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Remove goto, limit skipping to NMP

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This patch simplifies the control flow in search(), removing an if
and a goto. A side effect of the patch is that Stockfish is now a
little bit more selective at low depths, because we allow razoring,
futility pruning and probcut pruning after a null move.

passed STC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 32035 W: 6523 L: 6422 D: 19090
http://tests.stockfishchess.org/tests/view/5af142ca0ebc597fb3d39bb6

passed LTC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 41431 W: 6187 L: 6097 D: 29147
http://tests.stockfishchess.org/tests/view/5af148770ebc597fb3d39bc1

Ideas for further work:

• Use the nodes credit opened by the patch (the increased selectivity)
  to try somewhat higher razoring, futility or probcut margins at [0..4].

Bench: 4855031
This commit is contained in:
Joost VandeVondele 2018-05-09 01:40:32 +02:00 • committed by Stéphane Nicolet
parent de0889612f
commit 4d647428d8
1 changed files with 11 additions and 12 deletions
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23
src/search.cpp
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@ -682,12 +682,12 @@ namespace {
}
}
// Step 6. Evaluate the position statically
// Step 6. Static evaluation of the position
if (inCheck)
{
ss->staticEval = eval = VALUE_NONE;
improving = false;
goto moves_loop;
goto moves_loop; // Skip early pruning when in check
}
else if (ttHit)
{
@ -710,13 +710,7 @@ namespace {
ss->staticEval, TT.generation());
}
improving = ss->staticEval >= (ss-2)->staticEval
||(ss-2)->staticEval == VALUE_NONE;
if (ss->excludedMove || !pos.non_pawn_material(pos.side_to_move()))
goto moves_loop;
// Step 7. Razoring (skipped when in check, ~2 Elo)
// Step 7. Razoring (~2 Elo)
if ( !PvNode
&& depth < 3 * ONE_PLY
&& eval <= alpha - RazorMargin[depth / ONE_PLY])
@ -727,7 +721,10 @@ namespace {
return v;
}
// Step 8. Futility pruning: child node (skipped when in check, ~30 Elo)
improving = ss->staticEval >= (ss-2)->staticEval
|| (ss-2)->staticEval == VALUE_NONE;
// Step 8. Futility pruning: child node (~30 Elo)
if ( !rootNode
&& depth < 7 * ONE_PLY
&& eval - futility_margin(depth, improving) >= beta
@ -740,6 +737,8 @@ namespace {
&& (ss-1)->statScore < 30000
&& eval >= beta
&& ss->staticEval >= beta - 36 * depth / ONE_PLY + 225
&& !ss->excludedMove
&& pos.non_pawn_material(pos.side_to_move())
&& (ss->ply >= thisThread->nmp_ply || ss->ply % 2 != thisThread->nmp_odd))
{
assert(eval - beta >= 0);
@ -779,7 +778,7 @@ namespace {
}
}
// Step 10. ProbCut (skipped when in check, ~10 Elo)
// Step 10. ProbCut (~10 Elo)
// If we have a good enough capture and a reduced search returns a value
// much above beta, we can (almost) safely prune the previous move.
if ( !PvNode
@ -817,7 +816,7 @@ namespace {
}
}
// Step 11. Internal iterative deepening (skipped when in check, ~2 Elo)
// Step 11. Internal iterative deepening (~2 Elo)
if ( depth >= 8 * ONE_PLY
&& !ttMove)
{