lower_switch.c

/*
 * Copyright (C) 1995-2008 University of Karlsruhe.  All right reserved.
 *
 * This file is part of libFirm.
 *
 * This file may be distributed and/or modified under the terms of the
 * GNU General Public License version 2 as published by the Free Software
 * Foundation and appearing in the file LICENSE.GPL included in the
 * packaging of this file.
 *
 * Licensees holding valid libFirm Professional Edition licenses may use
 * this file in accordance with the libFirm Commercial License.
 * Agreement provided with the Software.
 *
 * This file is provided AS IS with NO WARRANTY OF ANY KIND, INCLUDING THE
 * WARRANTY OF DESIGN, MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE.
 */

/**
 * @file
 * @brief   Lowering of Switches if necessary or advantageous.
 * @author  Moritz Kroll
 * @version $Id$
 */

#include "config.h"

#include <limits.h>

#include "array_t.h"
#include "ircons.h"
#include "irgopt.h"
#include "irgwalk.h"
#include "irnode_t.h"
#include "irouts.h"
#include "irpass_t.h"
#include "lowering.h"

#define foreach_out_irn(irn, i, outirn) for (i = get_irn_n_outs(irn) - 1;\
	i >= 0 && (outirn = get_irn_out(irn, i)); --i)

typedef struct walk_env {
	unsigned         spare_size;        /**< the allowed spare size for table switches */
	int              changed;           /**< indicates whether a change was performed */
} walk_env_t;

typedef struct case_data {
	long     value;
	ir_node *target;
} case_data_t;

typedef struct ifcas_env {
	ir_node  *sel;
	int       defindex;
	ir_node **defusers;                 /**< the Projs pointing to the default case */
} ifcas_env_t;

/**
 * Evaluate a switch and decide whether we should build a table switch.
 *
 * @param cond       The Cond node representing the switch.
 * @param spare_size Allowed spare size for table switches in machine words.
 *                   (Default in edgfe: 128)
 */
static int should_do_table_switch(ir_node *cond, unsigned spare_size)
{
	long     default_pn;
	int      i;
	ir_node *proj;
	long switch_min = LONG_MAX, switch_max = LONG_MIN;
	unsigned long spare, num_cases = 0;

	/* TODO: Minimum size for jump table? */
	if (get_irn_n_outs(cond) <= 4)
		return 0;

	default_pn = get_Cond_default_proj(cond);

	foreach_out_irn(cond, i, proj) {
		long pn = get_Proj_proj(proj);
		if (pn == default_pn)
			continue;

		if (pn < switch_min)
			switch_min = pn;
		if (pn > switch_max)
			switch_max = pn;
		++num_cases;
	}

	/*
	 * Here we have: num_cases and [switch_min, switch_max] interval.
	 * We do an if-cascade if there are too many spare numbers.
	 */
	spare = (unsigned long) switch_max - (unsigned long) switch_min - num_cases + 1;
	return spare < spare_size;
}

static int casecmp(const void *a, const void *b)
{
	const case_data_t *cda = a;
	const case_data_t *cdb = b;

	/*
	 * Enforce unsigned sorting. Signed comparison will behave differently for
	 * 32-bit values, depending on sizeof(long). This will make the resulting
	 * array deterministic.
	 */
	return ((unsigned long)cda->value > (unsigned long)cdb->value) -
	       ((unsigned long)cda->value < (unsigned long)cdb->value);
}

/**
 * Creates an if cascade realizing binary search.
 */
static void create_if_cascade(ifcas_env_t *env, ir_node *curblock,
                              case_data_t *curcases, int numcases)
{
    ir_mode *cmp_mode;
    ir_node *cmp_sel;
    ir_node *sel_block;

    /* Get the mode and sel node for the comparison. */
    cmp_mode  = get_irn_mode(env->sel);
    cmp_sel   = env->sel;
    sel_block = get_nodes_block(cmp_sel);

    /*
     * Make sure that an unsigned comparison is used, by converting the sel
     * node to an unsigned mode and using that mode for the constants, too.
     * This is important, because the qsort applied to the case labels uses
     * an unsigned comparison and both comparison methods have to match.
     */
    if (mode_is_signed(cmp_mode))
    {
        cmp_mode = find_unsigned_mode(cmp_mode);
        cmp_sel  = new_r_Conv(sel_block, cmp_sel, cmp_mode);
    }

	assert(numcases >= 0);

	set_cur_block(curblock);

	if (numcases == 0) {
		/* zero cases: "goto default;" */
		env->defusers[env->defindex++] = new_Jmp();
	} else if (numcases == 1) {
		/* only one case: "if (sel == val) goto target else goto default;" */
		ir_node *val  = new_Const_long(cmp_mode, curcases[0].value);
		ir_node *cmp  = new_Cmp(cmp_sel, val);
		ir_node *proj = new_Proj(cmp, mode_b, pn_Cmp_Eq);
		ir_node *cond = new_Cond(proj);
		set_Block_cfgpred(curcases[0].target, 0, new_Proj(cond, mode_X, pn_Cond_true));
		env->defusers[env->defindex++] = new_Proj(cond, mode_X, pn_Cond_false);
	} else if (numcases == 2) {
		/* only two cases: "if (sel == val[0]) goto target[0];" */
		ir_node *val  = new_Const_long(cmp_mode, curcases[0].value);
		ir_node *cmp  = new_Cmp(cmp_sel, val);
		ir_node *proj = new_Proj(cmp, mode_b, pn_Cmp_Eq);
		ir_node *cond = new_Cond(proj);
		ir_node *in[1];
		ir_node *neblock;

		set_Block_cfgpred(curcases[0].target, 0, new_Proj(cond, mode_X, pn_Cond_true));
		in[0] = new_Proj(cond, mode_X, pn_Cond_false);
		neblock = new_Block(1, in);
		set_cur_block(neblock);

