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Align ponder move and final PV output

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In master for various reasons the given ponder move may get out of sync
with (the second move of) the last PV sent to the GUI:
1. If a ponder move is extracted from the TT despite bestmove leading to
a game ending draw.
2. If a ponder move is extracted from the TT from a fail high/low, since
after the ponder move extraction no new PV is sent to the GUI.
3. If `syzygy_extend_pv()` from within the call to `pv()` changes the PV
after the ponder move has been selected.
4. If the PV roll-back to protect mated-in scores differs in the second
move from the final PV sent to the GUI.

This PR keeps ponder move and the last printed PV in sync.

Fixes #6676.

closes https://github.com/official-stockfish/Stockfish/pull/6679

No functional change
This commit is contained in:
Robert Nurnberg @ elitebook
2026-04-02 20:30:12 +02:00
committed by Disservin
parent d173a0655d
commit b9f3273af1
1 changed files with 25 additions and 28 deletions
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src/search.cpp
+25 -28
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@@ -239,14 +239,18 @@ void Search::Worker::start_searching() {
main_manager()->bestPreviousScore = bestThread->rootMoves[0].score;
main_manager()->bestPreviousAverageScore = bestThread->rootMoves[0].averageScore;
// Send again PV info if we have a new best thread
if (bestThread != this)
std::string ponder;
bool extractedPonder = false;
if (bestThread->rootMoves[0].pv.size() == 1)
extractedPonder = bestThread->rootMoves[0].extract_ponder_from_tt(tt, rootPos);
// Send again PV info if we have a new best thread or extracted a ponder move.
if (bestThread != this || extractedPonder)
main_manager()->pv(*bestThread, threads, tt, bestThread->completedDepth);
std::string ponder;
if (bestThread->rootMoves[0].pv.size() > 1
|| bestThread->rootMoves[0].extract_ponder_from_tt(tt, rootPos))
// In rare cases, pv() may change the ponder move through syzygy_extend_pv().
if (bestThread->rootMoves[0].pv.size() > 1)
ponder = UCIEngine::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
auto bestmove = UCIEngine::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
@@ -262,9 +266,7 @@ void Search::Worker::iterative_deepening() {
Move pv[MAX_PLY + 1];
Depth lastBestMoveDepth = 0;
Value lastBestScore = -VALUE_INFINITE;
std::vector<Move> lastBestPV;
Depth lastBestMoveDepth = 0;
Value alpha, beta;
Value bestValue = -VALUE_INFINITE;
@@ -428,8 +430,7 @@ void Search::Worker::iterative_deepening() {
// PV that cannot be trusted, i.e. it can be delayed or refuted if we
// would have had time to fully search other root-moves. Thus here we
// suppress any exact mated-in/TB loss output and, if we do, below pick
// the score/PV from the previously completed iteration with the most
// recent bestmove change.
// the score/PV from the previous iteration.
&& !(threads.stop && is_loss(rootMoves[0].uciScore)
&& rootMoves[0].score == rootMoves[0].uciScore))
main_manager()->pv(*this, threads, tt, rootDepth);
@@ -440,7 +441,11 @@ void Search::Worker::iterative_deepening() {
if (!threads.stop)
{
completedDepth = rootDepth;
completedDepth = rootDepth;
if (lastIterationPV.empty() || rootMoves[0].pv[0] != lastIterationPV[0])
lastBestMoveDepth = rootDepth;
lastIterationPV = rootMoves[0].pv;
}
@@ -451,14 +456,12 @@ void Search::Worker::iterative_deepening() {
{
// Bring the last best move to the front for best thread selection.
// For an aborted d1 search we label the loss score as inexact.
if (!lastBestPV.empty())
if (!lastIterationPV.empty())
{
Utility::move_to_front(rootMoves,
[&lastBestPV = std::as_const(lastBestPV)](const auto& rm) {
return rm == lastBestPV[0];
});
rootMoves[0].pv = lastBestPV;
rootMoves[0].score = rootMoves[0].uciScore = lastBestScore;
Utility::move_to_front(rootMoves, [&lastPV = std::as_const(lastIterationPV)](
const auto& rm) { return rm == lastPV[0]; });
rootMoves[0].pv = lastIterationPV;
rootMoves[0].score = rootMoves[0].uciScore = rootMoves[0].previousScore;
}
else
{
@@ -468,12 +471,6 @@ void Search::Worker::iterative_deepening() {
main_manager()->pv(*this, threads, tt, rootDepth);
}
}
else if (lastBestPV.empty() || rootMoves[0].pv[0] != lastBestPV[0])
{
lastBestPV = rootMoves[0].pv;
lastBestScore = rootMoves[0].score;
lastBestMoveDepth = rootDepth;
}
// Have we found a "mate in x" after a completed iteration?
if (limits.mate && !threads.stop
@@ -2223,10 +2220,10 @@ bool RootMove::extract_ponder_from_tt(const TranspositionTable& tt, Position& po
pos.do_move(pv[0], st, &tt);
auto [ttHit, ttData, ttWriter] = tt.probe(pos.key());
if (ttHit)
if (!pos.is_draw(1))
{
if (MoveList<LEGAL>(pos).contains(ttData.move))
auto [ttHit, ttData, ttWriter] = tt.probe(pos.key());
if (ttHit && MoveList<LEGAL>(pos).contains(ttData.move))
pv.push_back(ttData.move);
}