[libfirm] / ir / be / becopyheur4.c

/**
 * This is the C implementation of the trivial mst algo
 * originally written in Java by Sebastian Hack.
 * Performs simple copy minimzation.
 *
 * @author Christian Wuerdig
 * @date   27.04.2007
 * @id     $Id$
 */

#ifdef HAVE_CONFIG_H
#include "config.h"
#endif /* HAVE_CONFIG_H */

#include <float.h>

#include "array.h"
#include "irnode.h"
#include "bitset.h"
#include "raw_bitset.h"
#include "irphase_t.h"
#include "pqueue.h"
#include "pset_new.h"
#include "xmalloc.h"
#include "pdeq.h"

#include "bearch.h"
#include "beifg.h"
#include "be_t.h"
#include "becopyopt_t.h"
#include "irbitset.h"

#define COL_COST_INFEASIBLE       DBL_MAX
#define AFF_NEIGHBOUR_FIX_BENEFIT 128.0
#define NEIGHBOUR_CONSTR_COSTS    64.0

typedef struct _col_cost_t {
        int    col;
        double cost;
} col_cost_t;

typedef struct _aff_chunk_t {
        bitset_t *nodes;
        double   weight;
        unsigned weight_consistent : 1;
} aff_chunk_t;

typedef struct _aff_edge_t {
        ir_node *src;
        ir_node *tgt;
        double  weight;
} aff_edge_t;

/* main coalescing environment*/
typedef struct _co_mst_env_t {
        int              n_regs;         /**< number of regs in class */
        int              k;              /**< number of non-ignore registers in class */
        bitset_t         *ignore_regs;   /**< set containing all global ignore registers */
        ir_phase         ph;             /**< phase object holding data for nodes */
        pqueue           *chunks;        /**< priority queue for chunks */
        pset_new_t       chunkset;       /**< set holding all chunks */
        be_ifg_t         *ifg;           /**< the interference graph */
        const arch_env_t *aenv;          /**< the arch environment */
        copy_opt_t       *co;            /**< the copy opt object */
} co_mst_env_t;

/* stores coalescing related information for a node */
typedef struct _co_mst_irn_t {
        ir_node     *irn;
        aff_chunk_t *chunk;
        bitset_t    *adm_colors;
        int         int_neigh;
        int         col;
        int         tmp_col;
        unsigned    fixed     : 1;
        unsigned    tmp_fixed : 1;
} co_mst_irn_t;


#define get_co_mst_irn(mst_env, irn) (phase_get_or_set_irn_data(&(mst_env)->ph, (irn)))

typedef int decide_func_t(co_mst_irn_t *node, int col);

static INLINE int get_mst_irn_col(co_mst_irn_t *node) {
        return node->tmp_fixed ? node->tmp_col : node->col;
}

/**
 * @return 1 if node @p node has color @p col, 0 otherwise.
 */
static int decider_has_color(co_mst_irn_t *node, int col) {
        return get_mst_irn_col(node) == col;
}

/**
 * @return 1 if node @p node has not color @p col, 0 otherwise.
 */
static int decider_hasnot_color(co_mst_irn_t *node, int col) {
        return get_mst_irn_col(node) != col;
}

/**
 * Always returns true.
 */
static int decider_always_yes(co_mst_irn_t *node, int col) {
        return 1;
}

/* compares two affinity edges */
static int cmp_aff_edge(const void *a, const void *b) {
        const aff_edge_t *e1 = a;
        const aff_edge_t *e2 = b;

        /* sort in descending order */
        return e1->weight < e2->weight ? 1 : -1;
}

/* compares to color-cost pairs */
static int cmp_col_cost(const void *a, const void *b) {
        const col_cost_t *c1 = a;
        const col_cost_t *c2 = b;

        return c1->cost < c2->cost ? -1 : 1;
}

/**
 * In case there is no phase information for irn, initialize it.
 */
static void *co_mst_irn_init(ir_phase *ph, ir_node *irn, void *old) {
        co_mst_irn_t *res = old ? old : phase_alloc(ph, sizeof(res[0]));
        co_mst_env_t *env = ph->priv;

        if (res != old) {
                void                      *neigh_it = be_ifg_neighbours_iter_alloca(env->ifg);
                const arch_register_req_t *req;
                ir_node                   *m;

                res->irn       = irn;
                res->chunk     = NULL;
                res->fixed     = 0;
                res->tmp_fixed = 0;
                res->tmp_col   = -1;
                res->int_neigh = 0;
                res->col       = arch_register_get_index(arch_get_irn_register(env->aenv, irn));

