kill nodes only reachable via out edges after backend transformation

[libfirm] / ir / be / beirgmod.c

/**
 * This file contains the following IRG modifications for be routines:
 * - backend dominance information
 * - SSA construction for a set of nodes
 * - insertion of Perm nodes
 * - empty block elimination
 * - a simple dead node elimination (set inputs of unreachable nodes to BAD)
 *
 * Author:      Sebastian Hack, Daniel Grund, Matthias Braun, Christian Wuerdig
 * Date:        04.05.2005
 * Copyright:   (c) Universitaet Karlsruhe
 * Licence:     This file protected by GPL -  GNU GENERAL PUBLIC LICENSE.
 * CVS-Id:      $Id$
 */
#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#include <stdlib.h>

#ifdef HAVE_MALLOC_H
 #include <malloc.h>
#endif
#ifdef HAVE_ALLOCA_H
 #include <alloca.h>
#endif

#include "hashptr.h"
#include "pdeq.h"
#include "pset.h"
#include "pmap.h"
#include "util.h"
#include "debug.h"

#include "irflag_t.h"
#include "ircons_t.h"
#include "irnode_t.h"
#include "ircons_t.h"
#include "irmode_t.h"
#include "irdom_t.h"
#include "iredges_t.h"
#include "irgopt.h"
#include "irprintf_t.h"
#include "irgwalk.h"

#include "be_t.h"
#include "bechordal_t.h"
#include "bearch.h"
#include "besched_t.h"
#include "belive_t.h"
#include "benode_t.h"
#include "beutil.h"
#include "beinsn_t.h"

#include "beirgmod.h"

#define DBG_MODULE "firm.be.irgmod"
#define DBG_LEVEL SET_LEVEL_0

/*
  ____                  _
 |  _ \  ___  _ __ ___ (_)_ __   __ _ _ __   ___ ___
 | | | |/ _ \| '_ ` _ \| | '_ \ / _` | '_ \ / __/ _ \
 | |_| | (_) | | | | | | | | | | (_| | | | | (_|  __/
 |____/ \___/|_| |_| |_|_|_| |_|\__,_|_| |_|\___\___|
 |  ___| __ ___  _ __ | |_(_) ___ _ __ ___
 | |_ | '__/ _ \| '_ \| __| |/ _ \ '__/ __|
 |  _|| | | (_) | | | | |_| |  __/ |  \__ \
 |_|  |_|  \___/|_| |_|\__|_|\___|_|  |___/

*/

/**
 * The dominance frontier for a graph.
 */
struct _be_dom_front_info_t {
        pmap *df_map;         /**< A map, mapping every block to a list of its dominance frontier blocks. */
        struct obstack obst;  /**< An obstack holding all the frontier data. */
};

/**
 * A wrapper for get_Block_idom.
 * This function returns the block itself, if the block is the start
 * block. Returning NULL would make any != comparison true which
 * suggests, that the start block is dominated by some other node.
 * @param bl The block.
 * @return The immediate dominator of the block.
 */
static INLINE ir_node *get_idom(ir_node *bl)
{
        ir_node *idom = get_Block_idom(bl);
        return idom == NULL ? bl : idom;
}

/**
 * Compute the dominance frontier for a given block.
 *
 * @param blk   the block where the calculation starts
 *
 * @return the list of all blocks in the dominance frontier of blk
 */
static ir_node **compute_df(ir_node *blk, be_dom_front_info_t *info)
{
        ir_node *c;
        const ir_edge_t *edge;
        ir_node **df_list = NEW_ARR_F(ir_node *, 0);
        ir_node **df;
        int len;

        /* Add local dominance frontiers */
        foreach_block_succ(blk, edge) {
                ir_node *y = edge->src;

                if (get_idom(y) != blk) {
                        ARR_APP1(ir_node *, df_list, y);
                }
        }

        /*
         * Go recursively down the dominance tree and add all blocks
         * into the dominance frontiers of the children, which are not
         * dominated by the given block.
         */
        for (c = get_Block_dominated_first(blk); c; c = get_Block_dominated_next(c)) {
                int i;
                ir_node **df_c_list = compute_df(c, info);

                for (i = ARR_LEN(df_c_list) - 1; i >= 0; --i) {
                        ir_node *w = df_c_list[i];
                        if (get_idom(w) != blk)
                                ARR_APP1(ir_node *, df_list, w);
                }
        }

