path: root/chapter12/src/main/java/com/seaofnodes/simple/GlobalCodeMotion.java

 package com.seaofnodes.simple;

import com.seaofnodes.simple.node.*;
import com.seaofnodes.simple.type.*;
import java.util.*;

public abstract class GlobalCodeMotion {

    // Arrange that the existing isCFG() Nodes form a valid CFG.  The
    // Node.use(0) is always a block tail (either IfNode or head of the
    // following block).  There are no unreachable infinite loops.
    public static void buildCFG( StopNode stop ) {
        schedEarly();
        Parser.SCHEDULED = true;
        schedLate(Parser.START);
    }

    // ------------------------------------------------------------------------
    // Backwards walk on the CFG only, looking for unreachable code - which has
    // to be an infinite loop.  Insert a bogus never-taken exit to Stop, so the
    // loop becomes reachable.  Also, set loop nesting depth
    static void fixLoops(StopNode stop) {
        // Backwards walk from Stop, looking for unreachable code
        BitSet visit = new BitSet();
        HashSet<CFGNode> unreach = new HashSet<>();
        unreach.add(Parser.START);
        for( Node ret : stop._inputs )
            ((ReturnNode)ret).walkUnreach(visit,unreach);
        if( unreach.isEmpty() ) return;

        // Forwards walk from unreachable, looking for loops with no exit test.
        visit.clear();
        for( CFGNode cfg : unreach )
            walkInfinite(cfg,visit,stop);
        // Set loop depth on remaining graph
        unreach.clear();
        visit.clear();
        for( Node ret : stop._inputs )
            ((ReturnNode)ret).walkUnreach(visit,unreach);
        assert unreach.isEmpty();
    }


    // Forwards walk over previously unreachable, looking for loops with no
    // exit test.
    static private void walkInfinite( CFGNode n, BitSet visit, StopNode stop ) {
        if( visit.get(n._nid) ) return; // Been there, done that
        visit.set(n._nid);
        if( n instanceof LoopNode loop )
            loop.forceExit(stop);
        for( Node use : n._outputs )
            if( use instanceof CFGNode cfg )
                walkInfinite(cfg,visit,stop);
    }

    // ------------------------------------------------------------------------
    private static void schedEarly() {
        ArrayList<CFGNode> rpo = new ArrayList<>();
        BitSet visit = new BitSet();
        _rpo_cfg(Parser.START, visit, rpo);
        // Reverse Post-Order on CFG
        for( int j=rpo.size()-1; j>=0; j-- ) {
            CFGNode cfg = rpo.get(j);
            cfg.loopDepth();
            for( Node n : cfg._inputs )
                _schedEarly(n,visit);
            // Strictly for dead infinite loops, we can have entire code blocks
            // not reachable from below - so we reach down, from above, one
            // step.  Since _schedEarly modifies the output arrays, the normal
            // region._outputs ArrayList iterator throws CME.  The extra edges
            // are always *added* after any Phis, so just walk the Phi prefix.
            if( cfg instanceof RegionNode region ) {
                int len = region.nOuts();
                for( int i=0; i<len; i++ )
                    if( region.out(i) instanceof PhiNode phi )
                        _schedEarly(phi,visit);
            }
        }
    }

    // Post-Order of CFG
    private static void _rpo_cfg(Node n, BitSet visit, ArrayList<CFGNode> rpo) {
        if( !(n instanceof CFGNode cfg) || visit.get(cfg._nid) )
            return;             // Been there, done that
        visit.set(cfg._nid);
        for( Node use : cfg._outputs )
            _rpo_cfg(use,visit,rpo);
        rpo.add(cfg);
    }

    private static void _schedEarly(Node n, BitSet visit) {
        if( n==null || visit.get(n._nid) ) return; // Been there, done that
        visit.set(n._nid);
        // Schedule not-pinned not-CFG inputs before self.  Since skipping
        // Pinned, this never walks the backedge of Phis (and thus spins around
        // a data-only loop, eventually attempting relying on some pre-visited-
        // not-post-visited data op with no scheduled control.
        for( Node def : n._inputs )
            if( def!=null && !def.isPinned() )
                _schedEarly(def,visit);
        // If not-pinned (e.g. constants, projections, phi) and not-CFG
        if( !n.isPinned() ) {
            // Schedule at deepest input
            CFGNode early = Parser.START; // Maximally early, lowest idepth
            for( int i=1; i<n.nIns(); i++ )
                if( n.in(i).cfg0().idepth() > early.idepth() )
                    early = n.in(i).cfg0(); // Latest/deepest input
            n.setDef(0,early);              // First place this can go
        }
    }

    // ------------------------------------------------------------------------
    private static void schedLate(StartNode start) {
        CFGNode[] late = new CFGNode[Node.UID()];
        Node[] ns = new Node[Node.UID()];
        _schedLate(start,ns,late);
        for( int i=0; i<late.length; i++ )
            if( ns[i] != null )
                ns[i].setDef(0,late[i]);
    }

