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JS8CBjTP6dqrNWXAQ7eAhycUh4rgAVtKdRoRz2vwxAeE8xiCo1iXlxbZz7gseSi5CHPxarcWz9g7Wsh0OVTWF22n8GFA2e0d6TLBYlyTx3rEEkpdsHYBjzc7iprNEeA/HoZjqCxohJ23VrASqlpvugEBRFSL+kzPR+SQXRal5UfBvizTn2LdRJAL5k2oXGfZ8t1t4v6WKVgrfkiS7xT9eplaPv9jj3+AGw2dmxs2YJWSHbJeUg4XyXJ+Q6vd2oAqPiJh9QbF+IJgcwX+nknKcgqxcaznC3UWbpcYmtY4HIIIN1SSHQFPN3bdYrjrOPdG+WUK/yBj1lOToXARIOPvt7u/PWjc14AFueLth8EV1XhStaKnGSS9CLuY7veRlmjwiAGvxc76/2SEhZ6MW8wcM4EpJyhXthRmaynIBUVbfcv0OQXEGvDDSJLff9OoMyNSMM0War9AarR4lfJHcs0hdb7T/vnIUPjJNRbUp4/q8n5BuZh96d30CisM4Wl40pOFTp87DBtK3XqV7Q6DSGSnL8CdpmQ/YT+YKknO1kBM9c0/KNkCPvkmb+n5hIebUvDwV7Bcw8MBsTyNZihf2EcQPWdhxTnqIItY1WbHo8Aaz/AJk7eEEw1eN7yoQWA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2026-09-29 11:16, Linus Torvalds wrote: > On Tue, 29 Sept 2026 at 08:04, Mathieu Desnoyers > 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