                /* set admissible registers */
                res->adm_colors = bitset_obstack_alloc(phase_obst(ph), env->n_regs);

                /* Exclude colors not assignable to the irn */
                req = arch_get_register_req(env->aenv, irn, -1);
                if (arch_register_req_is(req, limited))
                        rbitset_copy_to_bitset(req->limited, res->adm_colors);

                /* exclude global ignore registers as well */
                bitset_andnot(res->adm_colors, env->ignore_regs);

                /* calculate the number of interfering neighbours */
                be_ifg_foreach_neighbour(env->ifg, neigh_it, irn, m) {
                        if (! arch_irn_is(env->aenv, m, ignore))
                                res->int_neigh++;
                }

        }

        return res;
}

/**
 * Creates a new affinity chunk
 */
static INLINE aff_chunk_t *new_aff_chunk(co_mst_env_t *env) {
        aff_chunk_t *c = xmalloc(sizeof(*c));
        c->weight_consistent = 0;
        c->nodes             = bitset_irg_malloc(env->co->irg);
        pset_new_insert(&env->chunkset, c);
        return c;
}

/**
 * Frees all memory allocated by an affinity chunk.
 */
static INLINE void delete_aff_chunk(co_mst_env_t *env, aff_chunk_t *c) {
        pset_new_remove(&env->chunkset, c);
        bitset_free(c->nodes);
        free(c);
}

/**
 * Adds a node to an affinity chunk
 */
static INLINE void aff_chunk_add_node(aff_chunk_t *c, co_mst_irn_t *node) {
        c->weight_consistent = 0;
        node->chunk          = c;
        bitset_set(c->nodes, get_irn_idx(node->irn));
}

/**
 * Check if there are interference edges from c1 to c2.
 * @param env   The global co_mst environment
 * @param c1    A chunk
 * @param c2    Another chunk
 * @return 1 if there are interferences between nodes of c1 and c2, 0 otherwise.
 */
static INLINE int aff_chunks_interfere(co_mst_env_t *env, aff_chunk_t *c1, aff_chunk_t *c2) {
        void *nodes_it = be_ifg_nodes_iter_alloca(env->ifg);
        int  idx;

        /* check if there is a node in c1 having an interfering neighbour in c2 */
        bitset_foreach(c1->nodes, idx) {
                ir_node *n = get_idx_irn(env->co->irg, idx);
                ir_node *neigh;

                be_ifg_foreach_neighbour(env->ifg, nodes_it, n, neigh) {
                        if (bitset_is_set(c2->nodes, get_irn_idx(neigh)))
                                return 1;
                }
        }

        return 0;
}

/**
 * Let c1 absorb the nodes of c2 (only possible when there
 * are no interference edges from c1 to c2).
 * @return 1 if successful, 0 if not possible
 */
static INLINE int aff_chunk_absorb(co_mst_env_t *env, aff_chunk_t *c1, aff_chunk_t *c2) {
        if (! aff_chunks_interfere(env, c1, c2)) {
                bitset_or(c1->nodes, c2->nodes);
                c1->weight_consistent = 0;
                return 1;
        }
        return 0;
}

/**
 * Returns the affinity chunk of @p irn or creates a new
 * one with @p irn as element if there is none assigned.
 */
static INLINE aff_chunk_t *get_or_set_aff_chunk(co_mst_env_t *env, ir_node *irn) {
        co_mst_irn_t *node = get_co_mst_irn(env, irn);

        if (node->chunk == NULL) {
                node->chunk = new_aff_chunk(env);
                aff_chunk_add_node(node->chunk, node);
        }

        return node->chunk;
}

/**
 * Assures that the weight of the given chunk is consistent.
 */
static void aff_chunk_assure_weight(co_mst_env_t *env, aff_chunk_t *c) {
        if (! c->weight_consistent) {
                double w = 0.0;
                int    idx;

                bitset_foreach(c->nodes, idx) {
                        ir_node         *n  = get_idx_irn(env->co->irg, idx);
                        affinity_node_t *an = get_affinity_info(env->co, n);
                        co_mst_irn_t    *n1 = get_co_mst_irn(env, n);

