Should Stockfish continually investigate underpromotion in lines? (non-queen promotions)

[RogerThiede]

First, I'm curious if there is a simple classical/logical method to determine if promoting to a piece other than a queen is necessary. In my games, I would always promote a pawn to a queen unless it causes an immediate stalemate or when promoting to another piece leads to a checkmate in one. Are there positions in which more logic is needed beyond checking for a stalemate in one move or checkmate in one or two moves?

I have most 7man Tablebase WDL (.rtbw) files and use them during deep 60+ depth searches. I've discovered some Tablebase WDL misses on files like KBBBvKRN.rtbw and began to wonder what the TT resource consumption and unnecessary time was being spent on underpromotion lines.

Would it be more efficient to prune underpomotion lines if they don't lead to checkmate within two or three moves? While you could reach a KBBBvKRN position with the last move being an underpromotion to a Bishop, you could also reach it with a capture implying that the underpromotion to a Bishop happened much earlier. If Stockfish started with a position of 8 or more pieces, and reached a line with KBBBvKRN, that implies to me that it also has lines with KQBBvKRN in it's table lookups. How much resources are being used by those KBBBvKRN lines?

[MinetaS]

I've found a few cases where underpromotion is required; mainly to avoid stalemate. You may be able to find some examples from puzzle collections. Those happen quite rarely, though.

[RogerThiede]

@MinetaS, I already mentioned those types of positions.
Friendly repeat of the quesiton, "Are there positions in which more logic is needed beyond checking for a stalemate in one move or checkmate in one or two moves?"

[MinetaS]

@RogerThiede Sorry, I failed to elaborate my answer.

If promoting to a bishop or a rook leads to better result, then it means giving up partial dominance over squares is optimal on the position. I may come up with a few explaining logics, i.e. to avoid pinning an opponent's piece or restricting opponent's moves. Such cases aren't limited to one-move stalemate; sometimes it involves multiple moves to make a proper evaluation.

4b1k1/P4pPp/1R3P1P/2r5/8/1P6/1K6/8 w - - 0 1

Promoting to a queen will result in immediate stalemate followed by perpetual check of black's rook (probably 4+ moves).
SF gives a7-a8B on the position above after depth 26-28.

