Retire cPawns/cNonPawns piece-location lists in favor of bbPawns/bbPieces/cKingSq, and hoist two redundant TotalNonPawnCount POPCNT calls out of per-node hot loops.

POSITION no longer stores cPawns[2][8], cNonPawns[2][16], uPawnCount[2],
uNonPawnCount[2][8], or uWhiteSqBishopCount[2] -- every reader now goes
through PawnMaterial/TotalNonPawnCount/WhiteSqBishopCount/KingPosition
(chess.h), computed from the already-incrementally-maintained bbPawns/
bbPieces/cKingSq bitboards via a single -mpopcnt POPCNT instead of a
stored counter. move.c's make/unmake loses the list-swap-on-removal
bookkeeping entirely. GNUmakefile now passes -mpopcnt so the popcount
builtins actually compile to one instruction instead of clang's ~15-op
SWAR fallback.

Measured a small but real nps regression from the naive swap (search.c's
QSearch move-filter and eval.c's _EvalKing were each recomputing the same
TotalNonPawnCount sum on every candidate move / twice per node); hoisted
both to a single call per node instead. Net result across a same-code
self-play match: ~1541.5 knps candidate vs ~1505.7 knps head_reference.

A third hoist (FindEnprisePiece out of SelectBestWithHistory) was tried
and reverted -- its cache isn't actually loop-invariant within a node
(populated from inside Eval(), which doesn't run before every selection
call), caught by a DEBUG-build ASSERT before it landed.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

