From: Bill Wendling <morbo@google.com>
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>
Subject: Re: [PATCH v2] x86: use builtins to read eflags
Date: Thu, 3 Feb 2022 16:58:55 -0800 [thread overview]
Message-ID: <CAGG=3QUie3qMTo68e6pgNu=_NDof6eA+h6=VtREm8Do1memOUg@mail.gmail.com> (raw)
In-Reply-To: <87mtj7cwpa.ffs@tglx>
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
next prev parent reply other threads:[~2022-02-04 0:59 UTC|newest]
Thread overview: 63+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20211215211847.206208-1-morbo@google.com>
[not found] ` <87mtl1l63m.ffs@tglx>
2021-12-16 19:55 ` [PATCH] x86: use builtins to read eflags Bill Wendling
2021-12-17 12:48 ` Peter Zijlstra
2021-12-17 19:39 ` Thomas Gleixner
2022-03-14 23:09 ` H. Peter Anvin
2022-03-15 0:08 ` Bill Wendling
[not found] ` <20211215232605.GJ16608@worktop.programming.kicks-ass.net>
2021-12-16 20:00 ` Bill Wendling
2021-12-16 20:07 ` Nick Desaulniers
2021-12-29 2:12 ` [PATCH v2] " Bill Wendling
2022-01-27 20:56 ` Bill Wendling
2022-02-04 0:16 ` Thomas Gleixner
2022-02-04 0:58 ` Bill Wendling [this message]
2022-02-04 0:57 ` [PATCH v3] " Bill Wendling
2022-02-07 22:11 ` Nick Desaulniers
2022-02-08 9:14 ` David Laight
2022-02-08 23:18 ` Bill Wendling
2022-02-14 23:53 ` Nick Desaulniers
2022-02-10 22:31 ` [PATCH v4] " Bill Wendling
2022-02-11 16:40 ` David Laight
2022-02-11 19:25 ` Bill Wendling
2022-02-11 22:09 ` David Laight
2022-02-11 23:33 ` Bill Wendling
2022-02-12 0:24 ` Nick Desaulniers
2022-02-12 9:23 ` Bill Wendling
2022-02-15 0:33 ` Nick Desaulniers
2022-03-01 20:19 ` [PATCH v5] " Bill Wendling
2022-03-14 23:07 ` Bill Wendling
[not found] ` <AC3D873E-A28B-41F1-8BF4-2F6F37BCEEB4@zytor.com>
2022-03-15 7:19 ` Bill Wendling
2022-03-17 15:43 ` H. Peter Anvin
2022-03-17 18:00 ` Nick Desaulniers
2022-03-17 18:52 ` Linus Torvalds
2022-03-17 19:45 ` Bill Wendling
2022-03-17 20:13 ` Linus Torvalds
2022-03-17 21:10 ` Bill Wendling
2022-03-17 21:21 ` Linus Torvalds
2022-03-17 21:45 ` Bill Wendling
2022-03-17 22:51 ` Linus Torvalds
2022-03-17 23:14 ` Linus Torvalds
2022-03-17 23:19 ` Segher Boessenkool
2022-03-17 23:31 ` Linus Torvalds
2022-03-18 0:05 ` Segher Boessenkool
2022-03-17 22:37 ` Segher Boessenkool
2022-03-17 20:13 ` Florian Weimer
2022-03-17 20:36 ` Linus Torvalds
2022-03-18 0:25 ` Segher Boessenkool
2022-03-18 1:21 ` Linus Torvalds
2022-03-18 1:50 ` Linus Torvalds
2022-03-17 21:05 ` Andrew Cooper
2022-03-17 21:39 ` Linus Torvalds
2022-03-18 17:59 ` Andy Lutomirski
2022-03-18 18:19 ` Linus Torvalds
2022-03-18 21:48 ` Andrew Cooper
2022-03-18 23:10 ` Linus Torvalds
2022-03-18 23:42 ` Segher Boessenkool
2022-03-19 1:13 ` Linus Torvalds
2022-03-19 23:15 ` Andy Lutomirski
2022-03-18 22:09 ` Segher Boessenkool
2022-03-18 22:33 ` H. Peter Anvin
2022-03-18 22:36 ` David Laight
2022-03-18 22:47 ` H. Peter Anvin
2022-03-18 22:43 ` David Laight
2022-03-18 23:03 ` H. Peter Anvin
2022-03-18 23:04 ` Segher Boessenkool
2022-03-18 23:52 ` David Laight
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