    // Forwards post-order pass.  Schedule all outputs first, then draw a
    // idom-tree line from the LCA of uses to the early schedule.  Schedule is
    // legal anywhere on this line; pick the most control-dependent (largest
    // idepth) in the shallowest loop nest.
    private static void _schedLate(Node n, Node[] ns, CFGNode[] late) {
        if( late[n._nid]!=null ) return; // Been there, done that
        // These I know the late schedule of, and need to set early for loops
        if( n instanceof CFGNode cfg ) late[n._nid] = cfg.blockHead() ? cfg : cfg.cfg(0);
        if( n instanceof PhiNode phi ) late[n._nid] = phi.region();

        // Walk Stores before Loads, so we can get the anti-deps right
        for( Node use : n._outputs )
            if( isForwardsEdge(use,n) &&
                use._type instanceof TypeMem )
                _schedLate(use,ns,late);
        // Walk everybody now
        for( Node use : n._outputs )
            if( isForwardsEdge(use,n) )
                _schedLate(use,ns,late);
        // Already implicitly scheduled
        if( n.isPinned() ) return;
        // Need to schedule n

        // Walk uses, gathering the LCA (Least Common Ancestor) of uses
        CFGNode early = (CFGNode)n.in(0);
        assert early != null;
        CFGNode lca = null;
        for( Node use : n._outputs )
            lca = use_block(n,use, late)._idom(lca,null);

        // Loads may need anti-dependencies, raising their LCA
        if( n instanceof LoadNode load )
            lca = find_anti_dep(lca,load,early,late);

        // Walk up from the LCA to the early, looking for best place.  This is
        // lowest execution frequency, approximated by least loop depth and
        // deepest control flow.
        CFGNode best = lca;
        lca = lca.idom();       // Already found best for starting LCA
        for( ; lca != early.idom(); lca = lca.idom() )
            if( better(lca,best) )
                best = lca;
        assert !(best instanceof IfNode);
        ns  [n._nid] = n;
        late[n._nid] = best;
    }

    // Block of use.  Normally from late[] schedule, except for Phis, which go
    // to the matching Region input.
    private static CFGNode use_block(Node n, Node use, CFGNode[] late) {
        if( !(use instanceof PhiNode phi) )
            return late[use._nid];
        CFGNode found=null;
        for( int i=1; i<phi.nIns(); i++ )
            if( phi.in(i)==n )
                if( found==null ) found = phi.region().cfg(i);
                else Utils.TODO(); // Can be more than once
        assert found!=null;
        return found;
    }


    // Least loop depth first, then largest idepth
    private static boolean better( CFGNode lca, CFGNode best ) {
        return lca._loopDepth < best._loopDepth ||
                (lca.idepth() > best.idepth() || best instanceof IfNode);
    }

    // Skip iteration if a backedge
    private static boolean isForwardsEdge(Node use, Node def) {
        return use != null && def != null &&
            !(use.nIns()>2 && use.in(2)==def && (use instanceof LoopNode || (use instanceof PhiNode phi && phi.region() instanceof LoopNode)));
    }

    private static CFGNode find_anti_dep(CFGNode lca, LoadNode load, CFGNode early, CFGNode[] late) {
        // We could skip final-field loads here.
        // Walk LCA->early, flagging Load's block location choices
        for( CFGNode cfg=lca; early!=null && cfg!=early.idom(); cfg = cfg.idom() )
            cfg._anti = load._nid;
        // Walk load->mem uses, looking for Stores causing an anti-dep
        for( Node mem : load.mem()._outputs ) {
            switch( mem ) {
            case StoreNode st:
                lca = anti_dep(load,late[st._nid],st.cfg0(),lca,st);
                break;
            case PhiNode phi:
                // Repeat anti-dep for matching Phi inputs.
                // No anti-dep edges but may raise the LCA.
                for( int i=1; i<phi.nIns(); i++ )
                    if( phi.in(i)==load.mem() )
                        lca = anti_dep(load,phi.region().cfg(i),load.mem().cfg0(),lca,null);
                break;
            case LoadNode ld: break; // Loads do not cause anti-deps on other loads
            case ReturnNode ret: break; // Load must already be ahead of Return
            case NeverNode never: break;
            default: throw Utils.TODO();
            }
        }
        return lca;
    }

    //
    private static CFGNode anti_dep( LoadNode load, CFGNode stblk, CFGNode defblk, CFGNode lca, Node st ) {
        // Walk store blocks "reach" from its scheduled location to its earliest
        for( ; stblk != defblk.idom(); stblk = stblk.idom() ) {
            // Store and Load overlap, need anti-dependence
            if( stblk._anti==load._nid ) {
                lca = stblk._idom(lca,null); // Raise Loads LCA
                if( lca == stblk && st != null && Utils.find(st._inputs,load) == -1 ) // And if something moved,
                    st.addDef(load);   // Add anti-dep as well
                return lca;            // Cap this stores' anti-dep to here
            }
        }
        return lca;
    }

}