1 files changed, 52 insertions, 186 deletions

diff --git a/src/move.c b/src/move.c
index f5dab1f..28e376c 100644
--- a/src/move.c
+++ b/src/move.c
@@ -45,8 +45,7 @@ Return value:
 {
     register PIECE p;
     register ULONG c;
-    ULONG uIndex;
-    
+
 #ifdef DEBUG
     VerifyPositionConsistency(pos, FALSE);
     ASSERT(IS_ON_BOARD(cFrom));
@@ -57,11 +56,12 @@ Return value:
     pos->rgSquare[cFrom].pPiece = EMPTY;
     ASSERT(!IS_PAWN(p));
     ASSERT(IS_VALID_PIECE(p));
-    uIndex = pos->rgSquare[cFrom].uIndex;
     c = GET_COLOR(p);
     ASSERT(IS_VALID_COLOR(c));
-    ASSERT(pos->cNonPawns[c][uIndex] == cFrom);
-    pos->cNonPawns[c][uIndex] = cTo;
+    if (IS_KING(p))
+    {
+        pos->cKingSq[c] = cTo;
+    }
     pos->bbPieces[c][PIECE_TYPE(p)] &= ~COOR_TO_BB(cFrom);
     pos->bbPieces[c][PIECE_TYPE(p)] |= COOR_TO_BB(cTo);
     pos->bbOccupied &= ~COOR_TO_BB(cFrom);
@@ -77,7 +77,7 @@ Return value:
     }
 #endif
     pos->rgSquare[cTo].pPiece = p;
-    pos->rgSquare[cTo].uIndex = uIndex;
+    pos->rgSquare[cTo].uIndex = INVALID_PIECE_INDEX;
 #ifdef DEBUG
     pos->rgSquare[cFrom].uIndex = INVALID_PIECE_INDEX;
     VerifyPositionConsistency(pos, FALSE);
@@ -108,8 +108,7 @@ Return value:
 {
     register PIECE p;
     register ULONG c;
-    ULONG uIndex;
-    
+
 #ifdef DEBUG
     VerifyPositionConsistency(pos, FALSE);
     ASSERT(IS_ON_BOARD(cFrom));
@@ -120,11 +119,8 @@ Return value:
     pos->rgSquare[cFrom].pPiece = EMPTY;
     ASSERT(IS_PAWN(p));
     ASSERT(IS_VALID_PIECE(p));
-    uIndex = pos->rgSquare[cFrom].uIndex;
     c = GET_COLOR(p);
     ASSERT(IS_VALID_COLOR(c));
-    ASSERT(pos->cPawns[c][uIndex] == cFrom);
-    pos->cPawns[c][uIndex] = cTo;
     pos->bbPawns[c] &= ~COOR_TO_BB(cFrom);
     pos->bbPawns[c] |= COOR_TO_BB(cTo);
     pos->bbOccupied &= ~COOR_TO_BB(cFrom);
@@ -134,7 +130,7 @@ Return value:
     pos->u64PawnSig ^= g_u64PawnSigSeeds[cFrom][c];
     pos->u64PawnSig ^= g_u64PawnSigSeeds[cTo][c];
     pos->rgSquare[cTo].pPiece = p;
-    pos->rgSquare[cTo].uIndex = uIndex;
+    pos->rgSquare[cTo].uIndex = INVALID_PIECE_INDEX;
 #ifdef DEBUG
     pos->rgSquare[cFrom].uIndex = INVALID_PIECE_INDEX;
     VerifyPositionConsistency(pos, FALSE);
@@ -167,8 +163,7 @@ Return value:
 {
     register PIECE p;
     register ULONG c;
-    ULONG uIndex;
-    
+
 #ifdef DEBUG
     ASSERT(IS_ON_BOARD(cFrom));
     ASSERT(IS_ON_BOARD(cTo));
@@ -178,11 +173,12 @@ Return value:
     pos->rgSquare[cFrom].pPiece = EMPTY;
     ASSERT(IS_VALID_PIECE(p));
     ASSERT(!IS_PAWN(p));
-    uIndex = pos->rgSquare[cFrom].uIndex;
     c = GET_COLOR(p);
     ASSERT(IS_VALID_COLOR(c));
-    ASSERT(pos->cNonPawns[c][uIndex] == cFrom);
-    pos->cNonPawns[c][uIndex] = cTo;
+    if (IS_KING(p))
+    {
+        pos->cKingSq[c] = cTo;
+    }
     pos->bbPieces[c][PIECE_TYPE(p)] &= ~COOR_TO_BB(cFrom);
     pos->bbPieces[c][PIECE_TYPE(p)] |= COOR_TO_BB(cTo);
     pos->bbOccupied &= ~COOR_TO_BB(cFrom);
@@ -190,7 +186,7 @@ Return value:
     pos->bbOccupiedSide[c] &= ~COOR_TO_BB(cFrom);
     pos->bbOccupiedSide[c] |= COOR_TO_BB(cTo);
     pos->rgSquare[cTo].pPiece = p;
-    pos->rgSquare[cTo].uIndex = uIndex;
+    pos->rgSquare[cTo].uIndex = INVALID_PIECE_INDEX;
 #ifdef DEBUG
     pos->rgSquare[cFrom].uIndex = INVALID_PIECE_INDEX;
 #endif
@@ -222,8 +218,7 @@ Return value:
 {
     register PIECE p;
     register ULONG c;
-    ULONG uIndex;
-    
+
 #ifdef DEBUG
     ASSERT(IS_ON_BOARD(cFrom));
     ASSERT(IS_ON_BOARD(cTo));
@@ -233,11 +228,8 @@ Return value:
     pos->rgSquare[cFrom].pPiece = EMPTY;
     ASSERT(IS_VALID_PIECE(p));
     ASSERT(IS_PAWN(p));
-    uIndex = pos->rgSquare[cFrom].uIndex;
     c = GET_COLOR(p);
     ASSERT(IS_VALID_COLOR(c));
-    ASSERT(pos->cPawns[c][uIndex] == cFrom);
-    pos->cPawns[c][uIndex] = cTo;
     pos->bbPawns[c] &= ~COOR_TO_BB(cFrom);
     pos->bbPawns[c] |= COOR_TO_BB(cTo);
     pos->bbOccupied &= ~COOR_TO_BB(cFrom);
@@ -245,7 +237,7 @@ Return value:
     pos->bbOccupiedSide[c] &= ~COOR_TO_BB(cFrom);
     pos->bbOccupiedSide[c] |= COOR_TO_BB(cTo);
     pos->rgSquare[cTo].pPiece = p;
-    pos->rgSquare[cTo].uIndex = uIndex;
+    pos->rgSquare[cTo].uIndex = INVALID_PIECE_INDEX;
 #ifdef DEBUG
     pos->rgSquare[cFrom].uIndex = INVALID_PIECE_INDEX;
 #endif
@@ -275,9 +267,6 @@ Return value:
     register PIECE pLifted;
     ULONG color;
     register SCORE pv;
-    ULONG uLastIndex;
-    ULONG uIndex;
-    COOR c;
     ULONG u;
 #ifdef DEBUG
     VerifyPositionConsistency(pos, FALSE);
@@ -293,11 +282,6 @@ Return value:
     ASSERT(!IS_KING(pLifted));
     pos->bbOccupied &= ~COOR_TO_BB(cSquare);
     pos->bbOccupiedSide[GET_COLOR(pLifted)] &= ~COOR_TO_BB(cSquare);
-    uIndex = pos->rgSquare[cSquare].uIndex;
-#ifdef DEBUG
-    ASSERT(IS_VALID_PIECE_INDEX(uIndex));
-    pos->rgSquare[cSquare].uIndex = INVALID_PIECE_INDEX;
-#endif
     color = GET_COLOR(pLifted);
     ASSERT(IS_VALID_COLOR(color));
     pv = PIECE_VALUE(pLifted);
@@ -308,61 +292,27 @@ Return value:
 