        /* now copy the flexible array to the obstack */
        len = ARR_LEN(df_list);
        df = NEW_ARR_D(ir_node *, &info->obst, len);
        memcpy(df, df_list, len * sizeof(df[0]));
        DEL_ARR_F(df_list);

        pmap_insert(info->df_map, blk, df);
        return df;
}

be_dom_front_info_t *be_compute_dominance_frontiers(ir_graph *irg)
{
        be_dom_front_info_t *info = xmalloc(sizeof(*info));

        edges_assure(irg);
        obstack_init(&info->obst);
        info->df_map = pmap_create();
        assure_doms(irg);
        (void)compute_df(get_irg_start_block(irg), info);

        return info;
}

void be_free_dominance_frontiers(be_dom_front_info_t *info)
{
        obstack_free(&info->obst, NULL);
        pmap_destroy(info->df_map);
        free(info);
}

/* Get the dominance frontier of a block. */
ir_node **be_get_dominance_frontier(be_dom_front_info_t *info, ir_node *block)
{
        return pmap_get(info->df_map, block);
}

static void determine_phi_blocks(pset *copies, pset *copy_blocks, pset *phi_blocks, be_dom_front_info_t *df_info)
{
        ir_node *bl;
        waitq *worklist = new_waitq();
        FIRM_DBG_REGISTER(firm_dbg_module_t *dbg, DBG_MODULE);

        /*
         * Fill the worklist queue and the rest of the orig blocks array.
         */
        for (bl = pset_first(copy_blocks); bl; bl = pset_next(copy_blocks)) {
                waitq_put(worklist, bl);
        }

        while (!pdeq_empty(worklist)) {
                ir_node *bl  = waitq_get(worklist);
                ir_node **df = be_get_dominance_frontier(df_info, bl);
                int i;

                ir_node *y;

                DBG((dbg, LEVEL_3, "dom front of %+F\n", bl));
                DEBUG_ONLY(
                        for (i = ARR_LEN(df) - 1; i >= 0; --i)
                                DBG((dbg, LEVEL_3, "\t%+F\n", df[i]))
                );

                for (i = ARR_LEN(df) - 1; i >= 0; --i) {
                        y = df[i];
                        if (!pset_find_ptr(phi_blocks, y)) {
                                pset_insert_ptr(phi_blocks, y);

                                /*
                                 * Clear the link field of a possible phi block, since
                                 * the possibly created phi will be stored there. See,
                                 * search_def()
                                 */
                                set_irn_link(y, NULL);

                                if(!pset_find_ptr(copy_blocks, y))
                                        waitq_put(worklist, y);
                        }
                }
        }

        del_waitq(worklist);
}

/*
  ____ ____    _
 / ___/ ___|  / \
 \___ \___ \ / _ \
  ___) |__) / ___ \
 |____/____/_/   \_\
   ____                _                   _   _
  / ___|___  _ __  ___| |_ _ __ _   _  ___| |_(_) ___  _ __
 | |   / _ \| '_ \/ __| __| '__| | | |/ __| __| |/ _ \| '_ \
 | |__| (_) | | | \__ \ |_| |  | |_| | (__| |_| | (_) | | | |
  \____\___/|_| |_|___/\__|_|   \__,_|\___|\__|_|\___/|_| |_|

*/

/**
 * Find the copy of the given original node whose value is 'active'
 * at a usage.
 *
 * The usage is given as a node and a position. Initially, the given operand
 * points to a node for which copies were introduced. We have to find
 * the valid copy for this usage. This is done by traversing the
 * dominance tree upwards. If the usage is a phi function, we start
 * traversing from the predecessor block which corresponds to the phi
 * usage.
 *
 * @param usage       The node which uses the original node.
 * @param pos         The position of the argument which corresponds to the original node.
 * @param copies      A set containing all node which are copies from the original node.
 * @param copy_blocks A set containing all basic block in which copies of the original node are located.
 * @param phis        A set where all created phis are recorded.
 * @param phi_blocks  A set of all blocks where Phis shall be inserted (iterated dominance frontier).
 * @param mode        The mode for the Phi if one has to be created.
 * @return            The valid copy for usage.
 */
static ir_node *search_def(ir_node *usage, int pos, pset *copies, pset *copy_blocks, pset *phis, pset *phi_blocks, ir_mode *mode)
{
        ir_node *curr_bl;
        ir_node *start_irn;
        FIRM_DBG_REGISTER(firm_dbg_module_t *dbg, DBG_MODULE);

        curr_bl = get_nodes_block(usage);

