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 11:13, Roger Pau Monné wrote:
> 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.

Oh, good. If the compiler's happy with not being dealt an unsigned long,
even better. Maybe for consistency we want to use sizeof(void *) also in
pad's definition then.

Jan