linux-kernel - Re: [PATCH] [RFC] x86: avoid -mtune=atom with clang

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Date:   Tue, 19 Oct 2021 21:01:41 +0200
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Miguel Ojeda <ojeda@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>, llvm@...ts.linux.dev
Subject: Re: [PATCH] [RFC] x86: avoid -mtune=atom with clang

On Tue, Oct 19, 2021 at 7:11 PM Arnd Bergmann <arnd@...nel.org> wrote:
>
> +ifdef CONFIG_CC_IS_CLANG
> +        cflags-$(CONFIG_MATOM)         += -march=atom -mtune=generic
> +else
>          cflags-$(CONFIG_MATOM)         += -march=atom
> +endif

Is it possible to factor it out? Also, a comment with the LLVM version
we last checked would be good. Something like...

    cflags-$(CONFIG_MATOM)         += -march=atom

    # Atom optimizations do not seem well exercised in, at least, LLVM <= 13,
    # to the point of getting problems with register allocation.
    ifdef CONFIG_CC_IS_CLANG
            cflags-$(CONFIG_MATOM)         += -mtune=generic
    endif

Cheers,
Miguel