Re: [PATCH 1/7] xen/bitops: Cleanup ahead of rearrangements

[Shawn Anastasio <sanastasio@xxxxxxxxxxxxxxxxxxxxx>]

Hi Andrew,

On 3/13/24 12:27 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 )
> +

For improved diagnostics, I think it might make sense to explicitly
check if the expression can be evaluated at compile time and emit a
different error if not. Perhaps something like the following:

#define COMPILE_CHECK(fn, val, res)                                     \
    do {                                                                \
        __typeof__(fn(val)) actual = fn(val);                           \
        if ( !__builtin_constant_p(actual) )                            \
            asm (".error \"Unable to evaluate '" STR(fn(val)) "' at
compile time\"\n"); \
        else if ( actual != res )                                       \
            asm (".error \"Compile time check '" STR(fn(val) == res) "'
failed\""); \
    } while ( 0 )