Re: [PATCH for-4.19] x86/altcall: fix clang code-gen when using altcall in loop constructs

[Jan Beulich <jbeulich@xxxxxxxx>]

On 24.07.2024 10:28, Roger Pau Monné wrote:
> The only way I've found to cope with this is to use something like:
> 
> #define ALT_CALL_ARG(arg, n)                                            \
>     union {                                                             \
>         typeof(arg) e[(sizeof(long) + sizeof(arg) - 1) / sizeof(arg)];  \
>         unsigned long r;                                                \
>     } a ## n ## __  = {                                                 \
>         .e[0] = ({ BUILD_BUG_ON(sizeof(arg) > sizeof(void *)); (arg); })\
>     };                                                                  \
>     register unsigned long a ## n ## _ asm ( ALT_CALL_arg ## n ) =      \
>         a ## n ## __.r
> 
> An oversized array that ensures all the space of the long is covered
> by the array, but then we need an extra variable, as we would
> otherwise require modifying ALT_CALL{0..6}_OUT to use aX_.r instead of
> aX_.

I don't think we need to over-size the array. It just gets yet a little
more complex then:

#define ALT_CALL_ARG(arg, n)                                            \
    register union {                                                    \
        struct {                                                        \
            typeof(arg) e;                                              \
            char pad[sizeof(long) - sizeof(arg)];                       \
        } s;                                                            \
        unsigned long r;                                                \
    } a ## n ## _ asm ( ALT_CALL_arg ## n ) = {                         \
        .s.e = ({ BUILD_BUG_ON(sizeof(arg) > sizeof(void *)); (arg); }) \
    }

Jan