linux-kernel - Re: [PATCH] x86: use builtins to read eflags

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Date:   Fri, 17 Dec 2021 20:39:46 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Bill Wendling <morbo@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>, llvm@...ts.linux.dev,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: use builtins to read eflags

Bill,

On Thu, Dec 16 2021 at 11:55, Bill Wendling wrote:
> On Wed, Dec 15, 2021 at 4:57 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>> Emphasis on *can*. Just claiming that this might improve things does not
>> cut it. Where is the prove?
>>
> There are a few proofs. First, clang generates better code with the
> builtin.

which is best demonstrated by showing the before and after.

> Yes, that's because clang doesn't handle the "=rm" constraint
> in the same way that GCC does, but that's not really relevant (sure,
> clang should correct this, but that shouldn't prevent this patch from
> going, because builtins are generally better than inline assembly).
> Builtins exist for a reason. The compiler's able to understand what's
> going on and generate the appropriate code for it. It also gives the
> compiler more freedom for optimizations.
>
> Secondly, this one small function has had multiple changes since its
> creation, basically pinging back and forth trying to determine the
> best constraints to use:
>
>   6abcd98f x86: irqflags consolidation
>   f1f029c7 x86: fix assembly constraints in native_save_fl()
>   ab94fcf5 x86: allow "=rm" in native_save_fl()
>
> The information on which form to use already exists in the compiler.
> Using the builtin will save future churning and thus developers' time.

Why is the above and this

> The minimal version of GCC is now 5.1, which supports these builtins.
> That wasn't the case before.

not part of the change log to avoid maintainers having to ask exactly
these questions?

Thanks,

        tglx