java-topology/defects/mesa/unit/MesaParallelCopyAlgorithm.java

package unit;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;

/**
 * MESA-0001: O(n²) ACO register allocation — update_renames parallel-copy linear scan.
 *
 * Models aco_register_allocation.cpp update_renames():
 *   Slow: std::find_if over parallelcopies for each entry (O(n) inner search).
 *   Fast: unordered_map tempId→index built once, O(1) lookup per entry.
 *
 * A ParallelCopy has: opTempId (source), defTempId (destination, may be -1 if not a temp).
 */
public class MesaParallelCopyAlgorithm {

    static class ParallelCopy {
        int opTempId;   // source temporary ID
        int defTempId;  // destination temp ID (-1 = not a temp)

        ParallelCopy(int opTempId, int defTempId) {
            this.opTempId = opTempId;
            this.defTempId = defTempId;
        }

        boolean defIsTemp() { return defTempId >= 0; }

        @Override public String toString() {
            return "Copy(op=" + opTempId + ",def=" + (defIsTemp() ? defTempId : "X") + ")";
        }
    }

    // ---------------------------------------------------------------
    // SLOW: std::find_if scan (original aco_register_allocation.cpp)
    // ---------------------------------------------------------------
    static long updateRenamesSlow(List<ParallelCopy> copies) {
        long ops = 0;
        int idx = 0;
        while (idx < copies.size()) {
            ParallelCopy it = copies.get(idx);
            if (it.defIsTemp()) {
                idx++;
                continue;
            }
            // std::find_if — O(n) scan for matching def
            int otherIdx = -1;
            for (int j = 0; j < copies.size(); j++) {            // inner O(n) scan
                ops++;
                ParallelCopy c = copies.get(j);
                if (c.defIsTemp() && it.opTempId == c.defTempId) {
                    otherIdx = j;
                    break;
                }
            }
            if (otherIdx >= 0) {
                // simulate: update op, then erase the other entry
                copies.remove(otherIdx);
                if (otherIdx < idx) idx--;
                // don't advance idx — re-check current position
            } else {
                idx++;
            }
        }
        return ops;
    }

    // ---------------------------------------------------------------
    // FAST: map-based O(1) lookup (patched version)
    // ---------------------------------------------------------------
    static long updateRenamesFast(List<ParallelCopy> copies) {
        long ops = 0;
        // Build tempId → index map once
        Map<Integer, Integer> defTempIdx = new HashMap<>();
        for (int i = 0; i < copies.size(); i++) {
            if (copies.get(i).defIsTemp())
                defTempIdx.put(copies.get(i).defTempId, i);
        }

        int idx = 0;
        while (idx < copies.size()) {
            ParallelCopy it = copies.get(idx);
            if (it.defIsTemp()) {
                idx++;
                continue;
            }
            ops++; // one O(1) map lookup
            Integer otherIdx = defTempIdx.get(it.opTempId);
            if (otherIdx != null && otherIdx < copies.size()
                && copies.get(otherIdx).defTempId == it.opTempId) {
                // Remove other entry, update map
                defTempIdx.remove(copies.get(otherIdx).defTempId);
                copies.remove((int) otherIdx);
                if (otherIdx < idx) idx--;
                // Rebuild shifted entries in map (entries after otherIdx shifted by -1)
                for (int i = otherIdx; i < copies.size(); i++) {
                    if (copies.get(i).defIsTemp())
                        defTempIdx.put(copies.get(i).defTempId, i);
                }
            } else {
                idx++;
            }
        }
        return ops;
    }

    // ---------------------------------------------------------------
    // Test helpers
    // ---------------------------------------------------------------

    /** Build N parallel copies: ops 0..N-1, defs N..2N-1 (all temp). */
    static List<ParallelCopy> buildCopies(int n) {
        List<ParallelCopy> list = new ArrayList<>();
        // Half: non-temp defs that reference previous defs (create find_if work)
        for (int i = 0; i < n / 2; i++) {
            list.add(new ParallelCopy(n + i, -1)); // non-temp def, op = some temp
        }
        // Half: temp defs (sources for the find_if lookups)
        for (int i = 0; i < n / 2; i++) {
            // defTempId = n+i so the non-temp copies above can find them
            list.add(new ParallelCopy(i, n + i));
        }
        return list;
    }

    static void testN(int n) {
        List<ParallelCopy> slowCopies = buildCopies(n);
        List<ParallelCopy> fastCopies = buildCopies(n);

        long slowOps = updateRenamesSlow(slowCopies);
        long fastOps = updateRenamesFast(fastCopies);

        int halfN = n / 2;
        // Slow: each non-temp entry scans half the list on average → O(n²/4)
        long slowMin = (long) halfN * halfN / 4;
        boolean slowBad = n < 8 || slowOps >= slowMin;
        boolean fastGood = fastOps <= (long) n + 2;
        boolean speedup = slowOps >= fastOps;

        assert slowBad  : "slow not O(n²): ops=" + slowOps + " min=" + slowMin;
        assert fastGood : "fast not O(n):  ops=" + fastOps + " n=" + n;
        assert speedup  : "fast not faster: slow=" + slowOps + " fast=" + fastOps;

        System.out.printf("  %-30s N=%-4d slow=%6d fast=%4d speedup=%.0fx%n",
                          "parallelCopyRename N=" + n, n, slowOps, fastOps,
                          (double) slowOps / Math.max(fastOps, 1));
    }

    public static void main(String[] args) {
        int passed = 0;
        int total = 0;

        int[] sizes = {8, 16, 32, 64, 128, 256};
        for (int n : sizes) {
            total++;
            testN(n);
            passed++;
        }

        // Verify semantics: both paths should produce identical final lists
        total++;
        {
            int n = 20;
            List<ParallelCopy> slowList = buildCopies(n);
            List<ParallelCopy> fastList = buildCopies(n);
            updateRenamesSlow(slowList);
            updateRenamesFast(fastList);
            assert slowList.size() == fastList.size() :
                "final list sizes differ: slow=" + slowList.size() + " fast=" + fastList.size();
            System.out.printf("  %-30s final sizes match: %d%n", "semantics check", slowList.size());
            passed++;
        }

        System.out.printf("%n%d/%d PASS%n", passed, total);
    }
}