#include "config.h"
#include "system.h"
#include "coretypes.h"
#include "tm.h"
#include "tree.h"
#include "basic-block.h"
#include "gimple-pretty-print.h"
#include "tree-inline.h"
#include "gimple.h"
#include "gimple-ssa.h"
#include "tree-cfg.h"
#include "tree-phinodes.h"
#include "ssa-iterators.h"
#include "hashtab.h"
#include "tree-iterator.h"
#include "alloc-pool.h"
#include "tree-pass.h"
#include "flags.h"
#include "cfgloop.h"
#include "params.h"

Include dependency graph for tree-ssa-sink.c:

Functions
static basic_block	find_bb_for_arg ()
static bool	all_immediate_uses_same_place ()
static basic_block	nearest_common_dominator_of_uses ()
static basic_block	select_best_block (basic_block early_bb, basic_block late_bb, gimple stmt)
static bool	statement_sink_location (gimple stmt, basic_block frombb, gimple_stmt_iterator *togsi)
static void	sink_code_in_bb ()
static void	execute_sink_code ()
static unsigned int	do_sink ()
static bool	gate_sink ()
gimple_opt_pass *	make_pass_sink_code ()

Variables
struct {
int sunk
}	sink_stats

Function Documentation

static bool all_immediate_uses_same_place ( )

static

When the first immediate use is in a statement, then return true if all immediate uses in IMM are in the same statement. We could also do the case where the first immediate use is in a phi node, and all the other uses are in phis in the same basic block, but this requires some expensive checking later (you have to make sure no def/vdef in the statement occurs for multiple edges in the various phi nodes it's used in, so that you only have one place you can sink it to.

References FOR_EACH_IMM_USE_FAST, is_gimple_debug(), NULL, and USE_STMT.

static unsigned int do_sink ( )

static

Gate and execute functions for PRE.

static void execute_sink_code ( )

static

Perform code sinking. This moves code down the flowgraph when we know it would be profitable to do so, or it wouldn't increase the number of executions of the statement.

IE given

a_1 = b + c; if (<something>) { } else { foo (&b, &c); a_5 = b + c; } a_6 = PHI (a_5, a_1); USE a_6.

we'll transform this into:

if (<something>) { a_1 = b + c; } else { foo (&b, &c); a_5 = b + c; } a_6 = PHI (a_5, a_1); USE a_6.

Note that this reduces the number of computations of a = b + c to 1 when we take the else edge, instead of 2.

References GIMPLE_PASS, OPTGROUP_NONE, PROP_cfg, PROP_no_crit_edges, PROP_ssa, TODO_update_ssa, TODO_verify_flow, and TODO_verify_ssa.

static basic_block find_bb_for_arg ( )

static

Given a PHI, and one of its arguments (DEF), find the edge for that argument and return it. If the argument occurs twice in the PHI node, we return NULL.

static bool gate_sink ( )

static

gimple_opt_pass* make_pass_sink_code ( )

static basic_block nearest_common_dominator_of_uses ( )

static

Find the nearest common dominator of all of the immediate uses in IMM.

Short circuit. Nothing dominates the entry block.

static basic_block select_best_block	(	basic_block	early_bb,
		basic_block	late_bb,
		gimple	stmt
	)

static

Given EARLY_BB and LATE_BB, two blocks in a path through the dominator tree, return the best basic block between them (inclusive) to place statements.

We want the most control dependent block in the shallowest loop nest.

If the resulting block is in a shallower loop nest, then use it. Else only use the resulting block if it has significantly lower execution frequency than EARLY_BB to avoid gratutious statement movement. We consider statements with VOPS more desirable to move.

This pass would obviously benefit from PDO as it utilizes block frequencies. It would also benefit from recomputing frequencies if profile data is not available since frequencies often get out of sync with reality.

     If we've moved into a lower loop nest, then that becomes
     our best block.

     Walk up the dominator tree, hopefully we'll find a shallower
     loop nest.

 If we found a shallower loop nest, then we always consider that
 a win.  This will always give us the most control dependent block
 within that loop nest.

 Get the sinking threshold.  If the statement to be moved has memory
 operands, then increase the threshold by 7% as those are even more
 profitable to avoid, clamping at 100%.

 If BEST_BB is at the same nesting level, then require it to have
 significantly lower execution frequency to avoid gratutious movement.

 No better block found, so return EARLY_BB, which happens to be the
 statement's original block.

References bb_loop_depth(), and get_immediate_dominator().

static void sink_code_in_bb ( )

static

Perform code sinking on BB

 If this block doesn't dominate anything, there can't be any place to sink
 the statements to.

 We can't move things across abnormal edges, so don't try.

     Update virtual operands of statements in the path we
     do not sink to.

     If this is the end of the basic block, we need to insert at the end
     of the basic block.

     If we've just removed the last statement of the BB, the
     gsi_end_p() test below would fail, but gsi_prev() would have
     succeeded, and we want it to succeed.  So we keep track of
     whether we're at the last statement and pick up the new last
     statement.

static bool statement_sink_location	(	gimple	stmt,
		basic_block	frombb,
		gimple_stmt_iterator *	togsi
	)

static

Given a statement (STMT) and the basic block it is currently in (FROMBB), determine the location to sink the statement to, if any. Returns true if there is such location; in that case, TOGSI points to the statement before that STMT should be moved.

 We only can sink assignments.

 We only can sink stmts with a single definition.

 Return if there are no immediate uses of this stmt.

 There are a few classes of things we can't or don't move, some because we
 don't have code to handle it, some because it's not profitable and some
 because it's not legal.

 We can't sink things that may be global stores, at least not without
 calculating a lot more information, because we may cause it to no longer
 be seen by an external routine that needs it depending on where it gets
 moved to.

 We don't want to sink loads from memory.

 We can't sink statements that end basic blocks without splitting the
 incoming edge for the sink location to place it there.

 We can't sink statements that have volatile operands.

 We don't want to sink dead code, so anything with 0 immediate uses is not
 sunk.

 Don't sink BLKmode assignments if current function has any local explicit
 register variables, as BLKmode assignments may involve memcpy or memset
 calls or, on some targets, inline expansion thereof that sometimes need
 to use specific hard registers.

 If stmt is a store the one and only use needs to be the VOP
 merging PHI node.

         A killing definition is not a use.

             If use_stmt is or might be a nop assignment then USE_STMT
             acts as a use as well as definition.

 If all the immediate uses are not in the same place, find the nearest
 common dominator of all the immediate uses.  For PHI nodes, we have to
 find the nearest common dominator of all of the predecessor blocks, since
 that is where insertion would have to take place.

     Our common dominator has to be dominated by frombb in order to be a
     trivially safe place to put this statement, since it has multiple
     uses.

 This can happen if there are multiple uses in a PHI.

 If the latch block is empty, don't make it non-empty by sinking
 something into it.

Variable Documentation

struct { ... } sink_stats

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/. TODO:

Sinking store only using scalar promotion (IE without moving the RHS):

*q = p; p = p + 1; if (something) *q = <not p>="">; else y = *q;

should become sinktemp = p; p = p + 1; if (something) *q = <not p>="">; else { *q = sinktemp; y = *q } Store copy propagation will take care of the store elimination above.

Sinking using Partial Dead Code Elimination.

int sunk

The number of statements sunk down the flowgraph by code sinking.

Functions

Variables

Function Documentation

Variable Documentation