                        if (an != NULL) {
                                neighb_t *neigh;
                                co_gs_foreach_neighb(an, neigh) {
                                        ir_node      *m    = neigh->irn;
                                        int          m_idx = get_irn_idx(m);
                                        co_mst_irn_t *n2;

                                        /* skip ignore nodes */
                                        if (arch_irn_is(env->aenv, m, ignore))
                                                continue;

                                        n2 = get_co_mst_irn(env, m);

                                        /* record the edge in only one direction */
                                        if (idx < m_idx)
                                                w += (double)neigh->costs / (double)(1 + n1->int_neigh + n2->int_neigh);
                                }
                        }
                }

                c->weight            = w;
                c->weight_consistent = 1;
        }
}

/**
 * Build chunks of nodes connected by affinity edges.
 * We start at the heaviest affinity edge.
 * The chunks of the two edge-defining nodes will be
 * merged if there are no interference edges from one
 * chunk to the other.
 */
static void build_affinity_chunks(co_mst_env_t *env) {
        void        *nodes_it = be_ifg_nodes_iter_alloca(env->ifg);
        aff_edge_t  *edges    = NEW_ARR_F(aff_edge_t, 0);
        ir_node     *n;
        int         i;
        aff_chunk_t *curr_chunk;
        pset_new_iterator_t iter;

        /* at first we create the affinity edge objects */
        be_ifg_foreach_node(env->ifg, nodes_it, n) {
                int             n_idx = get_irn_idx(n);
                co_mst_irn_t    *n1;
                affinity_node_t *an;

                /* skip ignore nodes */
                if (arch_irn_is(env->aenv, n, ignore))
                        continue;

                n1 = get_co_mst_irn(env, n);
                an = get_affinity_info(env->co, n);

                if (an != NULL) {
                        neighb_t *neigh;
                        co_gs_foreach_neighb(an, neigh) {
                                ir_node      *m    = neigh->irn;
                                int          m_idx = get_irn_idx(m);
                                co_mst_irn_t *n2;

                                /* skip ignore nodes */
                                if (arch_irn_is(env->aenv, m, ignore))
                                        continue;

                                n2 = get_co_mst_irn(env, m);

                                /* record the edge in only one direction */
                                if (n_idx < m_idx) {
                                        aff_edge_t edge;

                                        edge.src    = n;
                                        edge.tgt    = m;
                                        edge.weight = (double)neigh->costs / (double)(1 + n1->int_neigh + n2->int_neigh);
                                        ARR_APP1(aff_edge_t, edges, edge);
                                }
                        }
                }
        }

        /* now: sort edges and build the affinity chunks */
        qsort(edges, ARR_LEN(edges), sizeof(edges[0]), cmp_aff_edge);
        for (i = 0; i < ARR_LEN(edges); ++i) {
                aff_chunk_t *c1 = get_or_set_aff_chunk(env, edges[i].src);
                aff_chunk_t *c2 = get_or_set_aff_chunk(env, edges[i].tgt);
                int         res = aff_chunk_absorb(env, c1, c2);

                /* if c2 was absorbed by c1, we can remove c2 */
                if (res) {
                        co_mst_irn_t *node = get_co_mst_irn(env, edges[i].tgt);
                        node->chunk = c1;
                        delete_aff_chunk(env, c2);
                }
        }

        /* now insert all chunks into a priority queue */
        foreach_pset_new(&env->chunkset, curr_chunk, iter) {
                aff_chunk_assure_weight(env, curr_chunk);
                pqueue_put(env->chunks, curr_chunk, curr_chunk->weight);
        }

        DEL_ARR_F(edges);
}

/**
 * Greedy collect affinity neighbours into thew new chunk @p chunk starting at node @p node.
 */
static void expand_chunk_from(co_mst_env_t *env, co_mst_irn_t *node, bitset_t *visited,
        aff_chunk_t *chunk, aff_chunk_t *orig_chunk, decide_func_t *decider, int col)
{
        waitq *nodes = new_waitq();

        /* init queue and chunk */
        waitq_put(nodes, node);
        bitset_set(visited, get_irn_idx(node->irn));
        aff_chunk_add_node(chunk, node);

        /* as long as there are nodes in the queue */
        while (! waitq_empty(nodes)) {
                co_mst_irn_t    *n    = waitq_get(nodes);
                affinity_node_t *an   = get_affinity_info(env->co, n->irn);
                int             n_idx = get_irn_idx(n->irn);

