From 11ab4cde071be92d4fbd188c4741010833ab8340 Mon Sep 17 00:00:00 2001
From: Carlos Esparza <ca2010es@gmail.com>
Date: Thu, 9 Oct 2025 12:10:46 -0700
Subject: [PATCH] Avoid unnecessary allocations in AccumulatorStack

replacing the vector with an array resulted in a .45+-.10 % speedup.

closes https://github.com/official-stockfish/Stockfish/pull/6352

no functional change
---
 src/nnue/network.h          |  2 --
 src/nnue/nnue_accumulator.h | 11 +++--------
 2 files changed, 3 insertions(+), 10 deletions(-)

diff --git a/src/nnue/network.h b/src/nnue/network.h
index c9358823b..6855831d5 100644
--- a/src/nnue/network.h
+++ b/src/nnue/network.h
@@ -106,8 +106,6 @@ class Network {
 
     template<IndexType Size>
     friend struct AccumulatorCaches::Cache;
-
-    friend class AccumulatorStack;
 };
 
 // Definitions of the network types
diff --git a/src/nnue/nnue_accumulator.h b/src/nnue/nnue_accumulator.h
index 56de0b7d0..7ff95b6ef 100644
--- a/src/nnue/nnue_accumulator.h
+++ b/src/nnue/nnue_accumulator.h
@@ -25,7 +25,6 @@
 #include <cstddef>
 #include <cstdint>
 #include <cstring>
-#include <vector>
 
 #include "../types.h"
 #include "nnue_architecture.h"
@@ -48,7 +47,7 @@ template<IndexType Size>
 struct alignas(CacheLineSize) Accumulator {
     std::int16_t               accumulation[COLOR_NB][Size];
     std::int32_t               psqtAccumulation[COLOR_NB][PSQTBuckets];
-    std::array<bool, COLOR_NB> computed;
+    std::array<bool, COLOR_NB> computed = {};
 };
 
 
@@ -142,10 +141,6 @@ struct AccumulatorState {
 
 class AccumulatorStack {
    public:
-    AccumulatorStack() :
-        accumulators(MAX_PLY + 1),
-        size{1} {}
-
     [[nodiscard]] const AccumulatorState& latest() const noexcept;
 
     void reset() noexcept;
@@ -178,8 +173,8 @@ class AccumulatorStack {
                                      const FeatureTransformer<Dimensions>& featureTransformer,
                                      const std::size_t                     end) noexcept;
 
-    std::vector<AccumulatorState> accumulators;
-    std::size_t                   size;
+    std::array<AccumulatorState, MAX_PLY + 1> accumulators;
+    std::size_t                               size = 1;
 };
 
 }  // namespace Stockfish::Eval::NNUE