Ensure ttValue != VALUE_NONE in singular extension search

The assert:

  assert(ttValue != VALUE_NONE);

Could fire for multiple reasons (although is very rare),
for instance after an IID we can have ttMove != MOVE_NONE
while ttValue is still set at VALUE_NONE.

But not only this, actually SMP is a source of corrupted
ttValue and anyhow we can detect the condition:

 ttMove != MOVE_NONE && ttValue == VALUE_NONE

even north of IID.

Reported by Ronald de Man.

It is so rare that bench didn't change.

bench: 7710548

diff --git a/src/search.cpp b/src/search.cpp
index acd042c0a8ceb99a5f0fdaca232778c19a722718..24fde88fc0ef5c1087cb43bdfd41a87d81c35f7f 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -700,6 +700,7 @@ moves_loop: // When in check and at SpNode search starts from here
                            &&  depth >= 8 * ONE_PLY
                            &&  abs(beta) < VALUE_KNOWN_WIN
                            &&  ttMove != MOVE_NONE
                            &&  depth >= 8 * ONE_PLY
                            &&  abs(beta) < VALUE_KNOWN_WIN
                            &&  ttMove != MOVE_NONE

+                           &&  ttValue != VALUE_NONE

                            && !excludedMove // Recursive singular search is not allowed
                            && (tte->bound() & BOUND_LOWER)
                            &&  tte->depth() >= depth - 3 * ONE_PLY;
                            && !excludedMove // Recursive singular search is not allowed
                            && (tte->bound() & BOUND_LOWER)
                            &&  tte->depth() >= depth - 3 * ONE_PLY;


@@ -766,8 +767,6 @@ moves_loop: // When in check and at SpNode search starts from here
           && !ext
           &&  pos.legal(move, ci.pinned))
       {
           && !ext
           &&  pos.legal(move, ci.pinned))
       {

-          assert(ttValue != VALUE_NONE);
-

           Value rBeta = ttValue - int(depth);
           ss->excludedMove = move;
           ss->skipNullMove = true;
           Value rBeta = ttValue - int(depth);
           ss->excludedMove = move;
           ss->skipNullMove = true;