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 <jbeul...@suse.com>
> Signed-off-by: Roger Pau Monné <roger....@citrix.com>
Reviewed-by: Jan Beulich <jbeul...@suse.com>
