#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 "gimple.h"
#include "gimple-ssa.h"
#include "tree-ssanames.h"
#include "tree-pass.h"
#include "langhooks.h"
#include "flags.h"

Include dependency graph for tree-nrv.c:

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	gate_pass_return_slot ()
gimple_opt_pass *	make_pass_nrv ()
static bool	dest_safe_for_nrv_p ()
static unsigned int	execute_return_slot_opt ()
gimple_opt_pass *	make_pass_return_slot ()

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.

DEST is available if it is not clobbered or used by the call.

References aggregate_value_p(), FOR_EACH_BB, gimple_call_fndecl(), gimple_call_lhs(), gimple_call_return_slot_opt_p(), gimple_call_set_return_slot_opt(), walk_stmt_info::gsi, gsi_end_p(), gsi_next(), gsi_start_bb(), gsi_stmt(), is_gimple_call(), and TREE_TYPE.

static unsigned int execute_return_slot_opt ( )

static

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.

Check if the location being assigned to is clobbered by the call.

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

static

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.

 No need to walk into types.

 Otherwise replace all occurrences of VAR with RESULT.

 Keep iterating.

References nrv_data::modified, and nrv_data::result.

static bool gate_pass_return_slot ( )

static

References execute(), and tree_nrv().

gimple_opt_pass* make_pass_nrv ( )

gimple_opt_pass* make_pass_return_slot ( )

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.

 If this function does not return an aggregate type in memory, then
 there is nothing to do.

 If a GIMPLE type is returned in memory, finalize_nrv_r might create
 non-GIMPLE.

 If the front end already did something like this, don't do it here.

 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.

 Look through each block for assignments to the RESULT_DECL.

             In a function with an aggregate return value, the
             gimplifier has changed all non-empty RETURN_EXPRs to
             return the RESULT_DECL.

             Now verify that this return statement uses the same value
             as any previously encountered return statement.

                 If we found a return statement using a different variable
                 than previous return statements, then we can not perform
                 NRV optimizations.

             The returned value must be a local automatic variable of the
             same type and alignment as the function's result.

              If there's any MODIFY of component of RESULT,
              then bail out.

 If dumping details, then note once and only the NRV replacement.

 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.

 Now walk through the function changing all references to VAR to be
 RESULT.

         If this is a copy from VAR to RESULT, remove it.

References DECL_ALIGN, DECL_CONTEXT, gcc_assert, get_base_address(), gimple_assign_copy_p(), gimple_assign_rhs1(), gimple_get_lhs(), gimple_has_lhs(), gimple_return_retval(), gsi_end_p(), gsi_next(), gsi_start_bb(), gsi_stmt(), NULL, TREE_ADDRESSABLE, TREE_CODE, TREE_STATIC, TREE_THIS_VOLATILE, TREE_TYPE, and useless_type_conversion_p().

Referenced by gate_pass_return_slot().

Data Structures

Functions

Function Documentation