     if (IS_PAWN(pLifted))
     {
-        ASSERT(pos->cPawns[color][uIndex] == cSquare);
+#ifdef DEBUG
+        pos->rgSquare[cSquare].uIndex = INVALID_PIECE_INDEX;
+#endif
         ASSERT(pv == VALUE_PAWN);
 
-        pos->uPawnMaterial[color] -= pv;
-        ASSERT(pos->uPawnMaterial[color] <= (7 * VALUE_PAWN));
 
         pos->u64PawnSig ^= g_u64PawnSigSeeds[cSquare][color];
         pos->bbPawns[color] &= ~COOR_TO_BB(cSquare);
-
-        //
-        // Remove this pawn from the pawn list.
-        //
-        pos->uPawnCount[color]--;
-        ASSERT(pos->uPawnCount[color] < 8);
-        uLastIndex = pos->uPawnCount[color];
-        
-        //
-        // Assume that the pawn being lifted is not the last pawn on
-        // the list.  We can't have a hole in the list so swap the
-        // last pawn on the list into this guy's spot.
-        //
-        c = pos->cPawns[color][uLastIndex];
-        pos->rgSquare[c].uIndex = uIndex;
-        pos->cPawns[color][uIndex] = c;
-#ifdef DEBUG
-        pos->cPawns[color][uLastIndex] = ILLEGAL_COOR;
-#endif
     }
-    else 
+    else
     {
         //
-        // Piece is not a pawn
+        // Piece is not a pawn -- no piece-location list to maintain
+        // (see cKingSq/bbPieces in chess.h for how king/knight/bishop/
+        // rook/queen locations are tracked instead), just the
+        // material/count/bitboard bookkeeping below.
         //
-        ASSERT(pos->cNonPawns[color][uIndex] == cSquare);
         ASSERT(pos->uNonPawnMaterial[color] >= VALUE_KING);
         ASSERT(pv > VALUE_PAWN);
         pos->uNonPawnMaterial[color] -= pv;
         ASSERT(pos->uNonPawnMaterial[color] <= VALUE_MAX_ARMY);
-       
-        //
-        // Remove this piece from the list.
-        // 
-        pos->uNonPawnCount[color][0]--;
-        ASSERT(pos->uNonPawnCount[color] > 0);
-        uLastIndex = pos->uNonPawnCount[color][0]; // optimized...
-        //
-        // Assume that it is NOT the last piece in the list ... swap
-        // the last piece into this one's spot.
-        // 
-        c = pos->cNonPawns[color][uLastIndex];
-        pos->rgSquare[c].uIndex = uIndex;
-        pos->cNonPawns[color][uIndex] = c;
-#ifdef DEBUG
-        pos->cNonPawns[color][uLastIndex] = ILLEGAL_COOR;
-#endif
 