		/* second part: "else if (sel == val[1]) goto target[1] else goto default;" */
		val  = new_Const_long(cmp_mode, curcases[1].value);
		cmp  = new_Cmp(cmp_sel, val);
		proj = new_Proj(cmp, mode_b, pn_Cmp_Eq);
		cond = new_Cond(proj);
		set_Block_cfgpred(curcases[1].target, 0, new_Proj(cond, mode_X, pn_Cond_true));
		env->defusers[env->defindex++] = new_Proj(cond, mode_X, pn_Cond_false);
	} else {
		/* recursive case: split cases in the middle */
		int midcase = numcases / 2;
		ir_node *val  = new_Const_long(cmp_mode, curcases[midcase].value);
		ir_node *cmp  = new_Cmp(cmp_sel, val);
		ir_node *proj = new_Proj(cmp, mode_b, pn_Cmp_Lt);
		ir_node *cond = new_Cond(proj);
		ir_node *in[1];
		ir_node *ltblock;
		ir_node *geblock;

		in[0] = new_Proj(cond, mode_X, pn_Cond_true);
		ltblock = new_Block(1, in);

		set_cur_block(curblock);
		in[0] = new_Proj(cond, mode_X, pn_Cond_false);
		geblock = new_Block(1, in);
		set_cur_block(geblock);

		create_if_cascade(env, ltblock, curcases, midcase);
		create_if_cascade(env, geblock, curcases + midcase, numcases - midcase);
	}
}

/**
 * Block-Walker: searches for Cond nodes with a non-boolean mode
 */
static void find_cond_nodes(ir_node *block, void *ctx)
{
	walk_env_t  *env = ctx;
	ir_node     *projx;
	ir_node     *cond;
	ir_node     *sel;
	ir_mode     *sel_mode;
	long         default_pn;
	int          i, j = 0, numcases;
	ir_node     *proj;
	case_data_t *cases;
	ir_node     *condblock;
	ir_node     *defblock = NULL;
	ifcas_env_t  ifcas_env;

	if (get_Block_n_cfgpreds(block) != 1)
		return;

	projx = get_Block_cfgpred(block, 0);
	if (!is_Proj(projx))
		return;
	assert(get_irn_mode(projx) == mode_X);

	cond = get_Proj_pred(projx);
	if (!is_Cond(cond))
		return;

	sel      = get_Cond_selector(cond);
	sel_mode = get_irn_mode(sel);

	if (sel_mode == mode_b)    /* not a switch? */
		return;

	if (should_do_table_switch(cond, env->spare_size))
		return;

	/*
	 * Switch should be transformed into an if cascade.
	 * So first order the cases, so we can do a binary search on them.
	 */
	env->changed = 1;

	numcases = get_irn_n_outs(cond) - 1;      // does not contain default case
	NEW_ARR_A(case_data_t, cases, numcases);

	default_pn = get_Cond_default_proj(cond);
	ifcas_env.sel = sel;
	ifcas_env.defindex = 0;
	NEW_ARR_A(ir_node*, ifcas_env.defusers, numcases);

	foreach_out_irn(cond, i, proj) {
		long pn = get_Proj_proj(proj);
		ir_node *target = get_irn_out(proj, 0);
		assert(get_Block_n_cfgpreds(target) == 1 && "Encountered critical edge in switch");

		if (pn == default_pn) {
			defblock = target;
			continue;
		}

		cases[j].value  = pn;
		cases[j].target = target;
		j++;
	}

	assert(defblock != NULL && "Switch without default proj");
	qsort(cases, numcases, sizeof(*cases), casecmp);

	/* Now create the if cascade */
	condblock = get_nodes_block(cond);
	create_if_cascade(&ifcas_env, condblock, cases, numcases);

	/* Connect new default case users */
	set_irn_in(defblock, ifcas_env.defindex, ifcas_env.defusers);
}

/**
 * Lowers all Switches (Cond nodes with non-boolean mode) depending on spare_size.
 * They will either remain the same or be converted into if-cascades.
 *
 * @param irg        The ir graph to be lowered.
 * @param spare_size Allowed spare size for table switches in machine words.
 *                   (Default in edgfe: 128)
 */
void lower_switch(ir_graph *irg, unsigned spare_size)
{
	walk_env_t env;
	ir_graph *rem = current_ir_graph;

	current_ir_graph = irg;

	env.changed    = 0;
	env.spare_size = spare_size;

	remove_critical_cf_edges(irg);
	assure_irg_outs(irg);

	irg_block_walk_graph(irg, find_cond_nodes, NULL, &env);

	if (env.changed) {
		/* control flow changed */
		set_irg_outs_inconsistent(irg);
		set_irg_doms_inconsistent(irg);
		set_irg_extblk_inconsistent(irg);
		set_irg_loopinfo_inconsistent(irg);
	}
	current_ir_graph = rem;
}

struct pass_t {
	ir_graph_pass_t pass;
	unsigned        spare_size;
};

/**
 * Wrapper for running lower_switch() as a pass.
 */
static int pass_wrapper(ir_graph *irg, void *context)
{
	struct pass_t *pass = context;

	lower_switch(irg, pass->spare_size);
	return 0;
}

/* creates a pass for lower_switch */
ir_graph_pass_t *lower_switch_pass(const char *name, unsigned spare_size)
{
	struct pass_t *pass = XMALLOCZ(struct pass_t);

	pass->spare_size = spare_size;
	return def_graph_pass_constructor(
		&pass->pass, name ? name : "lower_switch", pass_wrapper);
}