Re: [RFC PATCH 1/4] hazptr: Add initial implementation of hazard pointers

[Boqun Feng]

On Fri, Sep 27, 2024, at 9:30 AM, Mathieu Desnoyers wrote:
> On 2024-09-27 02:01, Boqun Feng wrote:
>> 	#define ADDRESS_EQ(var, expr)							\
>> 	({										\
>> 		bool _____cmp_res = (unsigned long)(var) == (unsigned long)(expr);	\
>> 											\
>> 		OPTIMIZER_HIDE_VAR(var);						\
>> 		_____cmp_res;								\
>> 	})
>
> If the goal is to ensure gcc uses the register populated by the
> second, I'm afraid it does not work. AFAIU, "hiding" the dependency
> chain does not prevent the SSA GVN optimization from combining the
> registers as being one and choosing one arbitrary source. "hiding"
> the dependency chain before or after the comparison won't help here.
>
> int fct_hide_var_compare(void)
> {
>      int *a, *b;
>
>      do {
>          a = READ_ONCE(p);
>          asm volatile ("" : : : "memory");
>          b = READ_ONCE(p);
>      } while (!ADDRESS_EQ(a, b));

Note that ADDRESS_EQ() only hide first parameter, so this should be ADDRESS_EQ(b, a).

Regards,
Boqun

>      return *b;
> }
>
> gcc 14.2 x86-64:
>
> fct_hide_var_compare:
>   mov    rax,QWORD PTR [rip+0x0]        # 67 <fct_hide_var_compare+0x7>
>   mov    rdx,QWORD PTR [rip+0x0]        # 6e <fct_hide_var_compare+0xe>
>   cmp    rax,rdx
>   jne    60 <fct_hide_var_compare>
>   mov    eax,DWORD PTR [rax]
>   ret
> main:
>   xor    eax,eax
>   ret
>
> gcc 14.2.0 ARM64:
>
> fct_hide_var_compare:
>          adrp    x0, .LANCHOR0
>          add     x0, x0, :lo12:.LANCHOR0
> .L12:
>          ldr     x1, [x0]
>          ldr     x2, [x0]
>          cmp     x1, x2
>          bne     .L12
>          ldr     w0, [x1]
>          ret
> p:
>          .zero   8
>
>
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com