[Bugfix] Add replacement of _compute_slot_mapping_kernel on CPU (#37987)

Signed-off-by: jiang1.li <jiang1.li@intel.com>

csrc/cpu/utils.cpp

@@ -189,3 +189,38 @@ ScratchPadManager* ScratchPadManager::get_scratchpad_manager() {
   return &manager;
 }
 }  // namespace cpu_utils
+
+void compute_slot_mapping_kernel_impl(const torch::Tensor query_start_loc,
+                                      const torch::Tensor positions,
+                                      const torch::Tensor block_table,
+                                      torch::Tensor slot_mapping,
+                                      const int64_t block_size) {
+  const int32_t req_num = query_start_loc.size(0) - 1;
+  const int64_t block_table_stride = block_table.stride(0);
+
+  const int32_t* __restrict__ query_start_loc_ptr =
+      query_start_loc.data_ptr<int32_t>();
+  const int64_t* __restrict__ positions_ptr = positions.data_ptr<int64_t>();
+  const int32_t* __restrict__ blocktable_ptr = block_table.data_ptr<int32_t>();
+  int64_t* __restrict__ slot_mapping_ptr = slot_mapping.data_ptr<int64_t>();
+
+#pragma omp parallel for
+  for (int32_t req_idx = 0; req_idx < req_num; ++req_idx) {
+    int32_t token_start_idx = query_start_loc_ptr[req_idx];
+    int32_t token_end_idx = query_start_loc_ptr[req_idx + 1];
+    int32_t token_num = token_end_idx - token_start_idx;
+    const int64_t* __restrict__ curr_position_ptr =
+        positions_ptr + token_start_idx;
+    int64_t* __restrict__ curr_slot_mapping_ptr =
+        slot_mapping_ptr + token_start_idx;
+    const int32_t* __restrict__ curr_block_table_ptr =
+        blocktable_ptr + req_idx * block_table_stride;
+
+    for (int32_t token_idx = 0; token_idx < token_num; ++token_idx) {
+      int64_t token_position = curr_position_ptr[token_idx];
+      int64_t block_id = curr_block_table_ptr[token_position / block_size];
+      curr_slot_mapping_ptr[token_idx] =
+          block_id * block_size + token_position % block_size;
+    }
+  }
+}