                /* check all affinity neighbors */
                if (an != NULL) {
                        neighb_t *neigh;
                        co_gs_foreach_neighb(an, neigh) {
                                ir_node      *m    = neigh->irn;
                                int          m_idx = get_irn_idx(m);
                                co_mst_irn_t *n2;

                                /* skip ignore nodes */
                                if (arch_irn_is(env->aenv, m, ignore))
                                        continue;

                                n2 = get_co_mst_irn(env, m);

                                if (n_idx < m_idx                                 &&
                                        ! bitset_is_set(visited, m_idx)               &&
                                        decider(n2, col)                              &&
                                        ! n2->fixed                                   &&
                                        ! aff_chunks_interfere(env, chunk, n2->chunk) &&
                                        bitset_is_set(orig_chunk->nodes, m_idx))
                                {
                                        /*
                                                following conditions are met:
                                                - neighbour is not visited
                                                - neighbour likes the color
                                                - neighbour has not yet a fixed color
                                                - the new chunk doesn't interfere with the chunk of the neighbour
                                                - neighbour belongs or belonged once to the original chunk
                                        */
                                        bitset_set(visited, m_idx);
                                        aff_chunk_add_node(chunk, n2);
                                        /* enqueue for further search */
                                        waitq_put(nodes, n2);
                                }
                        }
                }
        }

        del_waitq(nodes);
}

/**
 * Fragment the given chunk into chunks having given color and not having given color.
 */
static aff_chunk_t *fragment_chunk(co_mst_env_t *env, int col, aff_chunk_t *c, waitq *tmp) {
        bitset_t    *visited = bitset_irg_malloc(env->co->irg);
        int         idx;
        aff_chunk_t *best = NULL;

        bitset_foreach(c->nodes, idx) {
                ir_node       *irn;
                co_mst_irn_t  *node;
                aff_chunk_t   *tmp_chunk;
                decide_func_t *decider;

                if (bitset_is_set(visited, idx))
                        continue;

                irn  = get_idx_irn(env->co->irg, idx);
                node = get_co_mst_irn(env, irn);

                /* create a new chunk starting at current node */
                tmp_chunk = new_aff_chunk(env);
                waitq_put(tmp, tmp_chunk);
                decider = get_mst_irn_col(node) == col ? decider_has_color : decider_hasnot_color;
                expand_chunk_from(env, node, visited, tmp_chunk, c, decider, col);
                assert(bitset_popcnt(tmp_chunk->nodes) > 0 && "No nodes added to chunk");

                /* remember the local best */
                aff_chunk_assure_weight(env, tmp_chunk);
                if (! best || best->weight < tmp_chunk->weight)
                        best = tmp_chunk;
        }

        assert(best && "No chunk found?");
        bitset_free(visited);
        return best;
}

/**
 * Initializes an array of color-cost pairs.
 * Sets forbidden colors to costs COL_COST_INFEASIBLE and all others to @p c.
 */
static INLINE void col_cost_init(co_mst_env_t *env, col_cost_t *cost, double c) {
        int i;

        for (i = 0; i < env->n_regs; ++i) {
                cost[i].col = i;
                if (bitset_is_set(env->ignore_regs, i))
                        cost[i].cost = COL_COST_INFEASIBLE;
                else
                        cost[i].cost = c;
        }
}

/**
 * Initializes an array of color-cost pairs.
 * Sets all colors except color @p col to COL_COST_INFEASIBLE and @p col to 0.0
 */
static INLINE void col_cost_init_single(co_mst_env_t *env, col_cost_t *cost, int col) {
        assert(! bitset_is_set(env->ignore_regs, col) && "Attempt to use forbidden color.");
        col_cost_init(env, cost, COL_COST_INFEASIBLE);
        cost[col].col = 0;
        cost[0].col   = col;
        cost[0].cost  = 0.0;
}

/**
 * Resets the temporary fixed color of all nodes within wait queue @p nodes.
 * ATTENTION: the queue is empty after calling this function!
 */
static INLINE void reject_coloring(waitq *nodes) {
        while (! waitq_empty(nodes)) {
                co_mst_irn_t *n = waitq_get(nodes);
                n->tmp_fixed = 0;
        }
}