SF log

info depth 1 seldepth 1 multipv 1 score cp 966 nodes 119 nps 119000 tbhits 0 time 1 pv a7a8q
info depth 2 seldepth 2 multipv 1 score cp 1219 nodes 387 nps 387000 tbhits 0 time 1 pv a7a8r
info depth 3 seldepth 2 multipv 1 score cp 1485 nodes 542 nps 542000 tbhits 0 time 1 pv a7a8r
info depth 4 seldepth 4 multipv 1 score cp 1457 nodes 742 nps 742000 tbhits 0 time 1 pv a7a8r c5c2 b2b1
info depth 5 seldepth 4 multipv 1 score cp 1457 nodes 836 nps 836000 tbhits 0 time 1 pv a7a8r c5c2 b2b1
info depth 6 seldepth 5 multipv 1 score cp 1537 nodes 1856 nps 928000 tbhits 0 time 2 pv a7a8q c5e5 a8g2
info depth 7 seldepth 6 multipv 1 score cp 1591 nodes 2718 nps 1359000 tbhits 0 time 2 pv a7a8q c5e5 b2c3 e5e2
info depth 8 seldepth 9 multipv 1 score cp 1183 nodes 8107 nps 2702333 tbhits 0 time 3 pv a7a8q c5e5 b2c3 e5e3 c3c4 e3e1
info depth 9 seldepth 11 multipv 1 score cp 946 nodes 25403 nps 5080600 tbhits 0 time 5 pv a7a8b c5b5 b6b5 e8b5 b3b4 b5f1
info depth 10 seldepth 11 multipv 1 score cp 875 nodes 45927 nps 5103000 tbhits 0 time 9 pv a7a8b c5b5 b6b5 e8b5 a8e4 b5e8 e4d3 e8c6
info depth 11 seldepth 12 multipv 1 score cp 880 nodes 61799 nps 5149916 tbhits 1 time 12 pv a7a8b c5c8 a8e4 c8d8 b2a3 d8d4 b6b4
info depth 12 seldepth 17 multipv 1 score cp 890 nodes 362237 nps 6355035 tbhits 12 time 57 pv a7a8b c5c8 a8e4 c8d8 e4f5 d8d2 b2a3 d2d8 a3b4 d8a8 b4c3 a8d8 b3b4 e8a4 b6a6 a4d1 f5e4
info depth 13 seldepth 19 multipv 1 score cp 900 nodes 485259 nps 6470120 tbhits 20 time 75 pv a7a8b c5c8 a8e4 e8d7 b6d6 d7b5 e4f5 c8b8 b2c3 b5e2 d6c6 e2d1 b3b4 d1a4 c6c8 b8c8 f5c8
info depth 14 seldepth 25 multipv 1 score cp 900 nodes 710633 nps 6767933 tbhits 71 time 105 pv a7a8b c5b5 b6b5 e8b5 a8e4 b5d3 e4c6 d3e4 c6d7 e4h1 b2c3 h1e4
info depth 15 seldepth 27 multipv 1 score cp 802 nodes 1304605 nps 7090244 tbhits 160 time 184 pv a7a8b c5b5 b6d6 b5b8 a8d5 e8a4 b2c2 a4b3 d5b3 b8c8 c2d2 c8c2 d2e1 c2e2 e1f1 e2e1 f1f2 e1e2 f2f3 e2e3 f3f4 e3f3 f4g5
info depth 16 seldepth 22 multipv 1 score cp 866 nodes 1783033 nps 6884297 tbhits 1598 time 259 pv a7a8b c5b5 b6a6 b5h5 a8e4 h5h4 e4d3 h4h6 a6d6 h6h5 d6d8 h5e5 d3c4 h7h5 b2c3 h5h4 c4b5
info depth 17 seldepth 28 multipv 1 score cp 864 nodes 3892542 nps 6608730 tbhits 4505 time 589 pv b3b4 c5c8 b2b3 c8d8 b3c4 e8d7 c4c5 d8c8 c5d4 c8d8 d4c4 d7e6 c4c5 d8c8 b6c6 c8d8
info depth 18 seldepth 36 multipv 1 score cp 785 nodes 5728245 nps 6803141 tbhits 5205 time 842 pv b6a6 c5c8 b3b4 c8a8 b2c3 e8b5 a6a5 b5c6 b4b5 c6b7 c3b4 a8d8 a5a3 d8c8 b4a5 c8d8 a5b6 b7a8
