On 13/03/2024 5:27 pm, Andrew Cooper wrote:
> diff --git a/xen/common/bitops.c b/xen/common/bitops.c
> new file mode 100644
> index 000000000000..4c07191b4030
> --- /dev/null
> +++ b/xen/common/bitops.c
> @@ -0,0 +1,41 @@
> +#include <xen/bitops.h>
> +#include <xen/bug.h>
> +#include <xen/init.h>
> +
> +/* Hide a value from the optimiser. */
> +#define HIDE(x) ({ typeof(x) _x = x; asm volatile ( "" : "+r" (_x) ); _x; })
> +
> +/*
> + * Check that fn(val) can be calcuated by the compiler, and that it gives the
> + * expected answer.
> + */
> +#define COMPILE_CHECK(fn, val, res) \
> + do { \
> + if ( fn(val) != res ) \
> + asm (".error \"Compile time check '" STR(fn(val) == res) "'
> failed\""); \
> + } while ( 0 )
It turns out that Clang doesn't like this
https://gitlab.com/xen-project/people/andyhhp/xen/-/jobs/6387413632
despite it being capable of reducing the expression to a constant.
This also calls into question whether it's a viable replacement for
__bad_bitop_size() et al.
~Andrew
