[Perf] Use dummy M for weight prepacking on x86 (#35890)

Signed-off-by: Li, Tianmu <tianmu.li@intel.com>

csrc/cpu/dnnl_helper.cpp

@@ -237,13 +237,10 @@ W8A8MatMulPrimitiveHandler::W8A8MatMulPrimitiveHandler(const Args& args)
   };
   dnnl::memory::desc original_b_md({b_k_size_, b_n_size_}, b_type_,
                                    {b_k_stride_, b_n_stride_});
-#ifdef __aarch64__
+
   // dummy M size for prepacking weights
   // Prepacking weights improves performance and avoid runtime reorders
   constexpr dnnl_dim_t kProbeM = 128;
-#else
-  constexpr dnnl_dim_t kProbeM = DNNL_RUNTIME_DIM_VAL;
-#endif
 
   prepack_weight(args.b_ptr, original_b_md,
                  create_primitive_desc(
@@ -411,19 +408,17 @@ MatMulPrimitiveHandler::MatMulPrimitiveHandler(const Args& args)
   dnnl::memory::desc original_b_md({b_k_size_, b_n_size_}, b_type_,
                                    {b_k_stride_, b_n_stride_});
 
+  // dummy M size for prepacking weights
+  // Prepacking weights improves performance and avoid runtime reorders
+  constexpr dnnl_dim_t kProbeM = 128;
+
   prepack_weight(args.b_ptr, original_b_md,
                  create_primitive_desc(
-                     MSizeCacheKey{
-#ifdef VLLM_USE_ACL
-                         // Arm Compute Library (ACL) backend for oneDNN does
-                         // not support runtime
-                         // dimensions, so we set M to a default value
-                         .a_m_size = 128,
+                     MSizeCacheKey{// Use a concrete M so oneDNN's kernel
+                                   // selector can choose an optimally blocked
+                                   // weight layout.
+                                   .a_m_size = kProbeM,
                                    .a_m_stride = b_k_size_,
-#else
-                         .a_m_size = DNNL_RUNTIME_DIM_VAL,
-                         .a_m_stride = DNNL_RUNTIME_DIM_VAL,
-#endif
                                    .use_bias = false,
                                    .bias_type = dnnl::memory::data_type::undef},
                      true)