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cfg.c

/*
 * cfg.c
 *
 * A basic block is the longest sequence of instructions that we are
 * sure will be executed sequentially: no branches, no labels.
 *
 * The control-flow graph is a directed graph that reflects the
 * flow of execution between blocks.
 */


#include <stdlib.h>
#include <string.h>
#include "imc.h"
#include "optimizer.h"


static void propagate_need(Basic_block *bb, SymReg* r, int i);
static void bb_findadd_edge(Parrot_Interp, IMC_Unit *, Basic_block*, SymReg*);
static void mark_loop(Parrot_Interp, IMC_Unit *, Edge*);
static void init_basic_blocks(IMC_Unit *);
static void analyse_life_symbol(Parrot_Interp, IMC_Unit *, SymReg*);
static void analyse_life_block(Parrot_Interp, Basic_block*, SymReg*);
static void bb_add_edge(IMC_Unit *, Basic_block*, Basic_block*);
static void bb_remove_edge(IMC_Unit *, Edge*);
static Basic_block* make_basic_block(Interp *, IMC_Unit *, Instruction*);

/* Code: */

#define INVOKE_SUB_CALL 1
#define INVOKE_SUB_RET  2
#define INVOKE_SUB_LOOP 3
#define INVOKE_SUB_OTHER 4

static int
check_invoke_type(Interp *interp, IMC_Unit * unit, Instruction *ins)
{
    /*
     * 1) pcc sub call or yield
     */
    if (ins->type & (ITPCCSUB | ITPCCYIELD))
        return INVOKE_SUB_CALL;
    /*
     * inside another pcc_sub
     * 2) invoke = loop to begin
     */
    if (unit->instructions->r[0] && unit->instructions->r[0]->pcc_sub)
        return INVOKE_SUB_LOOP;
    /* 3) invoke P1 returns */
    if (ins->opsize == 2)
        return INVOKE_SUB_RET;
    /* 4) other usage */

    IMCC_INFO(interp)->dont_optimize = 1;  /* too complex, to follow */
    IMCC_INFO(interp)->optimizer_level &= ~OPT_PASM;
    return INVOKE_SUB_OTHER;
}

void
find_basic_blocks (Parrot_Interp interpreter, IMC_Unit * unit, int first)
{
    Basic_block *bb;
    Instruction *ins;
    SymHash *hsh = &unit->hash;
    SymReg * r;
    int nu = 0;
    int i;

    IMCC_info(interpreter, 2, "find_basic_blocks\n");
    init_basic_blocks(unit);
    for (i = 0; i < hsh->size; i++) {
        for (r = hsh->data[i]; r; r = r->next) {
            if (r && (r->type & VTADDRESS)) {
                r->last_ins = NULL;
            }
        }
    }

    /* XXX FIXME: Now the way to check for a sub is unit->type */
    ins = unit->instructions;
    if (first && ins->type == ITLABEL && ins->r[0]->type == VT_PCC_SUB) {
        IMCC_debug(interpreter, DEBUG_CFG, "pcc_sub %s nparams %d\n",
                ins->r[0]->name, ins->r[0]->pcc_sub->nargs);
        expand_pcc_sub(interpreter, unit, ins);
    }
    ins->index = i = 0;

    bb = make_basic_block(interpreter, unit, ins);
    if (ins->type & ITBRANCH) {
        SymReg * addr = get_branch_reg(bb->end);
        if (addr)
            addr->last_ins = ins;
    }
    for (ins = ins->next; ins; ins = ins->next) {
        ins->index = ++i;

        bb->end = ins;
        ins->bbindex = unit->n_basic_blocks - 1;

        if (ins->opnum == -1 && (ins->type & ITPCCSUB)) {
            if (first) {
                /* XXX FIXME: Now the way to check for a sub is unit->type */
                if (ins->type & ITLABEL) {
                    expand_pcc_sub_ret(interpreter, unit, ins);
                    ins->type &= ~ITLABEL;
                }
                else {
                    /* if this is a sub call expand it */
                    expand_pcc_sub_call(interpreter, unit, ins);
                }
                ins->type &= ~ITPCCSUB;
            }
        }
        else if (ins->type & ITLABEL) {
            /* set the labels address (ins) */
            ins->r[0]->first_ins = ins;
        }

