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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.07.2024 11:31, Roger Pau Monne wrote:
> Yet another clang code generation issue when using altcalls.
> 
> The issue this time is with using loop constructs around alternative_{,v}call
> instances using parameter types smaller than the register size.
> 
> Given the following example code:
> 
> static void bar(bool b)
> {
>     unsigned int i;
> 
>     for ( i = 0; i < 10; i++ )
>     {
>         int ret_;
>         register union {
>             bool e;
>             unsigned long r;
>         } di asm("rdi") = { .e = b };
>         register unsigned long si asm("rsi");
>         register unsigned long dx asm("rdx");
>         register unsigned long cx asm("rcx");
>         register unsigned long r8 asm("r8");
>         register unsigned long r9 asm("r9");
>         register unsigned long r10 asm("r10");
>         register unsigned long r11 asm("r11");
> 
>         asm volatile ( "call %c[addr]"
>                        : "+r" (di), "=r" (si), "=r" (dx),
>                          "=r" (cx), "=r" (r8), "=r" (r9),
>                          "=r" (r10), "=r" (r11), "=a" (ret_)
>                        : [addr] "i" (&(func)), "g" (func)
>                        : "memory" );
>     }
> }
> 
> See: https://godbolt.org/z/qvxMGd84q
> 
> Clang will generate machine code that only resets the low 8 bits of %rdi
> between loop calls, leaving the rest of the register possibly containing
> garbage from the use of %rdi inside the called function.  Note also that clang
> doesn't truncate the input parameters at the callee, thus breaking the psABI.
> 
> Fix this by turning the `e` element in the anonymous union into an array that
> consumes the same space as an unsigned long, as this forces clang to reset the
> whole %rdi register instead of just the low 8 bits.
> 
> Fixes: 2ce562b2a413 ('x86/altcall: use a union as register type for function 
> parameters on clang')
> Suggested-by: Jan Beulich <jbeulich@xxxxxxxx>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>