        DBG((dbg, LEVEL_1, "Searching valid def for use %+F at pos %d\n", usage, pos));
        /*
         * If the usage is in a phi node, search the copy in the
         * predecessor denoted by pos.
         */
        if(is_Phi(usage)) {
                curr_bl = get_Block_cfgpred_block(curr_bl, pos);
                start_irn = sched_last(curr_bl);
        } else {
                start_irn = sched_prev(usage);
        }

        /*
         * Traverse the dominance tree upwards from the
         * predecessor block of the usage.
         */
        while(curr_bl != NULL) {
                ir_node *phim;

            /*
                 * If this block contains a copy, search the block
                 * instruction by instruction. If nothing is found
                 * search for a not scheduled PhiM.
                 */
                if(pset_find_ptr(copy_blocks, curr_bl)) {
                        ir_node *irn;

                        /* Look at each instruction from last to first. */
                        sched_foreach_reverse_from(start_irn, irn) {

                                /* Take the first copy we find. */
                                if(pset_find_ptr(copies, irn))
                                        return irn;
                        }

                        for(phim = pset_first(copies); phim; phim = pset_next(copies)) {
                                if(!is_Phi(phim) || !(get_irn_mode(phim) == mode_M))
                                        continue;

                                if(get_nodes_block(phim) == curr_bl) {
                                        pset_break(copies);
                                        return phim;
                                }
                        }
                }

                if(pset_find_ptr(phi_blocks, curr_bl)) {
                        ir_node *phi = get_irn_link(curr_bl);

                        if(phi == NULL) {
                                int i, n_preds = get_irn_arity(curr_bl);
                                ir_graph *irg = get_irn_irg(curr_bl);
                                ir_node **ins = alloca(n_preds * sizeof(ins[0]));

                                for(i = 0; i < n_preds; ++i)
                                        ins[i] = new_r_Bad(irg);

                                phi = new_r_Phi(irg, curr_bl, n_preds, ins, mode);
                                DBG((dbg, LEVEL_2, "\tcreating phi %+F in %+F\n", phi, curr_bl));

                                set_irn_link(curr_bl, phi);
                                if(mode != mode_M)
                                        sched_add_after(curr_bl, phi);

                                for(i = 0; i < n_preds; ++i) {
                                        ir_node *arg = search_def(phi, i, copies, copy_blocks, phis, phi_blocks, mode);
                                        if(arg == NULL) {
                                                ir_node *irn;

                                                ir_fprintf(stderr, "no definition found for %+F at position %d\nCopies: ", phi, i);
                                                for(irn = pset_first(copies); irn; irn = pset_next(copies)) {
                                                        ir_fprintf(stderr, "%+F ", irn);
                                                }
                                                ir_fprintf(stderr, "\n\n");
                                                assert(arg && "no definition found");
                                        }
                                        DBG((dbg, LEVEL_2, "\t\t%+F(%d) -> %+F\n", phi, i, arg));
                                        set_irn_n(phi, i, arg);
                                }

                                if(phis != NULL)
                                        pset_insert_ptr(phis, phi);
                        }

                        return phi;
                }

                /* If were not done yet, look in the immediate dominator */
                curr_bl = get_Block_idom(curr_bl);
                if(curr_bl)
                        start_irn = sched_last(curr_bl);
        }

        return NULL;
}

static void fix_usages(pset *copies, pset *copy_blocks, pset *phi_blocks, pset *phis, pset *ignore_uses)
{
        int n_outs = 0;
        struct obstack obst;
        ir_node *irn;
        int i;
        struct out {
                ir_node *irn;
                int pos;
        } *outs;

        FIRM_DBG_REGISTER(firm_dbg_module_t *dbg, DBG_MODULE);

        obstack_init(&obst);

        /*
         * Put all outs into an array.
         * This is necessary, since the outs would be modified while
         * iterating on them what could bring the outs module in trouble.
         */
        for (irn = pset_first(copies); irn; irn = pset_next(copies)) {
                const ir_edge_t *edge;
                foreach_out_edge(irn, edge) {
                        if (!pset_find_ptr(ignore_uses, get_edge_src_irn(edge))) {
                                struct out tmp;
                                tmp.irn = get_edge_src_irn(edge);
                                tmp.pos = get_edge_src_pos(edge);
                                obstack_grow(&obst, &tmp, sizeof(tmp));
                                n_outs++;
                        }
                }
        }
        outs = obstack_finish(&obst);