        /* a LABEL starts a new basic block, but not, if we already have
         * a new one (last was a branch)
         */
        if (nu)
            nu = 0;
        else if ( (ins->type & ITLABEL)) {
            bb->end = ins->prev;
            bb = make_basic_block(interpreter, unit, ins);
        }
        /* a branch is the end of a basic block
         * so start a new one with the next instruction */
        if (ins->type & ITBRANCH) {
            SymReg * addr = get_branch_reg(bb->end);
            int found = 1;

            if (addr)
                addr->last_ins = ins;
            /* if we have a bsr, then consider it only as a branch,
             * when we have the target here
             * and it doesn't saveall - like P6C recursive bsr's
             *
             * ignore set_addr - no new basic block
             */
            if (!strcmp(ins->op, "bsr") || !strcmp(ins->op, "set_addr")) {
                char *name = ins->r[0]->name;
                r = get_sym(name);
                if (*ins->op == 'b') {  /* bsr */
                    Instruction * lab;
                    found = r != NULL && r->first_ins;
                    IMCC_debug(interpreter, DEBUG_CFG, "bsr %s local:%s\n",
                            name, found ? "yes": "no");
                    if (r) {
                        int j;
                        lab = r->first_ins;
                        if (lab)
                            for (j = 0, lab = lab->next; j < 5 && lab;
                                    lab = lab->next, j++) {
                                if (!strcmp(lab->op, "saveall")) {
                                    ins->type |= ITSAVES;
                                    lab->type |= ITSAVES;
                                    IMCC_debug(interpreter, DEBUG_CFG, "\ttype saveall\n");
                                    break;
                                }
                            }
                    }
                }
                else {
                    /* don't treat set_addr as jump source */
                    found = 0;
                }
            }
            if (found) {
                if (ins->next)
                    bb = make_basic_block(interpreter, unit, ins->next);
                nu = 1;
            }
        }
    }

    if (IMCC_INFO(interpreter)->debug & DEBUG_CFG) {
        dump_instructions(interpreter, unit);
        dump_labels(unit);
    }
}

static void
bb_check_set_addr(Parrot_Interp interpreter, IMC_Unit * unit,
        Basic_block *bb, SymReg *label)
{
    Instruction *ins;
    for (ins = unit->instructions; ins; ins = ins->next) {
             if (ins->opnum == PARROT_OP_set_addr_p_ic &&
                    !strcmp(label->name, ins->r[1]->name)) {
                IMCC_debug(interpreter, DEBUG_CFG, "set_addr %s\n",
                        ins->r[1]->name);

                /*
                 * connect this block with first and last block
                 */
                bb_add_edge(unit, unit->bb_list[0], bb);
                bb_add_edge(unit, unit->bb_list[unit->n_basic_blocks - 1], bb);
                /* and mark the instruction as being kind of a branch */
                bb->start->type |= ITADDR;
                break;
            }
    }
}

/* Once the basic blocks have been computed, build_cfg computes
   the dependencies between them. */
void
build_cfg(Parrot_Interp interpreter, IMC_Unit * unit)
{
    int i, j, changes;
    SymReg * addr;
    Basic_block *last = NULL, *bb;
    Edge *pred;

    IMCC_info(interpreter, 2, "build_cfg\n");
    for (i = 0; i < unit->n_basic_blocks; i++) {
        bb = unit->bb_list[i];

        /* if the block can fall-through */
        if (i > 0 && ! (last->end->type & IF_goto) )
            bb_add_edge(unit, last, bb);
        /* check first ins, if label try to find a set_addr op */
        if (bb->start->type & ITLABEL) {
            bb_check_set_addr(interpreter, unit, bb, bb->start->r[0]);
        }
        /* look if last instruction is a branch */
        addr = get_branch_reg(bb->end);
        if (addr)
            bb_findadd_edge(interpreter, unit, bb, addr);
        else if (!strcmp(bb->start->op, "invoke") ||
                !strcmp(bb->start->op, "invokecc")) {
            if (check_invoke_type(interpreter, unit, bb->start) ==
                    INVOKE_SUB_LOOP)
                bb_add_edge(unit, bb, unit->bb_list[0]);
        }
        if (!strcmp(bb->end->op, "ret")) {
            Instruction * sub;
            IMCC_debug(interpreter, DEBUG_CFG, "found ret in bb %d\n", i);
            /* now go back, find labels and connect these with
             * bsrs
             */
            /* this doesn't work, if this is a local backward sub call
             * the bsr isn't chained yet so the pred_list is empty
             *
             * s. #25948
             */
            if (!bb->pred_list) {
                for (j = i; j < unit->n_basic_blocks; j++) {
                    Basic_block * b_bsr = unit->bb_list[j];
                    if (!strcmp(b_bsr->end->op, "bsr")) {
                        addr = get_branch_reg(b_bsr->end);
                        if (addr)
                            bb_findadd_edge(interpreter, unit, b_bsr, addr);
                    }
                }
            }
            /* end #25948 */

