi_start, bsi_stmt(), CALL_EXPR_RETURN_SLOT_OPT, dest_safe_for_nrv_p(), TREE_CODE, and TREE_OPERAND. { 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; }

static tree finalize_nrv_r (   )  [static]

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. Definition at line 76 of file tree-nrv.c. References NULL_TREE, nrv_data::result, TYPE_P, and nrv_data::var. { 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; }

static tree finalize_nrv_r (  tree *  , int *  , void *  )  [static]

static unsigned int tree_nrv (   )  [static]

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. Definition at line 105 of file tree-nrv.c. References aggregate_value_p(), bsi_end_p(), bsi_next(), bsi_start, bsi_stmt(), current_function_decl, DECL_ALIGN, DECL_CONTEXT, DECL_RESULT, nrv_data::result, TREE_ADDRESSABLE, TREE_CODE, TREE_OPERAND, TREE_STATIC, TREE_THIS_VOLATILE, and TREE_TYPE. { 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; } } } 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; }

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Variable Documentation

struct tree_opt_pass pass_nrv

Initial value: { "nrv", NULL, tree_nrv, NULL, NULL, 0, TV_TREE_NRV, PROP_cfg, 0, 0, 0, TODO_dump_func | TODO_ggc_collect, 0 } Definition at line 217 of file tree-nrv.c.

struct tree_opt_pass pass_return_slot

Initial value: { "retslot", NULL, execute_return_slot_opt, NULL, NULL, 0, 0, PROP_ssa | PROP_alias, 0, 0, 0, 0, 0 } Definition at line 306 of file tree-nrv.c.

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