gcc/tree-nrv.c

/* Language independent return value optimizations
   Copyright (C) 2004-2021 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: */
  virtual bool gate (function *) { return optimize > 0; }

  virtual unsigned int execute (function *);

}; // 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");
    }

  /* 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_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: */
  virtual unsigned int execute (function *);

}; // 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);
}