            for (pred = bb->pred_list; pred; pred = pred->next) {
                int found = 0;
                SymReg *r;
                if (!strcmp(pred->from->end->op, "bsr")) {

                    r = pred->from->end->r[0];

                    sub = pred->to->start;
                    if ((sub->type & ITLABEL) &&
                            (!strcmp(sub->r[0]->name, r->name)))
                        found = 1;
invok:
                    j = pred->from->index;
                    if (found) {
                        int saves = 0;
                        IMCC_debug(interpreter, DEBUG_CFG,
                                "\tcalled from bb %d '%I'\n",
                                j, pred->from->end);
                        for (; sub && sub != bb->end; sub = sub->next) {
                            if (!strcmp(sub->op, "saveall"))
                                if (!(sub->type & ITSAVES)) {
                                    break;
                                }
                            unit->bb_list[sub->bbindex]->
                                flag |= BB_IS_SUB;
                            if (!strcmp(sub->op, "restoreall")) {
                                sub->type |= ITSAVES;
                                saves = 1;
                            }
                        }
                        if (!saves)
                            bb_add_edge(unit, bb,
                                    unit->bb_list[j+1]);
                        IMCC_debug(interpreter, DEBUG_CFG, "\tand does saveall %s\n",
                                saves ? "yes" : "no");
                    }
                }
                else if (!strcmp(pred->from->end->op, "invoke")) {
                    found = 1;
                    sub = pred->to->start;
                    goto invok;
                }
                if (!found)
                    IMCC_debug(interpreter, DEBUG_CFG, "\tcalled from unknown!\n");
            }
        }

        last = bb;
    }

    /* Decouple unreachable blocks (not the first block, with no predecessors)
     * from the CFG
     */
    do {
        changes = 0;
        for (i = 1; i < unit->n_basic_blocks; i++) {
            bb = unit->bb_list[i];
            if (!bb->pred_list) {
                /* Remove all successor edges of block bb */
                while (bb->succ_list) {
                    bb_remove_edge(unit, bb->succ_list);
                    IMCC_debug(interpreter, DEBUG_CFG,
                            "remove edge from bb: %d\n", bb->index);
                    changes = 1;
                }
            }
        }
    } while (changes);

    if (IMCC_INFO(interpreter)->debug & DEBUG_CFG)
        dump_cfg(unit);
}


/* find the placement of the label, and link the two nodes */

static void
bb_findadd_edge(Parrot_Interp interpreter, IMC_Unit * unit, Basic_block *from, SymReg *label)
{
    Instruction *ins;
    SymReg *r = find_sym(interpreter, label->name);

    if (r && (r->type & VTADDRESS) && r->first_ins)
        bb_add_edge(unit, from,
                unit->bb_list[r->first_ins->bbindex]);
    else {
        IMCC_debug(interpreter, DEBUG_CFG, "register branch %I ",
                from->end);
        /* XXX is probably only ok, if the invoke is "near" the
         *     set_addr ins
         */
        for (ins = from->end; ins; ins = ins->prev) {
            if ((ins->type & ITBRANCH) && !strcmp(ins->op, "set_addr") &&
                    ins->r[1]->first_ins) {
                bb_add_edge(unit, from, unit->
                                bb_list[ins->r[1]->first_ins->bbindex]);
                IMCC_debug(interpreter, DEBUG_CFG, "(%s) ", ins->r[1]->name);
                break;
            }
        }
        IMCC_debug(interpreter, DEBUG_CFG, "\n");
    }
}


int
blocks_are_connected(Basic_block *from, Basic_block *to)
{
    Edge *pred;
    for (pred = to->pred_list; pred != NULL; pred=pred->pred_next) {
      if (pred->from == from)
          return 1; /*already linked*/
    }
    return 0;
}


static void
bb_add_edge(IMC_Unit * unit, Basic_block *from, Basic_block *to)
{
    Edge *e;
    if (blocks_are_connected(from, to))
      return;

    /* we assume that the data is correct, and thus if the edge is not
       on the predecessors of 'from', it won't be on the successors
       of 'to'. */

    e = mem_sys_allocate(sizeof(Edge));

    e->succ_next = from->succ_list;
    e->from = from;

    e->pred_next = to->pred_list;
    e->to = to;

    from->succ_list = to->pred_list = e;

