this patch fixes a rare but reproducible segfault observed playing a
multi-threaded match, it is discussed somewhat in fishcooking.
From the core file, it could be observed that the issue was in qsearch, namely:
````
ss->pv[0] = MOVE_NONE;
````
and the backtrace shows the it arrives there via razoring, called from the rootNode:
````
(gdb) bt
alpha=-19, beta=682, depth=DEPTH_ZERO) at search.cpp:1247
````
Indeed, ss->pv can indeed by a nullptr at the rootNode. However, why is the
segfault so rare ?
The reason is that the condition that guards razoring:
````
(depth < 2 * ONE_PLY && eval <= alpha - RazorMargin)
````
is almost never true, since at the root alpha for depth < 5 is -VALUE_INFINITE.
Nevertheless with the new failHigh scheme, this is not guaranteed, and rootDepth > 5,
can still result in a depth < 2 search at the rootNode. If now another thread,
via the hash, writes a new low eval to the rootPos qsearch can be entered.
Rare but not unseen... I assume that some of the crashes in fishtest recently
might be due to this.
Closes https://github.com/official-stockfish/Stockfish/pull/1860
No functional change
void Thread::search() {
Stack stack[MAX_PLY+7], *ss = stack+4; // To reference from (ss-4) to (ss+2)
void Thread::search() {
Stack stack[MAX_PLY+7], *ss = stack+4; // To reference from (ss-4) to (ss+2)
Value bestValue, alpha, beta, delta;
Move lastBestMove = MOVE_NONE;
Depth lastBestMoveDepth = DEPTH_ZERO;
Value bestValue, alpha, beta, delta;
Move lastBestMove = MOVE_NONE;
Depth lastBestMoveDepth = DEPTH_ZERO;
std::memset(ss-4, 0, 7 * sizeof(Stack));
for (int i = 4; i > 0; i--)
(ss-i)->continuationHistory = &this->continuationHistory[NO_PIECE][0]; // Use as sentinel
std::memset(ss-4, 0, 7 * sizeof(Stack));
for (int i = 4; i > 0; i--)
(ss-i)->continuationHistory = &this->continuationHistory[NO_PIECE][0]; // Use as sentinel
bestValue = delta = alpha = -VALUE_INFINITE;
beta = VALUE_INFINITE;
bestValue = delta = alpha = -VALUE_INFINITE;
beta = VALUE_INFINITE;
}
// Step 7. Razoring (~2 Elo)
}
// Step 7. Razoring (~2 Elo)
- if ( depth < 2 * ONE_PLY
- && eval <= alpha - RazorMargin)
+ if ( !rootNode // The required rootNode PV handling is not available in qsearch
+ && depth < 2 * ONE_PLY
+ && eval <= alpha - RazorMargin)
return qsearch<NT>(pos, ss, alpha, beta);
improving = ss->staticEval >= (ss-2)->staticEval
return qsearch<NT>(pos, ss, alpha, beta);
improving = ss->staticEval >= (ss-2)->staticEval