From df01cb7ed3c92ec65a0a8d8c6ed870bc664c27bb Mon Sep 17 00:00:00 2001
From: Carlos Esparza <ca2010es@gmail.com>
Date: Thu, 9 Apr 2026 21:44:44 +0200
Subject: [PATCH] Avoid branching before `ValueList::push_back`

Instead of checking whether a threat index is valid and then writing to
a vector if it isn't, we can instead always write to the buffer of the
vector, and only increase the size of the vector if the index is valid.

This saves some branch mispredictions.

passed STC:
https://tests.stockfishchess.org/tests/view/69ceb1689f7a7e3fdfc9a44b
LLR: 2.95 (-2.94,2.94) <0.00,2.00>
Total: 205760 W: 53109 L: 52561 D: 100090
Ptnml(0-2): 603, 22552, 56076, 22992, 657

local speedtest shows
```
Result of 100 runs (cycles)
===========================
base (...ockfish.orig) =     296886  +/- 1645
test (...kfish.df0822) =     298862  +/- 1662
speedup %              =      +0.67  +/- 0.19
                         [95% CI; t-statistic]

p(speedup > 0)   =  1.0000
p(speedup > .5%) =  0.9562

CPU: 16 x Intel(R) Core(TM) Ultra 9 185H
Hyperthreading: on
```

closes https://github.com/official-stockfish/Stockfish/pull/6703

No functional change
---
 src/misc.h                         |  6 ++++++
 src/nnue/features/full_threats.cpp | 24 ++++++++----------------
 2 files changed, 14 insertions(+), 16 deletions(-)

diff --git a/src/misc.h b/src/misc.h
index d1c368fdd..d69f7ef38 100644
--- a/src/misc.h
+++ b/src/misc.h
@@ -201,6 +201,12 @@ class ValueList {
         assert(size_ < MaxSize);
         values_[size_++] = value;
     }
+    // pushes back value if value < max
+    void push_back_if_lt(const T& value, const T& max) {
+        assert(size_ < MaxSize);
+        values_[size_] = value;
+        size_ += (value < max);
+    }
     const T* begin() const { return values_; }
     const T* end() const { return values_ + size_; }
     const T& operator[](int index) const { return values_[index]; }
diff --git a/src/nnue/features/full_threats.cpp b/src/nnue/features/full_threats.cpp
index 6cae5d0ed..a62a664e0 100644
--- a/src/nnue/features/full_threats.cpp
+++ b/src/nnue/features/full_threats.cpp
@@ -22,7 +22,6 @@
 
 #include <array>
 #include <cassert>
-#include <cstddef>
 #include <cstdint>
 #include <initializer_list>
 #include <utility>
@@ -236,8 +235,7 @@ void FullThreats::append_active_indices(Color perspective, const Position& pos,
                     Piece     attacked = pos.piece_on(to);
                     IndexType index    = make_index(perspective, attacker, from, to, attacked, ksq);
 
-                    if (index < Dimensions)
-                        active.push_back(index);
+                    active.push_back_if_lt(index, Dimensions);
                 }
 
                 while (attacks_right)
@@ -247,8 +245,7 @@ void FullThreats::append_active_indices(Color perspective, const Position& pos,
                     Piece     attacked = pos.piece_on(to);
                     IndexType index    = make_index(perspective, attacker, from, to, attacked, ksq);
 
-                    if (index < Dimensions)
-                        active.push_back(index);
+                    active.push_back_if_lt(index, Dimensions);
                 }
 
                 // Set of pawns which are prevented from movement by a pawn in front of them
@@ -261,8 +258,7 @@ void FullThreats::append_active_indices(Color perspective, const Position& pos,
                     assert(type_of(attacked) == PAWN);
 
                     IndexType index = make_index(perspective, attacker, from, to, attacked, ksq);
-                    if (index < Dimensions)
-                        active.push_back(index);
+                    active.push_back_if_lt(index, Dimensions);
                 }
             }
             else
@@ -279,8 +275,7 @@ void FullThreats::append_active_indices(Color perspective, const Position& pos,
                         IndexType index =
                           make_index(perspective, attacker, from, to, attacked, ksq);
 
-                        if (index < Dimensions)
-                            active.push_back(index);
+                        active.push_back_if_lt(index, Dimensions);
                     }
                 }
             }
@@ -342,13 +337,10 @@ void FullThreats::append_changed_indices(Color                   perspective,
         auto&           insert = add ? added : removed;
         const IndexType index  = make_index(perspective, attacker, from, to, attacked, ksq);
 
-        if (index < Dimensions)
-        {
-            if (prefetchBase)
-                prefetch<PrefetchRw::READ, PrefetchLoc::LOW>(
-                  prefetchBase + static_cast<std::ptrdiff_t>(index) * prefetchStride);
-            insert.push_back(index);
-        }
+        if (prefetchBase)
+            prefetch<PrefetchRw::READ, PrefetchLoc::LOW>(reinterpret_cast<const void*>(
+              reinterpret_cast<uintptr_t>(prefetchBase) + index * prefetchStride));
+        insert.push_back_if_lt(index, Dimensions);
     }
 }