On 2026-09-29 11:16, Linus Torvalds wrote:
On Tue, 29 Sept 2026 at 08:04, Mathieu Desnoyers <[email protected]> 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
