refactor: use C++20 function std::popcount (#4389)

- Replace custom popcnt16 implementation with std::popcount from C++20
- Maintain compatibility with older compilers and MacOS by providing a
  conditional compilation fallback to __builtin_popcount and a lookup
  table method
- Move and inline related functions within SHAMapInnerNode for
  performance and readability

Signed-off-by: Manoj Doshi <mdoshi@ripple.com>

src/ripple/shamap/SHAMapInnerNode.h

@@ -28,6 +28,7 @@
 #include <ripple/shamap/impl/TaggedPointer.h>
 
 #include <atomic>
+#include <bit>
 #include <bitset>
 #include <cstdint>
 #include <limits>
@@ -194,12 +195,24 @@ public:
     makeCompressedInner(Slice data);
 };
 
+inline bool
+SHAMapInnerNode::isEmpty() const
+{
+    return isBranch_ == 0;
+}
+
 inline bool
 SHAMapInnerNode::isEmptyBranch(int m) const
 {
     return (isBranch_ & (1 << m)) == 0;
 }
 
+inline int
+SHAMapInnerNode::getBranchCount() const
+{
+    return popcnt16(isBranch_);
+}
+
 inline bool
 SHAMapInnerNode::isFullBelow(std::uint32_t generation) const
 {

src/ripple/shamap/impl/SHAMapInnerNode.cpp

@@ -256,18 +256,6 @@ SHAMapInnerNode::serializeWithPrefix(Serializer& s) const
         [&](SHAMapHash const& hh) { s.addBitString(hh.as_uint256()); });
 }
 
-bool
-SHAMapInnerNode::isEmpty() const
-{
-    return isBranch_ == 0;
-}
-
-int
-SHAMapInnerNode::getBranchCount() const
-{
-    return popcnt16(isBranch_);
-}
-
 std::string
 SHAMapInnerNode::getString(const SHAMapNodeID& id) const
 {
@@ -292,9 +280,9 @@ SHAMapInnerNode::setChild(int m, std::shared_ptr<SHAMapTreeNode> child)
 
     auto const dstIsBranch = [&] {
         if (child)
-            return isBranch_ | (1 << m);
+            return isBranch_ | (1u << m);
         else
-            return isBranch_ & ~(1 << m);
+            return isBranch_ & ~(1u << m);
     }();
 
     auto const dstToAllocate = popcnt16(dstIsBranch);

src/ripple/shamap/impl/TaggedPointer.h

@@ -22,6 +22,8 @@
 
 #include <ripple/shamap/SHAMapTreeNode.h>
 
+#include <array>
+#include <bit>
 #include <cstdint>
 #include <optional>
 
@@ -217,6 +219,31 @@ public:
     getChildIndex(std::uint16_t isBranch, int i) const;
 };
 
+[[nodiscard]] inline int
+popcnt16(std::uint16_t a)
+{
+#if __cpp_lib_bitops
+    return std::popcount(a);
+#elif defined(__clang__) || defined(__GNUC__)
+    return __builtin_popcount(a);
+#else
+    // fallback to table lookup
+    static auto constexpr const tbl = []() {
+        std::array<std::uint8_t, 256> ret{};
+        for (int i = 0; i != 256; ++i)
+        {
+            for (int j = 0; j != 8; ++j)
+            {
+                if (i & (1 << j))
+                    ret[i]++;
+            }
+        }
+        return ret;
+    }();
+    return tbl[a & 0xff] + tbl[a >> 8];
+#endif
+}
+
 }  // namespace ripple
 
 #endif

src/ripple/shamap/impl/TaggedPointer.ipp

@@ -22,6 +22,7 @@
 #include <ripple/shamap/impl/TaggedPointer.h>
 
 #include <array>
+#include <bit>
 
 #include <boost/pool/pool_alloc.hpp>
 
@@ -159,29 +160,6 @@ deallocateArrays(std::uint8_t boundaryIndex, void* p)
     freeArrayFuns[boundaryIndex](p);
 }
 
-[[nodiscard]] inline int
-popcnt16(std::uint16_t a)
-{
-#if defined(__clang__) || defined(__GNUC__)
-    return __builtin_popcount(a);
-#else
-    // fallback to table lookup
-    static auto constexpr const tbl = []() {
-        std::array<std::uint8_t, 256> ret{};
-        for (int i = 0; i != 256; ++i)
-        {
-            for (int j = 0; j != 8; ++j)
-            {
-                if (i & (1 << j))
-                    ret[i]++;
-            }
-        }
-        return ret;
-    }();
-    return tbl[a & 0xff] + tbl[a >> 8];
-#endif
-}
-
 // Used in `iterChildren` and elsewhere as the hash value for sparse arrays when
 // the hash isn't actually stored in the array.
 static SHAMapHash const zeroSHAMapHash;
@@ -285,7 +263,7 @@ TaggedPointer::getChildIndex(std::uint16_t isBranch, int i) const
 
     // mask sets all the bits >=i to zero and all the bits <i to
     // one.
-    auto const mask = (1 << i) - 1;
+    auto const mask = (1u << i) - 1;
     return popcnt16(isBranch & mask);
 }