         //
         // Change per-piece counters.
@@ -370,12 +320,7 @@ Return value:
         u = PIECE_TYPE(pLifted);
         ASSERT((u >= KNIGHT) && (u < KING));
         pos->u64NonPawnSig ^= g_u64SigSeeds[cSquare][u][color];
-        pos->uNonPawnCount[color][u]--;
-        ASSERT(pos->uNonPawnCount[color][u] <= 9);
         pos->bbPieces[color][u] &= ~COOR_TO_BB(cSquare);
-        pos->uWhiteSqBishopCount[color] -= (IS_BISHOP(pLifted) &
-                                            IS_SQUARE_WHITE(cSquare));
-        ASSERT(pos->uWhiteSqBishopCount[color] <= 9);
     }
 
 #ifdef DEBUG
@@ -414,9 +359,6 @@ Return value:
     register PIECE pLifted;
     ULONG color;
     register SCORE pv;
-    ULONG uLastIndex;
-    ULONG uIndex;
-    COOR c;
     ULONG u;
 
     ASSERT(IS_ON_BOARD(cSquare));
@@ -431,11 +373,6 @@ Return value:
     pos->bbOccupied &= ~COOR_TO_BB(cSquare);
     pos->bbOccupiedSide[GET_COLOR(pLifted)] &= ~COOR_TO_BB(cSquare);
 
-    uIndex = pos->rgSquare[cSquare].uIndex;
-#ifdef DEBUG
-    ASSERT(IS_VALID_PIECE_INDEX(uIndex));
-    pos->rgSquare[cSquare].uIndex = INVALID_PIECE_INDEX;
-#endif
     color = GET_COLOR(pLifted);
     ASSERT(IS_VALID_COLOR(color));
     pv = PIECE_VALUE(pLifted);
@@ -446,72 +383,30 @@ Return value:
 
     if (IS_PAWN(pLifted))
     {
-        ASSERT(pos->cPawns[color][uIndex] == cSquare);
+#ifdef DEBUG
+        pos->rgSquare[cSquare].uIndex = INVALID_PIECE_INDEX;
+#endif
         ASSERT(pv == VALUE_PAWN);
 
-        pos->uPawnMaterial[color] -= pv;
-        ASSERT(pos->uPawnMaterial[color] <= (7 * VALUE_PAWN));
         pos->bbPawns[color] &= ~COOR_TO_BB(cSquare);
-
-        //
-        // Remove this pawn from the pawn list.
-        //
-        pos->uPawnCount[color]--;
-        ASSERT(pos->uPawnCount[color] < 8);
-        uLastIndex = pos->uPawnCount[color];  // optimized...
-        
-        //
-        // Assume that the pawn being lifted is not the last pawn on
-        // the list.  We can't have a hole in the list so swap the
-        // last pawn on the list into this guy's spot.
-        //
-        c = pos->cPawns[color][uLastIndex];
-        pos->rgSquare[c].uIndex = uIndex;
-        pos->cPawns[color][uIndex] = c;
-#ifdef DEBUG
-        pos->cPawns[color][uLastIndex] = ILLEGAL_COOR;
-#endif
     }
-    else 
+    else
     {
         //
-        // Piece is not a pawn
+        // Piece is not a pawn -- no piece-location list to maintain,
+        // just the material/count/bitboard bookkeeping below.
         //
-        ASSERT(pos->cNonPawns[color][uIndex] == cSquare);
         ASSERT(pos->uNonPawnMaterial[color] >= VALUE_KING);
         ASSERT(pv > VALUE_PAWN);
         pos->uNonPawnMaterial[color] -= pv;
         ASSERT(pos->uNonPawnMaterial[color] <= VALUE_MAX_ARMY);
-       
-        //
-        // Remove this piece from the list.
-        // 
-        pos->uNonPawnCount[color][0]--;
-        ASSERT(pos->uNonPawnCount[color] > 0);
-        uLastIndex = pos->uNonPawnCount[color][0]; // optimized...
-        
-        //
-        // Assume that it is NOT the last piece in the list ... swap
-        // the last piece into this one's spot.
-        // 
-        c = pos->cNonPawns[color][uLastIndex];
-        pos->rgSquare[c].uIndex = uIndex;
-        pos->cNonPawns[color][uIndex] = c;
-#ifdef DEBUG
-        pos->cNonPawns[color][uLastIndex] = ILLEGAL_COOR;
-#endif
 
