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

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, Jul 24, 2024 at 10:41:43AM +0200, Jan Beulich wrote:
> 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); }) \
>     }

We could even simplify this, there's no need for the union anymore,
since struct size == sizeof(long) already:

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

The above seems to work for both the original issue and the new one.

Thanks, Roger.