linux-kernel - RE: [PATCH v5] x86: use builtins to read eflags

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Message-ID: <93EDA96C-0EAD-4C40-ABEA-28C8BF76A9F9@zytor.com>
Date:   Fri, 18 Mar 2022 15:47:49 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     David Laight <David.Laight@...LAB.COM>,
        "'Segher Boessenkool'" <segher@...nel.crashing.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
CC:     Andy Lutomirski <luto@...nel.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Bill Wendling <morbo@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-toolchains <linux-toolchains@...r.kernel.org>
Subject: RE: [PATCH v5] x86: use builtins to read eflags

On March 18, 2022 3:36:48 PM PDT, David Laight <David.Laight@...LAB.COM> wrote:
>From: Segher Boessenkool
>> Sent: 18 March 2022 22:09
>...
>> It generally is a very good idea to
>> have a redzone though, without it you pay much more than necessary for
>> frame setup and teardown in leaf functions (similar to some of what the
>> misnamed "shrink-wrapping" optimisation does, but the two are mostly
>> independent, the benefits add up).
>
>Are there really leaf functions that need to spill data to stack
>where the cost of setting up a stack frame is significant?
>
>I'd have thought that was relatively rare.
>
>	David
>
>-
>Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
>Registration No: 1397386 (Wales)
>
>

Yes.