info depth 19 seldepth 36 multipv 1 score cp 833 nodes 7783457 nps 7082308 hashfull 0 tbhits 5661 time 1099 pv b6a6 c5c8 b3b4 c8a8 b2b3 a8c8 a6a5 e8c6 b4b5 c8a8 b3b4 c6d7 b4c5 d7e8 b5b6 a8c8 c5d6 e8c6 a5a3 c6e4 a3e3 e4c6 e3e7 c6g2 e7c7 c8e8 b6b7 g2b7 c7b7
info depth 20 seldepth 35 multipv 1 score cp 713 nodes 9192626 nps 7232593 hashfull 1 tbhits 6161 time 1271 pv b6a6 c5c8 b3b4 c8d8 b2c3 d8c8 c3d4 c8d8 d4e3 d8c8 e3e4 c8a8 e4e5 a8c8 a6b6 c8d8 e5e4 e8c6 e4f4 c6b7 f4e3 b7c6 b4b5 c6e8
info depth 21 seldepth 37 multipv 1 score cp 758 nodes 13596739 nps 7294387 hashfull 1 tbhits 6634 time 1864 pv b6a6 c5c8 a6b6 c8c5 b6e6 c5c8 e6a6 e8b5 a6d6 c8a8 d6b6 a8c8 b2a3 b5e8 a3b4 c8c6 b6b7 c6e6 b7e7 e8c6 e7c7 e6e4 b4a5 e4e5 a5b6 e5e6 b3b4 c6a8 b6a5
info depth 22 seldepth 34 multipv 1 score cp 685 nodes 18222695 nps 7459146 hashfull 2 tbhits 6872 time 2443 pv b6a6 c5c8 a6a5 e8c6 b2c3 c8a8 c3c4 a8e8 a5c5 e8c8 c4b4 c8d8 c5e5 d8d4 b4c3 d4d8 e5e7 c6b7 b3b4 b7a6 e7d7 d8c8 c3d4 c8c4 d4e5
info depth 23 seldepth 39 multipv 1 score cp 682 nodes 20663342 nps 7494864 hashfull 2 tbhits 7035 time 2757 pv b6a6 c5c8 a6a5 e8c6 b2c3 c8d8 c3c4 d8e8 a5c5 e8c8 c4b4 c6e8 c5a5 e8c6 a5a6 c8d8 a6b6 c6b7 b6b5 b7c6 b5b8 c6e8 b8b7 d8c8 b7b6 c8d8 b6b8 d8d4 b4a3
info depth 24 seldepth 41 multipv 1 score cp 676 nodes 22615614 nps 7558694 hashfull 2 tbhits 7180 time 2992 pv b6a6 c5c8 a6a5 e8c6 b2c3 c8d8 c3c4 d8e8 a5c5 e8c8 c4b4 c8d8 c5e5 d8d4 b4c3 d4d8 e5e7 d8c8 c3d4 c6a4 b3b4 a4b5 e7c7 b5e8 c7b7 e8c6 b7b8 c6e8 d4e4 c8c4 e4d5 c4d4 d5c5
info depth 25 seldepth 51 multipv 1 score cp 675 nodes 24792381 nps 7619047 hashfull 2 tbhits 7249 time 3254 pv b6a6 c5c8 b2a3 c8d8 a3b4 d8a8 a6a5 e8a4 a5c5 a4e8 c5c7 a8d8 c7b7 e8c6 b7b8 c6e8 b4a3 d8c8 a7a8q c8b8 a8d5 e8c6 d5e5 b8a8 a3b2 a8c8 e5d4 c6d5 b3b4 d5c6 d4b6
info depth 26 seldepth 46 multipv 1 score cp 1014 nodes 34254661 nps 7285125 hashfull 3 tbhits 16155 time 4702 pv a7a8b c5c8 a8d5 c8d8 d5c4 d8d7 b2c3 d7c7 c3d4 c7d7 d4e5 d7c7 c4d5 c7c8 e5f4 c8d8 d5c4 d8d4 f4e3 d4g4 b6b8 g4g3 e3d4
info depth 27 seldepth 42 multipv 1 score cp 862 nodes 63200029 nps 6666669 hashfull 10 tbhits 69162 time 9480 pv a7a8b c5c8 a8e4 e8c6 e4f5 c6d7 f5d3 d7c6 b3b4 c6e4 d3a6 e4b7 a6e2 b7e4 b6d6 e4c6 e2d3 c6e4 d3a6 e4b7 a6b5 b7e4 d6d4 e4d3 b5d7 c8a8 d7g4 d3b5 d4d6 a8e8 g4f5 e8b8 b2c3 b8e8 c3d4 b5a4 d6d5
info depth 28 seldepth 54 multipv 1 score cp 675 nodes 105645317 nps 6250092 hashfull 16 tbhits 84032 time 16903 pv b6a6 c5c8 a6b6 c8d8 b2a3 d8a8 b6b7 a8d8 b7b8 d8c8 a7a8q c8b8 a8d5 e8c6 d5d6 b8c8 a3b2 c6b5 d6b6 b5c6 b6a5 c6d7 a5a6 d7e8 a6b7 c8a8 b3b4 a8b8 b7e7 e8c6 b2c3