    /* memory housekeeping */
    e->next = 0;
    if (unit->edge_list == 0)
      unit->edge_list = e;
    else {
      e->next = unit->edge_list;
      unit->edge_list = e;
    }
}

static void
bb_remove_edge(IMC_Unit * unit, Edge * edge)
{
    Edge *prev;

    if (edge->from->succ_list == edge) {
        edge->from->succ_list = edge->succ_next;
    } else {
        for (prev = edge->from->succ_list; prev; prev = prev->succ_next) {
            if (prev->succ_next == edge) {
                prev->succ_next = edge->succ_next;
            }
        }
    }

    if (edge->to->pred_list == edge) {
        edge->to->pred_list = edge->pred_next;
    } else {
        for (prev = edge->to->pred_list; prev; prev = prev->pred_next) {
            if (prev->pred_next == edge) {
                prev->pred_next = edge->pred_next;
            }
        }
    }

    if (unit->edge_list == edge) {
        unit->edge_list = edge->next;
      free(edge);
    } else {
        for (prev = unit->edge_list; prev; prev = prev->next) {
            if (prev->next == edge) {
                prev->next = edge->next;
              free(edge);
            }
        }
    }
}

static void
free_edge(IMC_Unit * unit)
{
    Edge *e, *next;
    for (e = unit->edge_list; e; ) {
      next = e->next;
      free(e);
      e = next;
    }
    unit->edge_list = 0;
}

int
edge_count(IMC_Unit * unit)
{
    Edge *e;
    int i;
    for (i=0, e = unit->edge_list; e; e = e->next, i++)
        {}
    return i;
}

void
life_analysis(Parrot_Interp interpreter, IMC_Unit * unit)
{
    int i;
    SymReg** reglist = unit->reglist;

    IMCC_info(interpreter, 2, "life_analysis\n");
    for (i = 0; i < unit->n_symbols; i++)
        analyse_life_symbol(interpreter, unit, reglist[i]);
}

static void
analyse_life_symbol(Parrot_Interp interpreter, IMC_Unit * unit, SymReg* r)
{
    int i;

#if IMC_TRACE_HIGH
    fprintf(stderr, "cfg.c: analyse_life_symbol(%s)\n", r->name);
#endif

    if (r->life_info)
        free_life_info(unit, r);
    r->life_info = mem_sys_allocate_zeroed(unit->n_basic_blocks *
            sizeof(Life_range*));

    /* First we make a pass to each block to gather the information
     * that can be obtained locally */

    for (i=0; i < unit->n_basic_blocks; i++) {
      analyse_life_block(interpreter, unit->bb_list[i], r);
    }

    /* Now we need to consider the relations between blocks */

    for (i=0; i < unit->n_basic_blocks; i++) {
      if (r->life_info[i]->flags & LF_use) {
            Instruction *ins, *prev;

            ins = unit->bb_list[i]->start;

            /*
             * if the previous instruction (the last of the previous block)
             * was a sub call, and the symbol is live/use here, it needs
             * allocation in the non-volatile register range
             */
            if (ins->prev) {
                prev = ins->prev;
                if (prev->type & (ITPCCSUB|ITPCCYIELD))
                    r->usage |= U_NON_VOLATILE;
                else if (prev->opnum == PARROT_OP_invoke_p_p ||
                         prev->opnum == PARROT_OP_invokecc_p)
                    r->usage |= U_NON_VOLATILE;
                else if (ins->type & ITADDR)
                    r->usage |= U_NON_VOLATILE;
            }

          /* This block uses r, so it must be live at
             the beginning */
          r->life_info[i]->flags |= LF_lv_in;

          /* propagate this info to every predecessor */
          propagate_need (unit->bb_list[i], r, i);
      }
    }
}

void
free_life_info(IMC_Unit * unit, SymReg *r)
{
    int i;
#if IMC_TRACE_HIGH
    fprintf(stderr, "free_life_into(%s)\n", r->name);
#endif
    if (r->life_info) {
        for (i=0; i < unit->n_basic_blocks; i++) {
            if (r->life_info[i])
                free(r->life_info[i]);
        }
        free(r->life_info);
        r->life_info = NULL;
    }
}

/* analyse_life_block studies the state of the var r
 * in the block bb.
 *
 * Its job is to set the flags LF_use, or LF_read,
 * and record the intervals inside the block where
 * the var is alive.
 */

static void
analyse_life_block(Parrot_Interp interpreter, Basic_block* bb, SymReg* r)
{
    Instruction* ins, *special;
    Life_range* l;
    int is_alias;