        /*
         * Search the valid def for each out and set it.
         */
        for(i = 0; i < n_outs; ++i) {
                ir_node *irn  = outs[i].irn;
                int pos       = outs[i].pos;
                ir_mode *mode = get_irn_mode(get_irn_n(irn, pos));

                ir_node *def;

                def = search_def(irn, pos, copies, copy_blocks, phis, phi_blocks, mode);
                DBG((dbg, LEVEL_2, "\t%+F(%d) -> %+F\n", irn, pos, def));

                if(def == NULL) {
                        ir_fprintf(stderr, "no definition found for %+F at position %d\nCopies: ", irn, pos);
                        for(irn = pset_first(copies); irn; irn = pset_next(copies)) {
                                ir_fprintf(stderr, "%+F ", irn);
                        }
                        ir_fprintf(stderr, "\n\n");
                        assert(def && "no definition found");
                }
                set_irn_n(irn, pos, def);
        }

        obstack_free(&obst, NULL);
}

#if 0
/**
 * Remove phis which are not necessary.
 * During place_phi_functions() phi functions are put on the dominance
 * frontiers blindly. However some of them will never be used (these
 * have at least one predecessor which is NULL, see search_def() for
 * this case). Since place_phi_functions() enters them into the
 * schedule, we have to remove them from there.
 *
 * @param copies The set of all copies made (including the phi functions).
 */
static void remove_odd_phis(pset *copies, pset *unused_copies)
{
        ir_node *irn;

        for(irn = pset_first(copies); irn; irn = pset_next(copies)) {
                if(is_Phi(irn)) {
                        int i, n;
                        int illegal = 0;

                        assert(sched_is_scheduled(irn) && "phi must be scheduled");
                        for(i = 0, n = get_irn_arity(irn); i < n && !illegal; ++i)
                                illegal = get_irn_n(irn, i) == NULL;

                        if(illegal) {
                                for(i = 0, n = get_irn_arity(irn); i < n; ++i)
                                        set_irn_n(irn, i, new_Bad());
                                sched_remove(irn);
                        }
                }
        }

        for(irn = pset_first(unused_copies); irn; irn = pset_next(unused_copies)) {
                for(i = 0, n = get_irn_arity(irn); i < n; ++i)
                        set_irn_n(irn, i, new_Bad());
                sched_remove(irn);
        }
}
#endif

void be_ssa_constr_phis_ignore(be_dom_front_info_t *info, be_lv_t *lv, int n, ir_node *nodes[], pset *phis, pset *ignore_uses)
{
        pset *irns = pset_new_ptr(n);
        int i;

        for(i = 0; i < n; ++i)
                pset_insert_ptr(irns, nodes[i]);
        be_ssa_constr_set_phis_ignore(info, lv, irns, phis, ignore_uses);
        del_pset(irns);
}

void be_ssa_constr_ignore(be_dom_front_info_t *info, be_lv_t *lv, int n, ir_node *nodes[], pset *ignore_uses)
{
        be_ssa_constr_phis_ignore(info, lv, n, nodes, NULL, ignore_uses);
}

void be_ssa_constr(be_dom_front_info_t *info, be_lv_t *lv, int n, ir_node *nodes[])
{
        pset *empty_set = be_empty_set();
        be_ssa_constr_ignore(info, lv, n, nodes, empty_set);
}

void be_ssa_constr_set_phis_ignore(be_dom_front_info_t *df, be_lv_t *lv, pset *nodes, pset *phis, pset *ignore_uses)
{
        int n                  = pset_count(nodes);
        pset *blocks           = pset_new_ptr(n);
        pset *phi_blocks       = pset_new_ptr(n);
        int save_optimize      = get_optimize();
        int save_normalize     = get_opt_normalize();
        int phis_set_created   = 0;
        FIRM_DBG_REGISTER(firm_dbg_module_t *dbg, DBG_MODULE);

        ir_node *irn;

        /* We need to collect the phi functions to compute their liveness. */
        if(lv && !phis) {
                phis_set_created = 1;
                phis = pset_new_ptr_default();
        }

        DBG((dbg, LEVEL_1, "Introducing following copies for:\n"));

        /* Fill the sets. */
        for(irn = pset_first(nodes); irn; irn = pset_next(nodes)) {
                ir_node *bl = get_nodes_block(irn);
                pset_insert_ptr(blocks, bl);
                DBG((dbg, LEVEL_1, "\t%+F in %+F\n", irn, bl));
        }

        /*
         * Disable optimization so that the phi functions do not
         * disappear.
         */
        set_optimize(0);
        set_opt_normalize(0);

