From 9f968446cbbe7630b3044526359baef4f67fa62d Mon Sep 17 00:00:00 2001
From: disservin <disservin.social@gmail.com>
Date: Wed, 28 Jan 2026 09:41:21 +0100
Subject: [PATCH] Improve usage of sf_assume

Make the sf_assume also have an effect with clang and help the compiler
proof it's side effect free by comparing against a variable.

https://tests.stockfishchess.org/tests/view/6979d14fa2c75f923be1e681
LLR: 2.93 (-2.94,2.94) <0.00,2.00>
Total: 63488 W: 16530 L: 16188 D: 30770
Ptnml(0-2): 167, 6904, 17277, 7212, 184

closes https://github.com/official-stockfish/Stockfish/pull/6573

No functional change
---
 src/misc.h                    | 10 ++++++-
 src/nnue/nnue_accumulator.cpp | 50 +++++++++++++++++++++--------------
 2 files changed, 39 insertions(+), 21 deletions(-)

diff --git a/src/misc.h b/src/misc.h
index 8adc72409..1471b1b6a 100644
--- a/src/misc.h
+++ b/src/misc.h
@@ -444,7 +444,9 @@ void move_to_front(std::vector<T>& vec, Predicate pred) {
     #define sf_always_inline
 #endif
 
-#if defined(__GNUC__) && !defined(__clang__)
+#if defined(__clang__)
+    #define sf_assume(cond) __builtin_assume(cond)
+#elif defined(__GNUC__)
     #if __GNUC__ >= 13
         #define sf_assume(cond) __attribute__((assume(cond)))
     #else
@@ -460,6 +462,12 @@ void move_to_front(std::vector<T>& vec, Predicate pred) {
     #define sf_assume(cond)
 #endif
 
+#ifdef __GNUC__
+    #define sf_unreachable() __builtin_unreachable()
+#else
+    #define sf_unreachable()
+#endif
+
 }  // namespace Stockfish
 
 template<std::size_t N>
diff --git a/src/nnue/nnue_accumulator.cpp b/src/nnue/nnue_accumulator.cpp
index 16af8d5f6..926e82d93 100644
--- a/src/nnue/nnue_accumulator.cpp
+++ b/src/nnue/nnue_accumulator.cpp
@@ -507,19 +507,23 @@ void double_inc_update(Color                                                   p
     assert(added.size() < 2);
     PSQFeatureSet::append_changed_indices(perspective, ksq, target_state.diff, removed, added);
 
-    assert(added.size() == 1);
-    assert(removed.size() == 2 || removed.size() == 3);
+    [[maybe_unused]] const int addedSize   = added.ssize();
+    [[maybe_unused]] const int removedSize = removed.ssize();
+
+    assert(addedSize == 1);
+    assert(removedSize == 2 || removedSize == 3);
 
     // Workaround compiler warning for uninitialized variables, replicated on
     // profile builds on windows with gcc 14.2.0.
-    // TODO remove once unneeded
-    sf_assume(added.size() == 1);
-    sf_assume(removed.size() == 2 || removed.size() == 3);
+    // Also helps with optimizations on some compilers.
+
+    sf_assume(addedSize == 1);
+    sf_assume(removedSize == 2 || removedSize == 3);
 
     auto updateContext =
       make_accumulator_update_context(perspective, featureTransformer, computed, target_state);
 
-    if (removed.size() == 2)
+    if (removedSize == 2)
     {
         updateContext.template apply<Add, Sub, Sub>(added[0], removed[0], removed[1]);
     }
@@ -593,35 +597,41 @@ void update_accumulator_incremental(
         updateContext.apply(added, removed);
     else
     {
-        assert(added.size() == 1 || added.size() == 2);
-        assert(removed.size() == 1 || removed.size() == 2);
-        assert((Forward && added.size() <= removed.size())
-               || (!Forward && added.size() >= removed.size()));
+        [[maybe_unused]] const int addedSize   = added.ssize();
+        [[maybe_unused]] const int removedSize = removed.ssize();
+
+        assert(addedSize == 1 || addedSize == 2);
+        assert(removedSize == 1 || removedSize == 2);
+        assert((Forward && addedSize <= removedSize) || (!Forward && addedSize >= removedSize));
 
         // Workaround compiler warning for uninitialized variables, replicated
         // on profile builds on windows with gcc 14.2.0.
-        // TODO remove once unneeded
-        sf_assume(added.size() == 1 || added.size() == 2);
-        sf_assume(removed.size() == 1 || removed.size() == 2);
+        // Also helps with optimizations on some compilers.
 
-        if ((Forward && removed.size() == 1) || (!Forward && added.size() == 1))
+        sf_assume(addedSize == 1 || addedSize == 2);
+        sf_assume(removedSize == 1 || removedSize == 2);
+
+        if (!(removedSize == 1 || removedSize == 2) || !(addedSize == 1 || addedSize == 2))
+            sf_unreachable();
+
+        if ((Forward && removedSize == 1) || (!Forward && addedSize == 1))
         {
-            assert(added.size() == 1 && removed.size() == 1);
+            assert(addedSize == 1 && removedSize == 1);
             updateContext.template apply<Add, Sub>(added[0], removed[0]);
         }
-        else if (Forward && added.size() == 1)
+        else if (Forward && addedSize == 1)
         {
-            assert(removed.size() == 2);
+            assert(removedSize == 2);
             updateContext.template apply<Add, Sub, Sub>(added[0], removed[0], removed[1]);
         }
-        else if (!Forward && removed.size() == 1)
+        else if (!Forward && removedSize == 1)
         {
-            assert(added.size() == 2);
+            assert(addedSize == 2);
             updateContext.template apply<Add, Add, Sub>(added[0], added[1], removed[0]);
         }
         else
         {
-            assert(added.size() == 2 && removed.size() == 2);
+            assert(addedSize == 2 && removedSize == 2);
             updateContext.template apply<Add, Add, Sub, Sub>(added[0], added[1], removed[0],
                                                              removed[1]);
         }