gcc/testsuite/gcc.target/aarch64/cpu-diagnostics-1.c

/* { dg-error "unknown" "" {target "aarch64*-*-*" } 0 } */
/* { dg-skip-if "do not override -mcpu" { *-*-* } { "-mcpu=*" } { "" } } */
/* { dg-options "-O2 -mcpu=dummy" } */

void f ()
{
  return;
}