        /*
         * Place the phi functions and reroute the usages.
         */
        determine_phi_blocks(nodes, blocks, phi_blocks, df);
        fix_usages(nodes, blocks, phi_blocks, phis, ignore_uses);

        /* reset the optimizations */
        set_optimize(save_optimize);
        set_opt_normalize(save_normalize);

        del_pset(phi_blocks);
        del_pset(blocks);

        /* Recompute the liveness (if wanted) of the original nodes, the copies and the inserted phis. */
        if(lv) {
#if 1
                foreach_pset(nodes, irn)
                        be_liveness_update(lv, irn);

                foreach_pset(phis, irn)
                        be_liveness_introduce(lv, irn);
#else
                be_liveness_recompute(lv);
#endif
        }

        /* Free the phi set of we created it. */
        if(phis_set_created)
                del_pset(phis);

}

void be_ssa_constr_set_phis(be_dom_front_info_t *df, be_lv_t *lv, pset *nodes, pset *phis)
{
        pset *empty_set = be_empty_set();
        be_ssa_constr_set_phis_ignore(df, lv, nodes, phis, empty_set);
}

void be_ssa_constr_set_ignore(be_dom_front_info_t *df, be_lv_t *lv, pset *nodes, pset *ignore_uses)
{
        be_ssa_constr_set_phis_ignore(df, lv, nodes, NULL, ignore_uses);
}

void be_ssa_constr_set(be_dom_front_info_t *info, be_lv_t *lv, pset *nodes)
{
        pset *empty_set = be_empty_set();
        be_ssa_constr_set_ignore(info, lv, nodes, empty_set);
}

/*
  ___                     _     ____
 |_ _|_ __  ___  ___ _ __| |_  |  _ \ ___ _ __ _ __ ___
  | || '_ \/ __|/ _ \ '__| __| | |_) / _ \ '__| '_ ` _ \
  | || | | \__ \  __/ |  | |_  |  __/  __/ |  | | | | | |
 |___|_| |_|___/\___|_|   \__| |_|   \___|_|  |_| |_| |_|

*/

ir_node *insert_Perm_after(const arch_env_t *arch_env,
                                                   be_lv_t *lv,
                                                   const arch_register_class_t *cls,
                                                   be_dom_front_info_t *dom_front,
                                                   ir_node *pos)
{
        ir_node *bl     = is_Block(pos) ? pos : get_nodes_block(pos);
        ir_graph *irg   = get_irn_irg(bl);
        pset *live      = pset_new_ptr_default();
        FIRM_DBG_REGISTER(firm_dbg_module_t *dbg, "be.node");

        ir_node *curr, *irn, *perm, **nodes;
        int i, n;

        DBG((dbg, LEVEL_1, "Insert Perm after: %+F\n", pos));

        be_liveness_nodes_live_at(lv, arch_env, cls, pos, live);

        n = pset_count(live);

        if(n == 0) {
                del_pset(live);
                return NULL;
        }

        nodes = xmalloc(n * sizeof(nodes[0]));

        DBG((dbg, LEVEL_1, "live:\n"));
        for(irn = pset_first(live), i = 0; irn; irn = pset_next(live), i++) {
                DBG((dbg, LEVEL_1, "\t%+F\n", irn));
                nodes[i] = irn;
        }
        del_pset(live);

        perm = be_new_Perm(cls, irg, bl, n, nodes);
        sched_add_after(pos, perm);
        free(nodes);

        curr = perm;
        for (i = 0; i < n; ++i) {
                ir_node *copies[2];
                ir_node *perm_op = get_irn_n(perm, i);
                const arch_register_t *reg = arch_get_irn_register(arch_env, perm_op);

                ir_mode *mode = get_irn_mode(perm_op);
                ir_node *proj = new_r_Proj(irg, bl, perm, mode, i);
                arch_set_irn_register(arch_env, proj, reg);

                sched_add_after(curr, proj);
                curr = proj;

                copies[0] = perm_op;
                copies[1] = proj;

                be_ssa_constr(dom_front, lv, 2, copies);
        }

        return perm;
}

struct _elr_closure_t {
        struct obstack obst;
        const be_chordal_env_t *cenv;
};

static void elr_split_walker(ir_node *bl, void *data)
{
        struct _elr_closure_t *c     = data;
        const be_chordal_env_t *cenv = c->cenv;
        const arch_env_t *aenv       = cenv->birg->main_env->arch_env;
        be_lv_t *lv                  = cenv->birg->lv;
        be_dom_front_info_t *dom_front = cenv->birg->dom_front;
        be_insn_t *insn;
        be_insn_env_t ie;