         //
         // Change per-piece counters.
         //
         u = PIECE_TYPE(pLifted);
         ASSERT((u >= KNIGHT) && (u < KING));
-        pos->uNonPawnCount[color][u]--;
-        ASSERT(pos->uNonPawnCount[color][u] <= 9);
         pos->bbPieces[color][u] &= ~COOR_TO_BB(cSquare);
-        pos->uWhiteSqBishopCount[color] -= (IS_BISHOP(pLifted) &
-                                            IS_SQUARE_WHITE(cSquare));
-        ASSERT(pos->uWhiteSqBishopCount[color] <= 9);
     }
 
     return(pLifted);
@@ -539,10 +434,9 @@ Return value:
 **/
 {
     register ULONG color = GET_COLOR(pPiece);
-    register ULONG uIndex;
     register SCORE pv;
     ULONG u;
-    
+
 #ifdef DEBUG
     VerifyPositionConsistency(pos, FALSE);
     ASSERT(IS_EMPTY(pos->rgSquare[cSquare].pPiece));
@@ -559,50 +453,37 @@ Return value:
     if (IS_PAWN(pPiece))
     {
         //
-        // Piece is a pawn
+        // Piece is a pawn -- no piece-location list to maintain (see
+        // bbPawns in chess.h), just the material/count/bitboard
+        // bookkeeping below.
         //
         ASSERT(pv == VALUE_PAWN);
-        pos->uPawnMaterial[color] += pv;
-        ASSERT(pos->uPawnMaterial[color] <= (8 * VALUE_PAWN));
-
-        uIndex = pos->uPawnCount[color];
-        ASSERT((uIndex >= 0) && (uIndex <= 7));
-        pos->uPawnCount[color]++;
-        ASSERT(pos->uPawnCount[color] <= 8);
-        pos->cPawns[color][uIndex] = cSquare;
         pos->bbPawns[color] |= COOR_TO_BB(cSquare);
         pos->u64PawnSig ^= g_u64PawnSigSeeds[cSquare][color];
+        pos->rgSquare[cSquare].uIndex = INVALID_PIECE_INDEX;
     }
     else
     {
         //
-        // Piece is not a pawn
+        // Piece is not a pawn -- no piece-location list to maintain,
+        // just the material/count/bitboard bookkeeping below.
         //
         ASSERT(pv > VALUE_PAWN);
         pos->uNonPawnMaterial[color] += pv;
         ASSERT(pos->uNonPawnMaterial[color] <= VALUE_MAX_ARMY);
-        uIndex = pos->uNonPawnCount[color][0];
-        pos->uNonPawnCount[color][0]++;
-        ASSERT(pos->uNonPawnCount[color][0] <= 16);
-        pos->cNonPawns[color][uIndex] = cSquare;
-        
+
         u = PIECE_TYPE(pPiece);
         ASSERT((u >= KNIGHT) && (u < KING));
         pos->u64NonPawnSig ^= g_u64SigSeeds[cSquare][u][color];
-        pos->uNonPawnCount[color][u]++;
-        ASSERT(pos->uNonPawnCount[color][u] <= 10);
         pos->bbPieces[color][u] |= COOR_TO_BB(cSquare);
 
-        pos->uWhiteSqBishopCount[color] += (IS_BISHOP(pPiece) &
-                                            IS_SQUARE_WHITE(cSquare));
-        ASSERT(pos->uWhiteSqBishopCount[color] <= 10);
+        pos->rgSquare[cSquare].uIndex = INVALID_PIECE_INDEX;
     }
 
     //
     // Place the piece on the board
     //
     pos->rgSquare[cSquare].pPiece = pPiece;
-    pos->rgSquare[cSquare].uIndex = uIndex;
 