    UNUSED(interpreter);
    l = make_life_range(r, bb->index);

    special = NULL;
    for (ins = bb->start; ins ; ins = ins->next) {
        if (ins==NULL) {
            IMCC_fatal(interpreter, 1,"analyse_life_block: "
                    "Index %i of %i has NULL instruction\n",
                    ins->index, bb->end->index);
        }
        if (ins->opnum == -1) {
            if (ins == bb->end)
                break;
            continue;
        }
        /*
         * if we have a setp_ind opcode, it may write all PMC
         * registers from 5..15
         */
        if (ins->opnum == PARROT_OP_setp_ind_i_p && r->set == 'P') {
            r->usage |= U_NON_VOLATILE;
        }

        /* restoreall and such */
        if (ins_writes2(ins, r->set))
            special = ins;

        /*
         * set p,p is basically a read - both are LF_use
         *
         * TODO live range coalescing
         */
        is_alias = (ins->type & ITALIAS) && ins->r[0] == r;

        if (instruction_reads(ins, r) || is_alias) {
            /* if instruction gets read after a special, consider
             * the first read of this instruction, like if a write
             * had happened at special, so that the reg doesn't pop into
             * life */
            if (! (l->flags & LF_def) ) {
                if (special) {
                    l->first_ins = special;
                    l->flags |= LF_def;
                    special = NULL;
                }
                else {
                    /* we read before having written before, so the var was
                     * live at the beginning of the block */
                    l->first_ins = bb->start;
                    l->flags |= LF_use;
                }
            }
            l->last_ins = ins;
        }

        if (!is_alias && instruction_writes(ins, r)) {

            l->flags |= LF_def;

            if (!l->first_ins) {
                l->first_ins = ins;
            }
            l->last_ins = ins;
        }
        if (ins == bb->end)
            break;
    }

    if (!l->last_ins)
      l->last_ins = l->first_ins;

    /* l->last can later be extended if it turns out
     * that another block needs the value resulting from this
     * computation */

}


static void
propagate_need(Basic_block *bb, SymReg* r, int i)
{
    Edge *edge;
    Basic_block *pred;
    Life_range *l;

    l = r->life_info[bb->index];
    /* l->last_ins = bb->end; XXX:leo why? */

    /* every predecessor of a LF_lv_in block must be in LF_lv_out
       and, unless itself is LV_def, this should be propagated to
       its predecessors themselves */

    for (edge=bb->pred_list; edge!=NULL; edge=edge->pred_next) {
      pred = edge->from;
      l = r->life_info[pred->index];

      if (l->flags & LF_lv_out) {
          /* this node has already been visited. Ignore it */
      }
      else {
            l->flags |= LF_lv_out;
          l->last_ins = pred->end;

          if (! (l->flags & LF_def) ) {
            l->flags |= LF_lv_in;
            l->first_ins = pred->start;
            l->last_ins  = pred->end;
                /* we arrived at block 0
                 *
                 * emit a warning if -w
                 * looking at some perl6 examples, where this warning
                 * is emitted, there seems always to be a code path
                 * where the var is not initialized, so this might
                 * even be correct :)
                 *
                 * XXX subroutines
                 */
#if 0
                if (pred->index == 0) {
                    Instruction *ins = r->life_info[i]->first_ins;
                    int bbi = ins->bbindex;
                    for ( ; ins && ins->bbindex == bbi; ins = ins->next)
                        if (instruction_reads(ins, r))
                            break;
                    IMCC_warning("propagate_need",
                            "'%s' might be used uninitialized in %s:%d\n",
                            r->name, function, ins->line);
                }
#endif
              propagate_need(pred, r, i);
          }
      }
    }
}

/*
 * Computes the dominators tree of the CFG.
 * Basic block A dominates B, if each path to B passes through A
 *
 * s. gcc:flow.c compute_dominators
 */

void
compute_dominators (Parrot_Interp interpreter, IMC_Unit * unit)
{
#define USE_BFS 0

#if !USE_BFS
    int i, change, pred_index;
#else
    int i, cur, len, succ_index;
    int *q;
    Set *visited;
#endif
    int n, b, runner, wrong;
    Edge *edge;
    Set** dominators;

    n = unit->n_basic_blocks;
    IMCC_info(interpreter, 2, "compute_dominators\n");
    dominators = unit->dominators = malloc(sizeof(Set*) * n);
    unit->idoms = malloc(sizeof(int) * n);

    dominators[0] = set_make(n);
    set_add(dominators[0], 0);
    for (i = 1; i < n; i++) {
        if (unit->bb_list[i]->pred_list) {
            dominators[i] = set_make_full(n);
        } else {
            dominators[i] = set_make(n);
            set_add(dominators[i], i);
        }
    }