        be_insn_env_init(&ie, cenv->birg, cenv->cls, &c->obst);

        for(insn = be_scan_insn(&ie, sched_first(bl)); !is_Block(insn->irn); insn = be_scan_insn(&ie, insn->next_insn)) {
                ir_node *pred = sched_prev(insn->irn);
                if(!is_Block(pred) && !is_Phi(insn->irn))
                        insert_Perm_after(aenv, lv, cenv->cls, dom_front, insn->irn);
        }
}

void extreme_liverange_splitting(struct _be_chordal_env_t *cenv)
{
        struct _elr_closure_t c;
        be_lv_t *lv = cenv->birg->lv;

        c.cenv = cenv;
        obstack_init(&c.obst);
        be_liveness_recompute(lv);
        irg_block_walk_graph(cenv->irg, elr_split_walker, NULL, &c);
        be_liveness_recompute(lv);
        obstack_free(&c.obst, NULL);
}

/**
 * Post-block-walker: Find blocks containing only one jump and
 * remove them.
 */
static void remove_empty_block(ir_node *block, void *data) {
        const ir_edge_t *edge, *next;
        ir_node *node;
        int *changed = data;
        ir_node *jump = NULL;

        assert(is_Block(block));

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

        sched_foreach(block, node) {
                if (! is_Jmp(node))
                        return;
                if (jump != NULL) {
                        /* we should never have 2 jumps in a block */
                        assert(0 && "We should never have 2 jumps in a block");
                        return;
                }
                jump = node;
        }

        if (jump == NULL)
                return;

        node = get_Block_cfgpred(block, 0);
        foreach_out_edge_safe(jump, edge, next) {
                ir_node *block = get_edge_src_irn(edge);
                int     pos    = get_edge_src_pos(edge);

                set_irn_n(block, pos, node);
        }

        set_Block_cfgpred(block, 0, new_Bad());
        sched_remove(jump);
        *changed = 1;
}

/* removes basic blocks that just contain a jump instruction */
int be_remove_empty_blocks(ir_graph *irg) {
        int changed = 0;

        irg_block_walk_graph(irg, remove_empty_block, NULL, &changed);
        if (changed) {
                /* invalidate analysis info */
                set_irg_doms_inconsistent(irg);
                set_irg_extblk_inconsistent(irg);
                set_irg_outs_inconsistent(irg);
        }
        return changed;
}

typedef struct _be_dead_out_env_t {
        ir_graph *irg;
        bitset_t *reachable;
        DEBUG_ONLY(firm_dbg_module_t *dbg);
} be_dead_out_env_t;

/**
 * Check if @p node has dead users, i.e. they are reachable only via outedges from @p node.
 * Set all ins of those users to BAD.
 */
static void kill_dead_out(ir_node *node, be_dead_out_env_t *env) {
        ir_graph        *irg = env->irg;
        const ir_edge_t *edge, *tmp_edge;

        if (irn_visited(node))
                return;
        mark_irn_visited(node);

        /* check all out edges */
        foreach_out_edge_safe(node, edge, tmp_edge) {
                ir_node *src = get_edge_src_irn(edge);

                if (! bitset_is_set(env->reachable, get_irn_idx(src))) {
                        /* BEWARE: do not kill anchors or TLS */
                        if (src != get_irg_globals(irg) && src != get_irg_tls(irg)) {
                                DBG((env->dbg, LEVEL_1, "killing %+F, only reachable from %+F\n", src, node));
                                be_kill_node(src);
                        }
                        continue;
                }

                /* descent */
                if (! is_Block(src)) {
                        kill_dead_out(src, env);
                }
        }
}

static void set_reachable(ir_node *node, void *data) {
        bitset_t *reachable = data;
        bitset_set(reachable, get_irn_idx(node));
}

/**
 * Set input of all nodes only reachable via out edges to BAD.
 */
void be_kill_dead_nodes(ir_graph *irg) {
        be_dead_out_env_t env;

        env.irg       = irg;
        env.reachable = bitset_alloca(get_irg_last_idx(irg));
        FIRM_DBG_REGISTER(env.dbg, "firm.be.killdead");

        /* collect all reachable nodes */
        irg_walk_in_or_dep_graph(irg, set_reachable, NULL, env.reachable);

        /* this is a forward walk (start -> end) */
        inc_irg_visited(irg);
        kill_dead_out(get_irg_start(irg), &env);
}