linux-kernel - Re: [PATCH v2] x86: use builtins to read eflags

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Date:   Thu, 27 Jan 2022 12:56:09 -0800
From:   Bill Wendling <morbo@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        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
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86: use builtins to read eflags

Bump for review.


On Tue, Dec 28, 2021 at 6:13 PM Bill Wendling <morbo@...gle.com> 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.
>
> Signed-off-by: Bill Wendling <morbo@...gle.com>
> ---
> v2: - Kept the original function to retain the out-of-line symbol.
>     - Improved the commit message.
>     - Note that I couldn't use Nick's suggestion of
>
>         return IS_ENABLED(CONFIG_X86_64) ? ...
>
>       because Clang complains about using __builtin_ia32_readeflags_u32 in
>       64-bit mode.
> ---
>  arch/x86/include/asm/irqflags.h | 19 +++++--------------
>  1 file changed, 5 insertions(+), 14 deletions(-)
>
> diff --git a/arch/x86/include/asm/irqflags.h b/arch/x86/include/asm/irqflags.h
> index c5ce9845c999..27f919ea7ac3 100644
> --- a/arch/x86/include/asm/irqflags.h
> +++ b/arch/x86/include/asm/irqflags.h
> @@ -19,20 +19,11 @@
>  extern inline unsigned long native_save_fl(void);
>  extern __always_inline unsigned long native_save_fl(void)
>  {
> -       unsigned long flags;
> -
> -       /*
> -        * "=rm" is safe here, because "pop" adjusts the stack before
> -        * it evaluates its effective address -- this is part of the
> -        * documented behavior of the "pop" instruction.
> -        */
> -       asm volatile("# __raw_save_flags\n\t"
> -                    "pushf ; pop %0"
> -                    : "=rm" (flags)
> -                    : /* no input */
> -                    : "memory");
> -
> -       return flags;
> +#ifdef CONFIG_X86_64
> +       return __builtin_ia32_readeflags_u64();
> +#else
> +       return __builtin_ia32_readeflags_u32();
> +#endif
>  }
>
>  static __always_inline void native_irq_disable(void)
> --
> 2.34.1.448.ga2b2bfdf31-goog
>