[CINN][Add Backend Pass Comment No.6] Add comment for longlong2int (#…

…70457)

paddle/cinn/optim/longlong2int.h

@@ -18,7 +18,73 @@
 namespace cinn {
 namespace optim {
 
-// Try to change the type of longlong to int in the expr.
+/**
+ * Converts int64 (long long) types to int32 in a block where possible.
+ *
+ * This pass is applicable in scenarios where the IR contains int64 types that
+ * can be safely represented as int32 without overflow.
+ *
+ * When applied, this pass will traverse the IR and convert int64 types to int32
+ * in various constructs, including:
+ * - Tensor shapes and indices
+ * - Loop variables and bounds
+ * - Buffer metadata (shapes, strides, offsets)
+ * - Comparison operations
+ *
+ * Overflow checking:
+ * The pass performs overflow checking primarily for nested for-loops. This
+ * focus on nested loops is based on the assumption that they are the most
+ * common source of potential overflows in typical computational kernels. The
+ * check considers:
+ * - The product of loop extents (iteration counts)
+ * - Whether loop bounds are constant and of index type
+ *
+ *
+ * Examples:
+ * 1. Loop variable conversion:
+ * Before conversion:
+ * {
+ *   ScheduleBlock(root_12)
+ *   {
+ *     attrs(tile_method:TileFirstGeneralTactic)
+ *     thread_bind[blockIdx.x] for (blockIdx.x, 0, 352)
+ *     {
+ *       thread_bind[threadIdx.x] for (threadIdx.x, 0, 256)
+ *       {
+ *         ScheduleBlock(var_2)
+ *         {
+ *           i0, i1, i2, i3 = axis.bind(idx / 4096, (idx % 4096) / 256, (idx %
+ * 256) / 16, idx % 16) read_buffers(_var[i0(0:22ll), i2(0:16ll)])
+ *           write_buffers(_var_2[i0(0:22ll), i1(0:16ll), i2(0:16ll),
+ * i3(0:16ll)])
+ *         var_2[i0, i1, i2, i3] = var[i0, i2, i3 + i1 * 16ll]
+ *         }
+ *       }
+ *     }
+ *   }
+ * }
+ *
+ * After conversion:
+ * {
+ *   ScheduleBlock(root_12)
+ *   {
+ *     attrs(tile_method:TileFirstGeneralTactic)
+ *     thread_bind[blockIdx.x] for (blockIdx.x, 0, 352)
+ *     {
+ *       thread_bind[threadIdx.x] for (threadIdx.x, 0, 256)
+ *       {
+ *         ScheduleBlock(var_2)
+ *         {
+ *           i0, i1, i2, i3 = axis.bind(idx / 4096, (idx % 4096) / 256, (idx %
+ * 256) / 16, idx % 16) read_buffers(_var[i0(0:22), i2(0:16)])
+ *           write_buffers(_var_2[i0(0:22), i1(0:16), i2(0:16),i3(0:16)])
+ *           var_2[i0, i1, i2, i3] = var[i0, i2, i3 + i1 * 16]
+ *         }
+ *       }
+ *     }
+ *   }
+ * }
+ */
 void TryCastLonglong2Int(Expr* expr);
 }  // namespace optim
 }  // namespace cinn