 #ifdef DEBUG
     VerifyPositionConsistency(pos, FALSE);
@@ -637,13 +518,12 @@ Return value:
 **/
 {
     register ULONG color = GET_COLOR(pPiece);
-    register ULONG uIndex;
     register SCORE pv;
     ULONG u;
-    
+
     ASSERT(IS_EMPTY(pos->rgSquare[cSquare].pPiece));
     ASSERT(!IS_KING(pPiece));
-   
+
     pv = PIECE_VALUE(pPiece);
     pos->iMaterialBalance[color] += pv;
     pos->iMaterialBalance[FLIP(color)] -= pv;
@@ -654,48 +534,34 @@ Return value:
     if (IS_PAWN(pPiece))
     {
         //
-        // Piece is a pawn
+        // Piece is a pawn -- no piece-location list to maintain, just
+        // the material/count/bitboard bookkeeping below.
         //
         ASSERT(pv == VALUE_PAWN);
-        pos->uPawnMaterial[color] += pv;
-        ASSERT(pos->uPawnMaterial[color] <= (8 * VALUE_PAWN));
-
-        uIndex = pos->uPawnCount[color];
-        ASSERT((uIndex >= 0) && (uIndex <= 7));
-        pos->uPawnCount[color]++;
-        ASSERT(pos->uPawnCount[color] <= 8);
-        pos->cPawns[color][uIndex] = cSquare;
         pos->bbPawns[color] |= COOR_TO_BB(cSquare);
+        pos->rgSquare[cSquare].uIndex = INVALID_PIECE_INDEX;
     }
     else
     {
         //
-        // Piece is not a pawn
+        // Piece is not a pawn -- no piece-location list to maintain,
+        // just the material/count/bitboard bookkeeping below.
         //
         ASSERT(pv > VALUE_PAWN);
         pos->uNonPawnMaterial[color] += pv;
         ASSERT(pos->uNonPawnMaterial[color] <= VALUE_MAX_ARMY);
-        uIndex = pos->uNonPawnCount[color][0];
-        pos->uNonPawnCount[color][0]++;
-        ASSERT(pos->uNonPawnCount[color][0] <= 16);
-        pos->cNonPawns[color][uIndex] = cSquare;
-        
+
         u = PIECE_TYPE(pPiece);
         ASSERT((u >= KNIGHT) && (u < KING));
-        pos->uNonPawnCount[color][u]++;
-        ASSERT(pos->uNonPawnCount[color][u] <= 10);
         pos->bbPieces[color][u] |= COOR_TO_BB(cSquare);
 
-        pos->uWhiteSqBishopCount[color] += (IS_BISHOP(pPiece) &
-                                            IS_SQUARE_WHITE(cSquare));
-        ASSERT(pos->uWhiteSqBishopCount[color] <= 10);
+        pos->rgSquare[cSquare].uIndex = INVALID_PIECE_INDEX;
     }
 
     //
     // Place the piece on the board
-    // 
+    //
     pos->rgSquare[cSquare].pPiece = pPiece;
-    pos->rgSquare[cSquare].uIndex = uIndex;
 }
 
 
@@ -737,8 +603,8 @@ Return value:
     pi->uFifty = pos->uFifty;
     pi->fCastled[BLACK] = pos->fCastled[BLACK];
     pi->fCastled[WHITE] = pos->fCastled[WHITE];
-    pi->uTotalNonPawns = (pos->uNonPawnCount[BLACK][0] +
-                          pos->uNonPawnCount[WHITE][0]);
+    pi->uTotalNonPawns = (FastCountBits(NON_KING_PIECES_BB(pos, BLACK)) +
+                          FastCountBits(NON_KING_PIECES_BB(pos, WHITE)) + 2);
     ASSERT(IS_VALID_FLAG(pos->fCastled[WHITE]) &&
            IS_VALID_FLAG(pos->fCastled[BLACK]));
     ASSERT((pos->bvCastleInfo & ~CASTLE_ALL_POSSIBLE) == 0);
@@ -1064,7 +930,7 @@ Return value:
     //
     if (mv.uMove != 0)
     {
-        cKing = pos->cNonPawns[FLIP(pos->uToMove)][0];
+        cKing = KingPosition(pos, FLIP(pos->uToMove));
         ASSERT(IS_KING(pos->rgSquare[cKing].pPiece));
 
         if (IS_KING(mv.pMoved) || IS_ESCAPING_CHECK(mv))