/**
 * Determines the costs for each color if it would be assigned to node @p node.
 */
static void determine_color_costs(co_mst_env_t *env, co_mst_irn_t *node, col_cost_t *costs) {
        affinity_node_t *an       = get_affinity_info(env->co, node->irn);
        void            *nodes_it = be_ifg_nodes_iter_alloca(env->ifg);
        neighb_t        *aff_neigh;
        ir_node         *int_neigh;
        int             idx;

        col_cost_init(env, costs, 0.0);

        /* calculate (negative) costs for affinity neighbours */
        co_gs_foreach_neighb(an, aff_neigh) {
                ir_node      *m     = aff_neigh->irn;
                co_mst_irn_t *neigh = get_co_mst_irn(env, m);
                double       c      = (double)aff_neigh->costs;

                /* calculate costs for fixed affinity neighbours */
                if (neigh->tmp_fixed || neigh->fixed) {
                        int col = get_mst_irn_col(neigh);
                        costs[col].cost -= c * AFF_NEIGHBOUR_FIX_BENEFIT;
                }
        }

        /* calculate (positive) costs for interfering neighbours */
        be_ifg_foreach_neighbour(env->ifg, nodes_it, node->irn, int_neigh) {
                co_mst_irn_t *neigh  = get_co_mst_irn(env, int_neigh);
                int          col     = get_mst_irn_col(neigh);
                int          col_cnt = bitset_popcnt(neigh->adm_colors);

                if (neigh->tmp_fixed || neigh->fixed) {
                        /* colors of fixed interfering neighbours are infeasible */
                        costs[col].cost = COL_COST_INFEASIBLE;
                }
                else if (col_cnt < env->k) {
                        /* calculate costs for constrained interfering neighbours */
                        double ratio = 1.0 - ((double)col_cnt / (double)env->k);

                        bitset_foreach_clear(neigh->adm_colors, idx) {
                                /* check only explicitly forbidden colors (skip global forbidden ones) */
                                if (! bitset_is_set(env->ignore_regs, idx)) {
                                        costs[col].cost += ratio * NEIGHBOUR_CONSTR_COSTS;
                                }
                        }
                }
        }

        /* set all not admissible colors to COL_COST_INFEASIBLE */
        bitset_foreach_clear(node->adm_colors, idx)
                costs[idx].cost = COL_COST_INFEASIBLE;
}

/* need forward declaration due to recursive call */
static int recolor_nodes(co_mst_env_t *env, co_mst_irn_t *node, col_cost_t *costs, waitq *changed_ones);

/**
 * Tries to change node to a color but @p explude_col.
 * @return 1 if succeeded, 0 otherwise.
 */
static int change_node_color_excluded(co_mst_env_t *env, co_mst_irn_t *node, int exclude_col, waitq *changed_ones) {
        int col = get_mst_irn_col(node);
        int res = 0;

        /* neighbours has already a different color -> good, temporary fix it */
        if (col != exclude_col) {
                node->tmp_fixed = 1;
                node->tmp_col   = col;
                waitq_put(changed_ones, node);
                return 1;
        }

        /* The node has the color it should not have _and_ has not been visited yet. */
        if (! (node->tmp_fixed || node->fixed)) {
                col_cost_t *costs = alloca(env->n_regs * sizeof(costs[0]));

                /* Get the costs for giving the node a specific color. */
                determine_color_costs(env, node, costs);

                /* Since the node must not have the not_col, set the costs for that color to "infinity" */
                costs[exclude_col].cost = COL_COST_INFEASIBLE;

                /* sort the colors according costs, cheapest first. */
                qsort(costs, env->n_regs, sizeof(costs[0]), cmp_col_cost);

                /* Try recoloring the node using the color list. */
                res = recolor_nodes(env, node, costs, changed_ones);
        }

        return res;
}

/**
 * Tries to bring node @p node to cheapest color and color all interfering neighbours with other colors.
 * ATTENTION: Expect @p costs already sorted by increasing costs.
 * @return 1 if coloring could be applied, 0 otherwise.
 */
static int recolor_nodes(co_mst_env_t *env, co_mst_irn_t *node, col_cost_t *costs, waitq *changed_ones) {
        int   i;
        waitq *local_changed = new_waitq();

        for (i = 0; i < env->n_regs; ++i) {
                void    *nodes_it = be_ifg_nodes_iter_alloca(env->ifg);
                int     tgt_col   = costs[i].col;
                int     neigh_ok  = 1;
                ir_node *neigh;

