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

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Date:   Fri, 18 Mar 2022 22:36:48 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     '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>,
        "H. Peter Anvin" <hpa@...or.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

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

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