Re: [PATCH hazptr 2/4] compiler.h: Introduce ptr_eq() to preserve address dependency

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2026-09-29 11:16, Linus Torvalds wrote:
> On Tue, 29 Sept 2026 at 08:04, Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
>>
>> I agree that doing it in asm would generate better code. Also, I've
>> been told that clang now preserves the address dependency in this
>> scenario, so perhaps we'd want to use preprocessor conditionals to
>> select how to implement ptr_eq based on:
>>
>> - architecture (allowing asm implementation overrides),
>> - compiler (e.g. if compiler is clang >= version X, just do a plain
>>     comparison). Likewise for gcc if it ever decides to fix this
>>     behavior.
>>
>> I think this "hide var" hack is a fallback which can be used as a
>> starting point, and then we can specialize based on architecture and
>> compiler.
>>
>> Or do you prefer this in a different order ?
> 
> Oh, if there's some sane way to tell that the compiler already honors
> address dependencies, then that should be done first and the whole
> thing should just become a simple
> 
>   (a) == (b)
> 
> for that situation - allowing the compiler then the freedom to do
> whatever (which can involve not using a register at all, but a compare
> to memory, or whatever - the compiler might have reasons to avoid the
> 'cmp' and use another sequence entirely)

For the records, here is the relevant compiler discussion:

https://github.com/llvm/llvm-project/issues/34577

and it seems like they still have unfixed scenarios,
which may or may not affect the hazptr use-case:

https://github.com/llvm/llvm-project/issues/220206

Considering this, I would not be inclined to jump to the conclusion
that using the compiler compare for hazptr acquire load/test/reload
is safe today without a careful analysis.

> 
> Then a "if we have an architecture fallback".
> 
> And then that OPTIMIZER_HIDE_VAR() thing as a last fallback.

OK,

Thanks!

Mathieu

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