                /* If the costs for that color (and all successive) are infinite, bail out we won't make it anyway. */
                if (costs[i].cost == COL_COST_INFEASIBLE) {
                        node->tmp_fixed = 0;
                        del_waitq(local_changed);
                        return 0;
                }

                /* Set the new color of the node and mark the node as temporarily fixed. */
                assert(! node->tmp_fixed && "Node must not have been temporary fixed.");
                node->tmp_fixed = 1;
                node->tmp_col   = tgt_col;

                assert(waitq_empty(local_changed) && "Node queue should be empty here.");
                waitq_put(local_changed, node);

                /* try to color all interfering neighbours with current color forbidden */
                be_ifg_foreach_neighbour(env->ifg, nodes_it, node->irn, neigh) {
                        co_mst_irn_t *nn = get_co_mst_irn(env, neigh);
                        /*
                                Try to change the color of the neighbor and record all nodes which
                                get changed in the tmp list. Add this list to the "changed" list for
                                that color. If we did not succeed to change the color of the neighbor,
                                we bail out and try the next color.
                        */
                        if (get_mst_irn_col(nn) == tgt_col) {
                                waitq *tmp = new_waitq();

                                /* try to color neighbour with tgt_col forbidden */
                                neigh_ok = change_node_color_excluded(env, nn, tgt_col, tmp);

                                /* join lists of changed nodes */
                                while (! waitq_empty(tmp))
                                        waitq_put(local_changed, waitq_get(tmp));
                                del_waitq(tmp);

                                if (! neigh_ok)
                                        break;
                        }
                }

                /*
                        We managed to assign the target color to all neighbors, so from the perspective
                        of the current node, every thing was ok and we can return safely.
                */
                if (neigh_ok) {
                        /* append the local_changed ones to global ones */
                        while (! waitq_empty(local_changed))
                                waitq_put(changed_ones, waitq_get(local_changed));
                        del_waitq(local_changed);
                        return 1;
                }
                else {
                        /* coloring of neighbours failed, so we try next color */
                        reject_coloring(local_changed);
                }
        }

        del_waitq(local_changed);
        return 0;
}

/**
 * Tries to bring node @p node and all it's neighbours to color @p tgt_col.
 * @return 1 if color @p col could be applied, 0 otherwise
 */
static int change_node_color(co_mst_env_t *env, co_mst_irn_t *node, int tgt_col, waitq *changed_ones) {
        int col = get_mst_irn_col(node);

        /* if node already has the target color -> good, temporary fix it */
        if (col == tgt_col) {
                if (! node->tmp_fixed) {
                        node->tmp_fixed = 1;
                        node->tmp_col   = tgt_col;
                        waitq_put(changed_ones, node);
                }
                return 1;
        }

        /*
                Node has not yet a fixed color and target color is admissible
                -> try to recolor node and it's affinity neighbours
        */
        if (! (node->fixed || node->tmp_fixed) && bitset_is_set(node->adm_colors, tgt_col)) {
                col_cost_t *costs = alloca(env->n_regs * sizeof(costs[0]));
                col_cost_init_single(env, costs, tgt_col);
                return recolor_nodes(env, node, costs, changed_ones);
        }

        return 0;
}

/**
 * Tries to color an affinity chunk (or at least a part of it).
 * Inserts uncolored parts of the chunk as a new chunk into the priority queue.
 */
static void color_aff_chunk(co_mst_env_t *env, aff_chunk_t *c) {
        aff_chunk_t *best_chunk   = NULL;
        int         best_color    = -1;
        waitq       *changed_ones = new_waitq();
        waitq       *tmp_chunks   = new_waitq();
        bitset_t    *visited;
        int         col, idx;

        /* check which color is the "best" for the given chunk */
        for (col = 0; col < env->k; ++col) {
                int         one_good = 0;
                aff_chunk_t *local_best;

                /* try to bring all nodes of given chunk to the current color. */
                bitset_foreach(c->nodes, idx) {
                        ir_node      *irn  = get_idx_irn(env->co->irg, idx);
                        co_mst_irn_t *node = get_co_mst_irn(env, irn);

                        assert(! node->fixed && "Node must not have a fixed color.");

                        one_good = change_node_color(env, node, col, changed_ones);