#if USE_BFS
    q = calloc(n, sizeof(int));
    visited = set_make (n);
    set_add(visited, 0);
    len=1;
    cur=0;

    while (cur < len) {
        for (edge = unit->bb_list[q[cur]]->succ_list;
                edge; edge = edge->succ_next) {
            succ_index = edge->to->index;
            set_intersec_inplace(dominators[succ_index], dominators[q[cur]]);
            set_add(dominators[succ_index],succ_index);

            if (!set_contains(visited, succ_index)) {
                set_add(visited, succ_index);
                q[len++] = succ_index;
            }
        }
        cur++;
    }
#else
    change = 1;
    while (change) {
        change = 0;

        /* TODO: This 'for' should be a breadth-first search for speed */
        for (i = 1; i < n; i++) {
            Set *s = set_copy (dominators[i]);

            for (edge=unit->bb_list[i]->pred_list;
                    edge; edge=edge->pred_next) {
                pred_index = edge->from->index;
                set_intersec_inplace(s, dominators[pred_index]);
            }

            set_add(s, i);

            if (! set_equal(dominators[i], s) ) {
                change = 1;
                set_free (dominators[i]);
                dominators[i] = s;
            }
            else
                set_free(s);
        }
    }
#endif
    /* calc idoms */
    unit->idoms[0] = unit->bb_list[0]->index;
    for (b = 1; b < n; b++) {
        unit->idoms[b] = 0;
        for (i = n - 1; i > 0; i--) {
            if (i != b && set_contains(dominators[b], i)) {
                wrong = 0;
                for (runner = 0; runner < n; runner++) {
                    if (runner != b && runner != i && set_contains(dominators[b], runner)) {
                        if (set_contains(dominators[runner], i)) {
                            wrong = 1;
                            break;
                        }
                    }
                }
                if (!wrong) {
                    unit->idoms[b] = unit->bb_list[i]->index;
                    break;
                }
            }
       }
   }

    if (IMCC_INFO(interpreter)->debug & DEBUG_CFG)
        dump_dominators(unit);
#if USE_BFS
    free(q);
    set_free(visited);
#endif
}

/* Algorithm to find dominance frontiers described in paper
 * "A Simple, Fast Dominance Algorithm", Cooper et al. (2001)
 */
void
compute_dominance_frontiers (Parrot_Interp interpreter, IMC_Unit * unit)
{
    int i, n, b, runner;
    Edge *edge;
    Set** dominance_frontiers;

    n = unit->n_basic_blocks;
    IMCC_info(interpreter, 2, "compute_dominance_frontiers\n");
    dominance_frontiers = unit->dominance_frontiers = malloc(sizeof(Set*) * n);

    dominance_frontiers[0] = set_make(n);
    for (i = 1; i < n; i++) {
        dominance_frontiers[i] = set_make(n);
    }

    /* for all nodes, b */
    for (b = 1; b < n; b++) {
        edge = unit->bb_list[b]->pred_list;
        /* if the number of predecessors of b >= 2 */
        if (edge && edge->pred_next) {
            /* for all predecessors, p, of b */
            for (; edge; edge = edge->pred_next) {
                /* runner = p */
                runner = edge->from->index;
                /* while runner != idoms[b] */
                while (runner >= 0 && runner != unit->idoms[b]) {
                    /* add b to runner's dominance frontier set */
                    set_add(unit->dominance_frontiers[runner], b);
                    /* runner = idoms[runner] */
                    if (runner == 0)
                        runner = -1;
                    else
                        runner = unit->idoms[runner];
                }
            }
        }
    }

    if (IMCC_INFO(interpreter)->debug & DEBUG_CFG)
        dump_dominance_frontiers(unit);
}

static void
free_dominators(IMC_Unit * unit)
{
    int i;

    if (!unit->dominators)
        return;
    for (i=0; i < unit->n_basic_blocks; i++) {
        set_free (unit->dominators[i]);
    }
    free(unit->dominators);
    unit->dominators = 0;
    free(unit->idoms);
}

static void
free_dominance_frontiers(IMC_Unit * unit)
{
    int i;

    if (!unit->dominance_frontiers)
        return;
    for (i=0; i < unit->n_basic_blocks; i++) {
        set_free (unit->dominance_frontiers[i]);
    }
    free(unit->dominance_frontiers);
    unit->dominance_frontiers = 0;
}


static void
sort_loops(Parrot_Interp interpreter, IMC_Unit * unit)
{
    int i, j, changed;
    Loop_info *li;
    int n_loops = unit->n_loops;
    Loop_info ** loop_info = unit->loop_info;

