linux-kernel - Re: [PATCH v4 14/18] static_call: Add static_cond

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Message-Id: <88AE3649-C2E6-4965-86B5-C8FE9D226293@amacapital.net>
Date:   Wed, 6 May 2020 12:57:02 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Jason Baron <jbaron@...mai.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Nadav Amit <namit@...are.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "H.J. Lu" <hjl.tools@...il.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH v4 14/18] static_call: Add static_cond_call()



> On May 6, 2020, at 10:23 AM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> On Wed, May 6, 2020 at 6:51 AM Peter Zijlstra <peterz@...radead.org> wrote:
>> 
>> I was hoping for:
>> 
>>        bar:                                    # @bar
>>                movl    %edi, .L_x$local(%rip)
>>                retq
>>        ponies:                                 # @ponies
>>                movq    .Lfoo$local(%rip), %rax
>>                testq   %rax, %rax
>>                jz      1f
>>                jmpq    *%rcx                   # TAILCALL
>>        1:
>>                retq
> 
> If you want to just avoid the 'cmov', the best way to do that is to
> insert a barrier() on one side of the if-statement.
> 
> That breaks the ability to turn the conditional jump into a cmov.

Having done this in the past, you can get potentially better code with asm volatile (“”); than with a barrier and its memory clobber. I don’t know if that will make a difference in this particular case.