Refactor global variables

This aims to remove some of the annoying global structure which Stockfish has.

Overall there is no major elo regression to be expected.

Non regression SMP STC (paused, early version):
https://tests.stockfishchess.org/tests/view/65983d7979aa8af82b9608f1
LLR: 0.23 (-2.94,2.94) <-1.75,0.25>
Total: 76232 W: 19035 L: 19096 D: 38101
Ptnml(0-2): 92, 8735, 20515, 8690, 84

Non regression STC (early version):
https://tests.stockfishchess.org/tests/view/6595b3a479aa8af82b95da7f
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 185344 W: 47027 L: 46972 D: 91345
Ptnml(0-2): 571, 21285, 48943, 21264, 609

Non regression SMP STC:
https://tests.stockfishchess.org/tests/view/65a0715c79aa8af82b96b7e4
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 142936 W: 35761 L: 35662 D: 71513
Ptnml(0-2): 209, 16400, 38135, 16531, 193

These global structures/variables add hidden dependencies and allow data
to be mutable from where it shouldn't it be (i.e. options). They also
prevent Stockfish from internal selfplay, which would be a nice thing to
be able to do, i.e. instantiate two Stockfish instances and let them
play against each other. It will also allow us to make Stockfish a
library, which can be easier used on other platforms.

For consistency with the old search code, `thisThread` has been kept,
even though it is not strictly necessary anymore. This the first major
refactor of this kind (in recent time), and future changes are required,
to achieve the previously described goals. This includes cleaning up the
dependencies, transforming the network to be self contained and coming
up with a plan to deal with proper tablebase memory management (see
comments for more information on this).

The removal of these global structures has been discussed in parts with
Vondele and Sopel.

closes https://github.com/official-stockfish/Stockfish/pull/4968

No functional change

src/tt.cpp

@@ -26,16 +26,13 @@
 #include <vector>
 
 #include "misc.h"
-#include "thread.h"
-#include "uci.h"
 
 namespace Stockfish {
 
-TranspositionTable TT;  // Our global transposition table
-
 // Populates the TTEntry with a new node's data, possibly
 // overwriting an old position. The update is not atomic and can be racy.
-void TTEntry::save(Key k, Value v, bool pv, Bound b, Depth d, Move m, Value ev) {
+void TTEntry::save(
+  Key k, Value v, bool pv, Bound b, Depth d, Move m, Value ev, uint8_t generation8) {
 
     // Preserve any existing move for the same position
     if (m || uint16_t(k) != key16)
@@ -49,7 +46,7 @@ void TTEntry::save(Key k, Value v, bool pv, Bound b, Depth d, Move m, Value ev)
 
         key16     = uint16_t(k);
         depth8    = uint8_t(d - DEPTH_OFFSET);
-        genBound8 = uint8_t(TT.generation8 | uint8_t(pv) << 2 | b);
+        genBound8 = uint8_t(generation8 | uint8_t(pv) << 2 | b);
         value16   = int16_t(v);
         eval16    = int16_t(ev);
     }
@@ -59,10 +56,7 @@ void TTEntry::save(Key k, Value v, bool pv, Bound b, Depth d, Move m, Value ev)
 // Sets the size of the transposition table,
 // measured in megabytes. Transposition table consists of a power of 2 number
 // of clusters and each cluster consists of ClusterSize number of TTEntry.
-void TranspositionTable::resize(size_t mbSize) {
-
-    Threads.main()->wait_for_search_finished();
-
+void TranspositionTable::resize(size_t mbSize, int threadCount) {
     aligned_large_pages_free(table);
 
     clusterCount = mbSize * 1024 * 1024 / sizeof(Cluster);
@@ -74,28 +68,25 @@ void TranspositionTable::resize(size_t mbSize) {
         exit(EXIT_FAILURE);
     }
 
-    clear();
+    clear(threadCount);
 }
 
 
 // Initializes the entire transposition table to zero,
 // in a multi-threaded way.
-void TranspositionTable::clear() {
-
+void TranspositionTable::clear(size_t threadCount) {
     std::vector<std::thread> threads;
 
-    for (size_t idx = 0; idx < size_t(Options["Threads"]); ++idx)
+    for (size_t idx = 0; idx < size_t(threadCount); ++idx)
     {
-        threads.emplace_back([this, idx]() {
+        threads.emplace_back([this, idx, threadCount]() {
             // Thread binding gives faster search on systems with a first-touch policy
-            if (Options["Threads"] > 8)
+            if (threadCount > 8)
                 WinProcGroup::bindThisThread(idx);
 
             // Each thread will zero its part of the hash table
-            const size_t stride = size_t(clusterCount / Options["Threads"]),
-                         start  = size_t(stride * idx),
-                         len =
-                           idx != size_t(Options["Threads"]) - 1 ? stride : clusterCount - start;
+            const size_t stride = size_t(clusterCount / threadCount), start = size_t(stride * idx),
+                         len = idx != size_t(threadCount) - 1 ? stride : clusterCount - start;
 
             std::memset(&table[start], 0, len * sizeof(Cluster));
         });