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

[Mathieu Desnoyers]

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));
    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