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

[Mathieu Desnoyers]

On 2024-09-27 21:23, Jonas Oberhauser wrote:
[...]
> That idea seems to be confirmed by this (atrocious, not to be copied!) 
> example:
>
> int fct_escape_address_of_b(void)
> {
>      int *a, *b;
>
>      do {
>          a = READ_ONCE(p);
>          asm volatile ("" : : : "memory");
>          b = READ_ONCE(p);
>      } while (a != b);
>
>      // really really hide b
>      int **p = &b;
>      OPTIMIZER_HIDE_VAR(p);
>
>      asm volatile ("" : : : "memory");
>      return *b;
> }
>
> This also does not generate any additional instructions, unlike just 
> using OPTIMIZER_HIDE_VAR(b).
>
> What is the advantage of defining OPTIMIZE_HIDE_VAR the way it currently 
> works instead of like above?

Did you try it on godbolt.org ? Does it have the intended effect ?
By the looks of it, you're just creating another version of @b called
"p", which is then never used and would be discarded by further
optimization. I'm unsure what you are trying to achieve here.

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com