linux-kernel - Re: [PATCH tip] x86/percpu: Rewrite arch_raw_cpu

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Message-ID: <CAHk-=wgvG1X60i5DQdExmS9DB3-0v3ByDyzLZdewvy6ptf1-+w@mail.gmail.com>
Date:   Fri, 13 Oct 2023 13:07:51 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     Sean Christopherson <seanjc@...gle.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Nadav Amit <namit@...are.com>,
        Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH tip] x86/percpu: Rewrite arch_raw_cpu_ptr()

On Fri, 13 Oct 2023 at 12:30, Uros Bizjak <ubizjak@...il.com> wrote:
>
> There are some other issues when memory access to the percpu area is
> implemented with an asm. An ongoing analysis shows that compilers
> can't CSE asm over basic-block boundaries, the CSE of asm is a very
> simple pattern matching through the BB.

Ahh. That explains the odd "partial CSE".

I was assuming that CSE was done on the SSA level with some dominance
analysis. Which presumably all the load simplification ends up doing.

              Linus