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 <87mtj7cwpa.ffs@tglx>
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From:   Bill Wendling <morbo@google.com>
Date:   Thu, 3 Feb 2022 16:58:55 -0800
Message-ID: <CAGG=3QUie3qMTo68e6pgNu=_NDof6eA+h6=VtREm8Do1memOUg@mail.gmail.com>
Subject: Re: [PATCH v2] x86: use builtins to read eflags
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Juergen Gross <jgross@suse.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>, llvm@lists.linux.dev,
        LKML <linux-kernel@vger.kernel.org>
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On Thu, Feb 3, 2022 at 4:16 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Tue, Dec 28 2021 at 18:12, Bill Wendling wrote:
> > GCC and Clang both have builtins to read and write the EFLAGS register.
> > This allows the compiler to determine the best way to generate this
> > code, which can improve code generation.
> >
> > This issue arose due to Clang's issue with the "=rm" constraint.  Clang
> > chooses to be conservative in these situations, and so uses memory
> > instead of registers. This is a known issue, which is currently being
> > addressed.
> >
> > However, using builtins is benefiical in general, because it removes the
> > burden of determining what's the way to read the flags register from the
> > programmer and places it on to the compiler, which has the information
> > needed to make that decision. Indeed, this piece of code has had several
> > changes over the years, some of which were pinging back and forth to
> > determine the correct constraints to use.
> >
> > With this change, Clang generates better code:
> >
> > Original code:
> >         movq    $0, -48(%rbp)
> >         #APP
> >         # __raw_save_flags
> >         pushfq
> >         popq    -48(%rbp)
> >         #NO_APP
> >         movq    -48(%rbp), %rbx
> >
> > New code:
> >         pushfq
> >         popq    %rbx
> >         #APP
> >
> > Note that the stack slot in the original code is no longer needed in the
> > new code, saving a small amount of stack space.
>
> This still lacks any information about the effect on GCC. There is a
> world outside clang. It's not my job to validate that.
>
I never said you were the one required to validate that. I'm not sure
where you got that idea from. There of course is no change to GCC's
output. I updated the commit message and sent out v3. PTAL.

-bw