    for (i = 0; i < n_loops; i++) {
        loop_info[i]->size = 0;
        for (j = 0; j < unit->n_basic_blocks; j++)
            if (set_contains(loop_info[i]->loop, j))
                loop_info[i]->size++;
    }
    changed = 1;
    while (changed) {
        changed = 0;
        for (i = 0; i < n_loops-1; i++)
            if (loop_info[i]->size < loop_info[i+1]->size) {
                li = loop_info[i];
                loop_info[i] = loop_info[i+1];
                loop_info[i+1] = li;
                changed = 1;
            }
    }
    /* set depth, it's incorrect til now, as it did depend on the
     * order of finding loops
     */
    for (i = 0; i < n_loops-1; i++) {
        int first = -1, last = 0;
        loop_info[i]->depth = 1;
        /* we could also take the depth of the first contained
         * block, but below is a check, that a inner loop is fully
         * contained in a outer loop
         */
        for (j = 0; j < unit->n_basic_blocks; j++)
            if (set_contains(loop_info[i+1]->loop, j)) {
                if (first < 0)
                    first = j;
                last = j;
            }
        for (j = i + 1; j < n_loops; j++) {
            if (set_contains(loop_info[i]->loop, first) &&
                    !set_contains(loop_info[i]->loop, last)) {
                IMCC_debug(interpreter, DEBUG_CFG, "sort_loops",
                        "loop %d contains first but not"
                        "last of outer loop %d\n", j, i);
            }
            if (set_contains(loop_info[i]->loop, last) &&
                    !set_contains(loop_info[i]->loop, first)) {
                IMCC_debug(interpreter, DEBUG_CFG, "sort_loops",
                        "loop %d contains last but not"
                        "first of outer loop %d\n", j, i);
            }
            loop_info[j]->depth = loop_info[i]->depth + 1;
        }
    }
}


/*
 * Searches for loops in the CFG. We search for edges that
 * go from a node to one of its dominators.
 */

void
find_loops (Parrot_Interp interpreter, IMC_Unit * unit)
{
    int i, succ_index;
    Set* dom;
    Edge* edge;

    IMCC_info(interpreter, 2, "find_loops\n");
    for (i = 0; i < unit->n_basic_blocks; i++) {
      dom = unit->dominators[i];

      for (edge=unit->bb_list[i]->succ_list;
                edge != NULL; edge=edge->succ_next) {
          succ_index = edge->to->index;

          if (set_contains(dom, succ_index) ) {
            mark_loop(interpreter, unit, edge);
          }
        }
    }

    sort_loops(interpreter, unit);
    if (IMCC_INFO(interpreter)->debug & DEBUG_CFG)
        dump_loops(unit);
}

/*
 * For loop_info, finds the natural preheader of the loop, if any, and returns
 * its index, otherwise returns -1.  A natural preheader exists if there is
 * only one predecessor to the loop header outside of the loop body, and if it
 * always transfers control directly to the header.
 */
int
natural_preheader (IMC_Unit * unit, Loop_info* loop_info)
{
    int preheader = -1;
    Edge* edge;

    for (edge = unit->bb_list[loop_info->header]->pred_list; edge; edge = edge->pred_next) {
        if (!set_contains(loop_info->loop, edge->from->index)) {
            if (preheader == -1 && unit->bb_list[edge->from->index]->succ_list->to->index == loop_info->header &&
                    !unit->bb_list[edge->from->index]->succ_list->succ_next) {
                preheader = unit->bb_list[edge->from->index]->index;
                continue;
            } else {
                return -1;
            }
        }
    }
    return preheader;
}

/* Increases the loop_depth of all the nodes in a loop */

static void
mark_loop (Parrot_Interp interpreter, IMC_Unit * unit, Edge* e)
{
    Set* loop;
    Set* exits;
    Basic_block *header, *footer, *enter;
    int i;
    Edge *edge;
    int n_loops;
    Loop_info ** loop_info;

    header =  e->to;
    footer =  e->from;
    enter = 0;
    /* look from where loop was entered */

    for (i = 0, edge=header->pred_list; edge; edge=edge->pred_next)
        if (footer != edge->from) {
            enter = edge->from;
            i++;
        }

