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

[Boqun Feng <boqun.feng@gmail.com>]

On Fri, Sep 27, 2024 at 09:37:50AM +0800, Boqun Feng wrote:
> 
> 
> 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

Note it's not hiding the dependency, rather the equality,

> > registers as being one and choosing one arbitrary source. "hiding"

after OPTIMIZER_HIDE_VAR(var), compiler doesn't know whether 'var' is
equal to 'expr' anymore, because OPTIMIZER_HIDE_VAR(var) uses "=r"(var)
to indicate the output is overwritten. So when 'var' is referred later,
compiler cannot use the register for a 'expr' value or any other
register that has the same value, because 'var' may have a different
value from the compiler's POV.

> > 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).
> 

I replaced ADDRESS_EQ(a, b) with ADDRESS_EQ(b, a), and the compile
result shows it can prevent the issue:

gcc 14.2 x86-64:

fct_hide_var_compare:
.L2:
        mov     rcx, QWORD PTR p[rip]
        mov     rdx, QWORD PTR p[rip]
        mov     rax, rdx
        cmp     rcx, rdx
        jne     .L2
        mov     eax, DWORD PTR [rax]
        ret

gcc 14.2.0 ARM64:

fct_hide_var_compare:
        adrp    x2, p
        add     x2, x2, :lo12:p
.L2:
        ldr     x3, [x2]
        ldr     x1, [x2]
        mov     x0, x1
        cmp     x3, x1
        bne     .L2
        ldr     w0, [x0]
        ret

Link to godbolt:

	https://godbolt.org/z/a7jsfzjxY

Regards,
Boqun

> 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
>