Simplify bitboards

This PR:
- retires the unused distance() functions in bitboard.h including the 4K SquareDistance LUT,
- makes the 64K PopCnt16 LUT constexpr and only compiles it in if we don't use  a popcount intrinsic (we can't fully remove the LUT because it's faster than the intrinsic in the absence of a popcount machine instruction),
- retires Bitboards::init().

Passed STC:
https://tests.stockfishchess.org/tests/view/6a48e6d3f97ff95f78795a8f
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 162016 W: 42054 L: 41976 D: 77986
Ptnml(0-2): 383, 17375, 45454, 17373, 423

I didn't run LTC because the changes only really impact workers without popcount and there aren't any present on fishtest ATM. The correctness of the constexpr PopCnt16 LUT was verified locally with a x86-64 build; the build yields the same signature as master.

closes https://github.com/official-stockfish/Stockfish/pull/6952

No functional change

src/bitboard.cpp

@@ -18,12 +18,8 @@
 
 #include "bitboard.h"
 
-#include <bitset>
-
 namespace Stockfish {
 
-u8 PopCnt16[1 << 16];
-
 // Returns an ASCII representation of a bitboard suitable
 // to be printed to standard output. Useful for debugging.
 std::string Bitboards::pretty(Bitboard b) {
@@ -45,10 +41,4 @@ std::string Bitboards::pretty(Bitboard b) {
     return s;
 }
 
-// Initializes the popcount table at startup.
-void Bitboards::init() {
-    for (unsigned i = 0; i < (1 << 16); ++i)
-        PopCnt16[i] = u8(std::bitset<16>(i).count());
-}
-
 }  // namespace Stockfish

src/bitboard.h

@@ -21,8 +21,8 @@
 
 #include <algorithm>
 #include <cassert>
-#include <cstring>
 #include <string>
+#include <type_traits>
 
 #include "types.h"
 #include "misc.h"
@@ -31,7 +31,6 @@ namespace Stockfish {
 
 namespace Bitboards {
 
-void        init();
 std::string pretty(Bitboard b);
 
 }  // namespace Stockfish::Bitboards
@@ -63,8 +62,6 @@ constexpr Bitboard Rank6BB = Rank1BB << (8 * 5);
 constexpr Bitboard Rank7BB = Rank1BB << (8 * 6);
 constexpr Bitboard Rank8BB = Rank1BB << (8 * 7);
 
-extern u8 PopCnt16[1 << 16];
-
 constexpr Bitboard square_bb(Square s) {
     assert(is_ok(s));
     return 1ULL << s;
@@ -130,26 +127,39 @@ constexpr Bitboard pawn_single_push_bb(Color c, Bitboard b) {
     return c == WHITE ? shift<NORTH>(b) : shift<SOUTH>(b);
 }
 
-inline int edge_distance(File f) { return std::min(f, File(FILE_H - f)); }
+constexpr int edge_distance(File f) { return std::min(f, File(FILE_H - f)); }
 
-constexpr int constexpr_popcount(Bitboard b) {
-    b = b - ((b >> 1) & 0x5555555555555555ULL);
-    b = (b & 0x3333333333333333ULL) + ((b >> 2) & 0x3333333333333333ULL);
-    b = (b + (b >> 4)) & 0x0F0F0F0F0F0F0F0FULL;
-    return static_cast<int>((b * 0x0101010101010101ULL) >> 56);
+
+template<typename T>
+constexpr int constexpr_popcount(T v) {
+    static_assert(std::is_integral_v<T>, "constexpr_popcount is undefined for non-integral types");
+
+    if constexpr (sizeof(T) <= 8)
+    {
+        u64 b = static_cast<std::make_unsigned_t<T>>(v);
+
+        b = b - ((b >> 1) & 0x5555555555555555ULL);
+        b = (b & 0x3333333333333333ULL) + ((b >> 2) & 0x3333333333333333ULL);
+        b = (b + (b >> 4)) & 0x0F0F0F0F0F0F0F0FULL;
+
+        return static_cast<int>((b * 0x0101010101010101ULL) >> 56);
+    }
+    else
+    {
+        int result = 0;
+
+        for (; v; v >>= static_cast<T>(1))
+            if (v & static_cast<T>(1))
+                ++result;
+
+        return result;
+    }
 }
 
 // Counts the number of non-zero bits in a bitboard.
 inline int popcount(Bitboard b) {
 
-#ifndef USE_POPCNT
-
-    u16 indices[4];
-    std::memcpy(indices, &b, sizeof(b));
-    return PopCnt16[indices[0]] + PopCnt16[indices[1]] + PopCnt16[indices[2]]
-         + PopCnt16[indices[3]];
-
-#elif defined(_MSC_VER)
+#ifdef _MSC_VER
 
     return int(_mm_popcnt_u64(b));
 

src/main.cpp

@@ -21,7 +21,6 @@
 #include <utility>
 
 #include "attacks.h"
-#include "bitboard.h"
 #include "misc.h"
 #include "position.h"
 #include "tune.h"
@@ -40,7 +39,6 @@ __attribute__((used)) // keep main alive
 int main(int argc, char* argv[]) {
     std::cout << engine_info() << std::endl;
 
-    Bitboards::init();
     Attacks::init();
     Position::init();
 

src/types.h

@@ -76,7 +76,7 @@
         #define IS_64BIT
     #endif
 
-    #if defined(USE_POPCNT) && defined(_MSC_VER)
+    #if defined(_MSC_VER)
         #include <nmmintrin.h>  // Microsoft header for _mm_popcnt_u64()
     #endif