    IMCC_debug(interpreter, DEBUG_CFG, "loop from %d to %d, entered from %d\n",
            footer->index, header->index, enter ? enter->index : -1 );
    if (i != 1) {
        if (i==0) {
            if (header->index)
                IMCC_debug(interpreter, DEBUG_CFG,"\tdead code\n");
            else
                IMCC_debug(interpreter, DEBUG_CFG, "\tsub start\n");
        }
        else
            IMCC_debug(interpreter, DEBUG_CFG,
                    "\tcan't determine loop entry block (%d found)\n" ,i);
    }

    loop = set_make(unit->n_basic_blocks);
    set_add(loop, footer->index);
    set_add(loop, header->index);

    footer->loop_depth++;

    if (header != footer) {
        header->loop_depth++;
        search_predecessors_not_in (footer, loop);
    }

    exits = set_make(unit->n_basic_blocks);
    for (i = 1; i < unit->n_basic_blocks; i++) {
        if (set_contains(loop, i)) {
            for (edge = unit->bb_list[i]->succ_list; edge; edge = edge->succ_next) {
                if (!set_contains(loop, edge->to->index)) {
                    set_add(exits, i);
                }
            }
        }
    }

    /* now 'loop' contains the set of nodes inside the loop.
    */
    n_loops = unit->n_loops;
    loop_info = unit->loop_info;
    if (!loop_info)
        loop_info = unit->loop_info = mem_sys_allocate(
                (n_loops+1)*sizeof(Loop_info *));
    else
        loop_info = unit->loop_info = mem_sys_realloc(loop_info,
                (n_loops+1)*sizeof(Loop_info *));
    loop_info[n_loops] = mem_sys_allocate(sizeof(Loop_info));
    loop_info[n_loops]->loop = loop;
    loop_info[n_loops]->exits = exits;
    loop_info[n_loops]->depth = footer->loop_depth;
    loop_info[n_loops]->n_entries = i;
    loop_info[n_loops]->header = header->index;
    loop_info[n_loops]->preheader = natural_preheader(unit, loop_info[n_loops]);
    unit->n_loops++;
}

static void
free_loops(IMC_Unit * unit)
{
    int i;
    for (i = 0; i < unit->n_loops; i++) {
        set_free(unit->loop_info[i]->loop);
        set_free(unit->loop_info[i]->exits);
        free(unit->loop_info[i]);
    }
    if (unit->loop_info)
        free(unit->loop_info);
    unit->n_loops = 0;
    unit->loop_info = 0;
}

void
search_predecessors_not_in(Basic_block *node, Set* s)
{
   Edge *edge;
   Basic_block *pred;

   for (edge = node->pred_list; edge != NULL; edge = edge->pred_next) {
      pred = edge->from;

      if (! set_contains(s, pred->index) ) {
         set_add(s, pred->index);
         pred->loop_depth++;
         search_predecessors_not_in (pred, s);
      }
   }

}
/*** Utility functions ***/

static void
init_basic_blocks(IMC_Unit * unit)
{
    if (unit->bb_list != NULL)
        clear_basic_blocks(unit);
    unit->bb_list =
        calloc(unit->bb_list_size = 256,
            sizeof(Basic_block*) );
    unit->n_basic_blocks = 0;
    unit->edge_list = 0;
}

void
clear_basic_blocks(IMC_Unit * unit)
{
    int i;
    if (unit->bb_list) {
        for (i=0; i < unit->n_basic_blocks; i++)
            free(unit->bb_list[i]);
        free(unit->bb_list);
        unit->bb_list = NULL;
    }
    free_edge(unit);
    free_dominators(unit);
    free_dominance_frontiers(unit);
    free_loops(unit);
}

static Basic_block*
make_basic_block(Interp *interpreter, IMC_Unit * unit, Instruction* ins)
{
    Basic_block *bb;
    int n;

    if (ins == NULL) {
        PANIC("make_basic_block: called with NULL argument\n");
    }

    bb = mem_sys_allocate(sizeof(Basic_block));

    bb->start = ins;
    bb->end = ins;

    bb->pred_list = NULL;
    bb->succ_list = NULL;
    n = unit->n_basic_blocks;
    ins->bbindex = bb->index = n;
    bb->loop_depth = 0;
    if (n == unit->bb_list_size) {
        unit->bb_list_size *= 2;
        unit->bb_list =
            mem_sys_realloc(unit->bb_list,
                unit->bb_list_size*sizeof(Basic_block*) );
    }
    unit->bb_list[n] = bb;
    unit->n_basic_blocks++;

    return bb;
}

Life_range *
make_life_range(SymReg *r, int idx)
{
   Life_range *l;

   l = mem_sys_allocate_zeroed(sizeof(Life_range));
   r->life_info[idx] = l;
   return l;
}

/*
 * Local variables:
 * c-indentation-style: bsd
 * c-basic-offset: 4
 * indent-tabs-mode: nil
 * End:
 *
 * vim: expandtab shiftwidth=4:
*/

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