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
>
