compiler: Use __typeof_unqual__() for __unqual_scalar_typeof()

The recent changes to get_unaligned() resulted in a new sparse warning: net/rds/ib_cm.c:96:35: sparse: sparse: incorrect type in argument 1 (different modifiers) @@ expected void * @@ got restricted __be64 const * @@ net/rds/ib_cm.c:96:35: sparse: expected void * net/rds/ib_cm.c:96:35: sparse: got restricted __be64 const * The updated get_unaligned_t() uses __unqual_scalar_typeof() to get an unqualified type. This works correctly for the compilers, but fails for sparse when the data type is __be64 (or any other __beNN variant). On sparse runs (C=[12]) __beNN types are annotated with __attribute__((bitwise)). That annotation allows sparse to detect incompatible operations on __beNN variables, but it also prevents sparse from evaluating the _Generic() in __unqual_scalar_typeof() and map __beNN to a unqualified scalar type, so it ends up with the default, i.e. the original qualified type of a 'const __beNN' pointer. That then ends up as the first pointer argument to builtin_memcpy(), which obviously causes the above sparse warnings. The sparse git tree supports typeof_unqual() now, which allows to use it instead of the _Generic() based __unqual_scalar_typeof(). With that sparse correctly evaluates the unqualified type and keeps the __beNN logic intact. The downside is that this requires a top of tree sparse build and an old sparse version will emit a metric ton of incomprehensible error messages before it dies with a segfault. Therefore implement a sanity check which validates that the checker is available and capable of handling typeof_unqual(). Emit a warning if not so the user can take informed action. [ tglx: Move the evaluation of USE_TYPEOF_UNQUAL to compiler_types.h so it is set before use and implement the sanity checker ]

PZPeter Zijlstra committed 10 months ago