[CPU][Bugfix] Fix Apple Silicon M1 compilation failure (#28681)

Signed-off-by: mgoin <mgoin64@gmail.com>

csrc/cpu/cpu_attn_impl.hpp

@@ -5,6 +5,10 @@
 #include <type_traits>
 #include <cstddef>
 
+#if defined(__APPLE__)
+  #include <sys/sysctl.h>
+#endif
+
 #include "cpu_types.hpp"
 #include "scratchpad_manager.h"
 #include "cpu_attn_macros.h"
@@ -741,9 +745,21 @@ class AttentionScheduler {
 
   static int64_t get_available_l2_size() {
     static int64_t size = []() {
+#if defined(__APPLE__)
+      // macOS doesn't have _SC_LEVEL2_CACHE_SIZE. Use sysctlbyname.
+      int64_t l2_cache_size = 0;
+      size_t len = sizeof(l2_cache_size);
+      if (sysctlbyname("hw.l2cachesize", &l2_cache_size, &len, NULL, 0) == 0 &&
+          l2_cache_size > 0) {
+        return l2_cache_size >> 1;  // use 50% of L2 cache
+      }
+      // Fallback if sysctlbyname fails
+      return 128 * 1024 >> 1;  // use 50% of 128KB
+#else
       long l2_cache_size = sysconf(_SC_LEVEL2_CACHE_SIZE);
       TORCH_CHECK_NE(l2_cache_size, -1);
       return l2_cache_size >> 1;  // use 50% of L2 cache
+#endif
     }();
     return size;
   }
@@ -816,10 +832,14 @@ struct VecTypeTrait<float> {
   using vec_t = vec_op::FP32Vec16;
 };
 
+// ARM only supports BF16 with ARMv8.6-A extension
+#if (defined(__aarch64__) && !defined(ARM_BF16_SUPPORT))
+#else
 template <>
 struct VecTypeTrait<c10::BFloat16> {
   using vec_t = vec_op::BF16Vec16;
 };
+#endif
 
 #if !defined(__powerpc__)
 template <>
@@ -1588,9 +1608,17 @@ class AttentionMainLoop {
 
               if (use_sink) {
                 alignas(64) float s_aux_fp32[16];
+#if defined(__aarch64__) && !defined(ARM_BF16_SUPPORT)
+                // ARM without native BF16 support: manual conversion
+                for (int i = 0; i < 16; ++i) {
+                  s_aux_fp32[i] = static_cast<float>(curr_s_aux[i]);
+                }
+#else
+                // All other platforms have BF16Vec16 available
                 vec_op::BF16Vec16 vec_bf16(curr_s_aux);
                 vec_op::FP32Vec16 vec_fp32(vec_bf16);
                 vec_fp32.save(s_aux_fp32);
+#endif
 
                 float* __restrict__ curr_sum_buffer = sum_buffer;
                 float* __restrict__ curr_max_buffer = max_buffer;