From 4436524a3e6f37b68d0f23683681b249a4edcf52 Mon Sep 17 00:00:00 2001
From: Timothy Herchen <timothy.herchen@gmail.com>
Date: Mon, 6 Jul 2026 11:31:28 +0200
Subject: [PATCH] add rbit fallback

GCC 12.2 added `__rbitll` intrinsic to `arm_acle.h`, so we can add a fallback for older versions

Clang 10 (our new minimum supported version) is unaffected by all this

closes https://github.com/official-stockfish/Stockfish/pull/6949

No functional change
---
 src/attacks.h | 16 ++++++++++++++--
 src/misc.h    |  4 ++++
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/src/attacks.h b/src/attacks.h
index de29914c9..2a4c77843 100644
--- a/src/attacks.h
+++ b/src/attacks.h
@@ -43,12 +43,24 @@ void init();
 #ifdef USE_HYPERBOLA_QUINT
 
 inline Bitboard reverse_bb(Bitboard bb) {
-    #ifdef __aarch64__
+    #if __has_builtin(__builtin_bitreverse64)
+    return __builtin_bitreverse64(bb);
+    #else
+        #ifdef __aarch64__
+            #if defined(__GNUC__) && !defined(__clang__) \
+              && (__GNUC__ < 12 || (__GNUC__ == 12 && __GNUC_MINOR__ < 2))
+    // no rbit in arm_acle.h
+    Bitboard out;
+    asm("rbit %0, %1" : "=r"(out) : "r"(bb));
+    return out;
+            #else
     return __rbitll(bb);
-    #else  // loongarch
+            #endif
+        #else  // loongarch
     Bitboard out;
     asm("bitrev.d %0, %1" : "=r"(out) : "r"(bb));
     return out;
+        #endif
     #endif
 }
 
diff --git a/src/misc.h b/src/misc.h
index 20c73bd43..db2c03f1c 100644
--- a/src/misc.h
+++ b/src/misc.h
@@ -561,6 +561,10 @@ void move_to_front(std::vector<T>& vec, Predicate pred) {
 }
 }
 
+#ifndef __has_builtin
+    #define __has_builtin(x) 0
+#endif
+
 #if defined(__GNUC__)
     #define sf_always_inline __attribute__((always_inline))
 #elif defined(_MSC_VER)