info depth 29 seldepth 43 multipv 1 score cp 782 nodes 114930804 nps 6189724 hashfull 17 tbhits 88676 time 18568 pv a7a8b c5c8 a8e4 e8c6 e4f5 c6d7 f5d3 d7c6 b3b4 c6e4 d3c4 e4c6 c4e2 c8a8 e2d1 c6f3 d1b3 f3d5 b3c2 d5e4 c2a4 e4d5 b6d6 d5e4 d6d1 e4c6 a4c2 c6a4 c2a4 a8c8 a4c6 c8b8 b2c3 b8e8
info depth 30 seldepth 48 multipv 1 score cp 783 nodes 124534910 nps 6147443 hashfull 18 tbhits 91717 time 20258 pv a7a8b c5c8 a8e4 e8c6 e4f5 c6d7 f5d3 d7c6 b3b4 c6e4 d3c4 e4c6 c4b3 c6d5 b3a4 c8a8 a4d1 d5c6 d1b3 c6d5 b3a4 d5f3 b6d6 f3e2 d6d5 a8b8 b2c3 e2b5 a4c2 b8c8 c3b2 b5d3 c2d1 c8b8 b2c3 b8c8 c3d4 c8c4 d4e3 c4e4 e3d2
info depth 31 seldepth 59 multipv 1 score cp 1445 nodes 272128206 nps 5259227 hashfull 40 tbhits 516083 time 51743 pv a7a8b c5c8 a8e4 c8d8 b3b4 d8d2 b2c3 d2d8 b6b7 d8c8 c3b2 c8d8 b7e7 d8b8 b2c3 b8c8 c3d4 c8d8 d4e5 e8a4 e4h7 g8h7 e7f7 a4b3 f7f8 d8d5 e5e4 d5b5 f8h8 h7g6 g7g8q b3g8 h8g8 g6f7 h6h7 f7e6
info depth 32 seldepth 51 multipv 1 score cp 2095 nodes 807532745 nps 4311186 hashfull 132 tbhits 2859727 time 187311 pv a7a8b c5c8 a8e4 e8c6 e4c6 c8a8 c6a4 a8e8 b2a3 e8a8 b6e6 f7e6 b3b4 g8f7 a3b3 a8c8 a4d7 c8b8 d7c6 f7g6 c6e4 g6h6 b3a4 b8c8 b4b5 c8b8 b5b6 b8c8 b6b7 c8d8 a4b5 d8b8 b5b6 b8d8 b6c7 d8e8 c7d6 e8b8 d6e6 b8d8 e6e7 d8d4 g7g8q d4e4 e7f7 e4b4 g8f8 h6h5 f8b4
info depth 33 seldepth 60 multipv 1 score cp 4965 nodes 10563920002 nps 4585673 hashfull 960 tbhits 43145123 time 2303679 pv a7a8b c5c8 a8e4 e8d7 e4d5 d7c6 d5c6 c8a8 c6a4 a8e8 b2a3 e8a8 b6e6 f7e6 b3b4 g8f7 a3b3 a8c8 a4d7 c8b8 d7c6 f7g6 c6e4 g6h6 b3a4 b8c8 b4b5 c8d8 b5b6
info depth 34 seldepth 35 multipv 1 score cp 4965 nodes 10566284817 nps 4585593 hashfull 960 tbhits 43169760 time 2304235 pv a7a8b c5c8 a8e4 e8d7 e4d5 d7c6 d5c6 c8a8 c6a4 a8e8 b2a3 e8a8 b6e6 f7e6 b3b4 g8f7 a3b3 a8c8 a4d7 c8b8 d7c6 f7g6 c6e4 g6h6 b3a4 b8c8 b4b5 c8d8 b5b6 d8c8 b6b7 c8d8 a4b5 d8b8 b5b6
info depth 35 seldepth 36 multipv 1 score cp 4965 nodes 10569460815 nps 4585497 hashfull 960 tbhits 43208178 time 2304976 pv a7a8b c5c8 a8e4 e8d7 e4d5 d7c6 d5c6 c8a8 c6a4 a8e8 b2a3 e8a8 b6e6 f7e6 b3b4 g8f7 a3b3 a8c8 a4d7 c8b8 d7c6 f7g6 c6e4 g6h6 b3a4 b8c8 b4b5 c8d8 b5b6 d8c8 b6b7 c8d8 a4b4 d8d4 b4a5 d4d8

Edit:
If white decided to promote a pawn to a queen, white will lose the queen.

1. a8=Q Rc2+ 2. Ka1 Rc1+ 3. Kb2 Rc2+ 4. Ka1 Rc1+ 5. Ka2 Rc2+ 6. Ka3 Ra2+ 7. Kb4 Rxa8 *

[RogerThiede]

This is a very intriguing position for promotions. Thank you for taking the time to locate this position.

[vondele]

I believe we have tried to search under promotion lines less deep, but that didn't lead to an Elo gain. I believe the search already deals with this efficiently enough. However, this is not an issue, feel free to test a patch on fishtest that explores these ideas.