                        if (one_good)
                                break;
                }

                /* try next color when failed */
                if (! one_good)
                        continue;

                /* fragment the chunk according to the coloring */
                local_best = fragment_chunk(env, col, c, tmp_chunks);

                /* check if the local best is global best */
                if (local_best) {
                        aff_chunk_assure_weight(env, local_best);

                        if (! best_chunk || best_chunk->weight < local_best->weight) {
                                /* kill the old best */
                                if (best_chunk)
                                        delete_aff_chunk(env, best_chunk);
                                best_chunk = local_best;
                                best_color = col;
                        }
                }

                /* reject the coloring and bring the coloring to the initial state */
                reject_coloring(changed_ones);
        }

        /* free all intermediate created chunks except best one */
        while (! waitq_empty(tmp_chunks)) {
                aff_chunk_t *tmp = waitq_get(tmp_chunks);
                if (tmp != best_chunk)
                        delete_aff_chunk(env, tmp);
        }
        del_waitq(tmp_chunks);

        /* return if coloring failed */
        if (! best_chunk) {
                delete_aff_chunk(env, c);
                del_waitq(changed_ones);
                return;
        }

        /* get the best fragment from the best list and color it */
        bitset_foreach(best_chunk->nodes, idx) {
                ir_node      *irn  = get_idx_irn(env->co->irg, idx);
                co_mst_irn_t *node = get_co_mst_irn(env, irn);
                int          res;

                res = change_node_color(env, node, col, changed_ones);
                assert(res && "Coloring failed");
                node->fixed = 1;
                node->col   = node->tmp_col;
                node->chunk = best_chunk;
        }

        /* fix colors */
        while (! waitq_empty(changed_ones)) {
                co_mst_irn_t *n = waitq_get(changed_ones);
                n->fixed = 1;
                n->col   = n->tmp_col;
        }

        /* remove the nodes in best chunk from original chunk */
        bitset_andnot(c->nodes, best_chunk->nodes);

        /* fragment the remaining chunk */
        visited = bitset_irg_malloc(env->co->irg);
        bitset_or(visited, best_chunk->nodes);
        bitset_foreach(c->nodes, idx) {
                if (! bitset_is_set(visited, idx)) {
                        aff_chunk_t  *new_chunk = new_aff_chunk(env);
                        ir_node      *irn       = get_idx_irn(env->co->irg, idx);
                        co_mst_irn_t *node      = get_co_mst_irn(env, irn);

                        expand_chunk_from(env, node, visited, new_chunk, c, decider_always_yes, 0);
                        aff_chunk_assure_weight(env, new_chunk);
                        pqueue_put(env->chunks, new_chunk, new_chunk->weight);
                }
        }

        /* clear obsolete chunks and free some memory */
        delete_aff_chunk(env, c);
        delete_aff_chunk(env, best_chunk);
        bitset_free(visited);
        del_waitq(changed_ones);
}

/**
 * Main driver for mst safe coalescing algorithm.
 */
int co_solve_heuristic_mst(copy_opt_t *co)
{
        unsigned     n_regs       = co->cenv->cls->n_regs;
        bitset_t     *ignore_regs = bitset_alloca(n_regs);
        unsigned     k;
        co_mst_env_t mst_env;

        /* init phase */
        phase_init(&mst_env.ph, "co_mst", co->irg, PHASE_DEFAULT_GROWTH, co_mst_irn_init, &mst_env);

        k = be_put_ignore_regs(co->cenv->birg, co->cenv->cls, ignore_regs);
        k = n_regs - k;

        mst_env.n_regs      = n_regs;
        mst_env.k           = k;
        mst_env.chunks      = new_pqueue();
        mst_env.co          = co;
        mst_env.ignore_regs = ignore_regs;
        mst_env.ifg         = co->cenv->ifg;
        pset_new_init(&mst_env.chunkset);

        /* build affinity chunks */
        build_affinity_chunks(&mst_env);

        /* color chunks as long as there are some */
        while (! pqueue_empty(mst_env.chunks)) {
                aff_chunk_t *chunk = pqueue_get(mst_env.chunks);
                color_aff_chunk(&mst_env, chunk);
        }

        /* free allocated memory */
        del_pqueue(mst_env.chunks);
        phase_free(&mst_env.ph);
        pset_new_destroy(&mst_env.chunkset);

        return 0;
}