Use stable sort to make sure bench with TB yields same results everywhere.

mstembera · Fanael · commit 289fbcc9a1db · 2021-03-07T14:29:34.000+01:00
std::sort() is not stable so different implementations can produce different results: use the stable version instead. Observed for '8/6k1/5r2/8/8/8/1K6/Q7 w - - 0 1' yielding different bench results for gcc and MSVC and 3-4-5 syzygy TB prior to this patch. closes official-stockfish#3083 No functional change.
diff --git a/src/search.cpp b/src/search.cpp
@@ -1939,7 +1939,7 @@ void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
     if (RootInTB)
     {
         // Sort moves according to TB rank
-        std::sort(rootMoves.begin(), rootMoves.end(),
+        std::stable_sort(rootMoves.begin(), rootMoves.end(),
                   [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
 
         // Probe during search only if DTZ is not available and we are winning
diff --git a/src/syzygy/tbprobe.cpp b/src/syzygy/tbprobe.cpp
@@ -759,7 +759,7 @@ Ret do_probe_table(const Position& pos, T* entry, WDLScore wdl, ProbeState* resu
     if (entry->hasPawns) {
         idx = LeadPawnIdx[leadPawnsCnt][squares[0]];
 
-        std::sort(squares + 1, squares + leadPawnsCnt, pawns_comp);
+        std::stable_sort(squares + 1, squares + leadPawnsCnt, pawns_comp);
 
         for (int i = 1; i < leadPawnsCnt; ++i)
             idx += Binomial[i][MapPawns[squares[i]]];
@@ -860,7 +860,7 @@ Ret do_probe_table(const Position& pos, T* entry, WDLScore wdl, ProbeState* resu
 
     while (d->groupLen[++next])
     {
-        std::sort(groupSq, groupSq + d->groupLen[next]);
+        std::stable_sort(groupSq, groupSq + d->groupLen[next]);
         uint64_t n = 0;
 
         // Map down a square if "comes later" than a square in the previous

Original file line number	Diff line number	Diff line change
`@@ -1939,7 +1939,7 @@ void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {`
`1939`	`1939`	`if (RootInTB)`
`1940`	`1940`	`{`
`1941`	`1941`	`// Sort moves according to TB rank`
`1942`		`- std::sort(rootMoves.begin(), rootMoves.end(),`
	`1942`	`+ std::stable_sort(rootMoves.begin(), rootMoves.end(),`
`1943`	`1943`	`[](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );`
`1944`	`1944`
`1945`	`1945`	`// Probe during search only if DTZ is not available and we are winning`