LKML Archive on lore.kernel.org
 help / color / Atom feed
From: Willy Tarreau <w@1wt.eu>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Matthew Wilcox <willy@infradead.org>,
	Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
	Wedson Almeida Filho <wedsonaf@google.com>,
	Miguel Ojeda <ojeda@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	rust-for-linux@vger.kernel.org,
	Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	Linux Doc Mailing List <linux-doc@vger.kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 00/13] [RFC] Rust support
Date: Sat, 17 Apr 2021 13:46:23 +0200
Message-ID: <20210417114623.GA15120@1wt.eu> (raw)
In-Reply-To: <YHrDwdQwEk2mSQWa@hirez.programming.kicks-ass.net>

On Sat, Apr 17, 2021 at 01:17:21PM +0200, Peter Zijlstra wrote:
> Well, I think the rules actually make sense, at the point in the syntax
> tree where + happens, we have 'unsigned char' and 'int', so at that
> point we promote to 'int'. Subsequently 'int' gets shifted and bad
> things happen.

That's always the problem caused by signedness being applied to the
type while modern machines do not care about that and use it during
(or even after) the operation instead :-/

We'd need to define some macros to zero-extend and sign-extend some
values to avoid such issues. I'm sure this would be more intuitive
than trying to guess how many casts (and in what order) to place to
make sure an operation works as desired.

> The 'unsigned long' doesn't happen until quite a bit later.
> 
> Anyway, the rules are imo fairly clear and logical, but yes they can be
> annoying. The really silly thing here is that << and >> have UB at all,
> and I would love a -fwrapv style flag that simply defines it. Yes it
> will generate worse code in some cases, but having the UB there is just
> stupid.

I'd also love to have a UB-less mode with well defined semantics for
plenty of operations that are known to work well on modern machines,
like integer wrapping, bit shifts ignoring higher bits etc. Lots of
stuff we often have to write useless code for, just to please the
compiler.

> That of course doesn't help your case here, it would simply misbehave
> and not be UB.
> 
> Another thing the C rules cannot really express is a 32x32->64
> multiplication, some (older) versions of GCC can be tricked into it, but
> mostly it just doesn't want to do that sanely and the C rules are
> absolutely no help there.

For me the old trick of casting one side as long long still works:

  unsigned long long mul3264(unsigned int a, unsigned int b)
  {
        return (unsigned long long)a * b;
  }

i386:
  00000000 <mul3264>:
     0: 8b 44 24 08           mov    0x8(%esp),%eax
     4: f7 64 24 04           mull   0x4(%esp)
     8: c3                    ret    

x86_64:
  0000000000000000 <mul3264>:
     0: 89 f8                 mov    %edi,%eax
     2: 89 f7                 mov    %esi,%edi
     4: 48 0f af c7           imul   %rdi,%rax
     8: c3                    retq   

Or maybe you had something else in mind ?

Willy

  reply index

Thread overview: 186+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-14 18:45 ojeda
2021-04-14 18:45 ` [PATCH 01/13] kallsyms: Support "big" kernel symbols (2-byte lengths) ojeda
2021-04-14 19:44   ` Matthew Wilcox
2021-04-14 19:59     ` Miguel Ojeda
2021-04-14 18:45 ` [PATCH 02/13] kallsyms: Increase maximum kernel symbol length to 512 ojeda
2021-04-14 23:48   ` Nick Desaulniers
2021-04-14 18:45 ` [PATCH 03/13] Makefile: Generate CLANG_FLAGS even in GCC builds ojeda
2021-04-14 18:59   ` Nathan Chancellor
2021-04-15 10:18     ` Miguel Ojeda
2021-04-14 23:46   ` Nick Desaulniers
2021-04-15  0:47     ` Miguel Ojeda
2021-04-14 18:45 ` [PATCH 04/13] Kbuild: Rust support ojeda
2021-04-14 23:19   ` Nick Desaulniers
2021-04-15  0:43     ` Miguel Ojeda
2021-04-15 18:03       ` Nick Desaulniers
2021-04-16 12:23         ` Miguel Ojeda
2021-04-17 19:35       ` Masahiro Yamada
2021-04-16 13:38   ` Peter Zijlstra
2021-04-16 17:05     ` Linus Torvalds
2021-04-16 17:47       ` Miguel Ojeda
2021-04-16 18:09         ` Al Viro
2021-04-16 18:57           ` Miguel Ojeda
2021-04-16 20:22             ` Willy Tarreau
2021-04-16 20:34               ` Connor Kuehl
2021-04-16 20:58                 ` Willy Tarreau
2021-04-16 21:39                   ` Miguel Ojeda
2021-04-16 22:04                     ` Willy Tarreau
2021-04-16 22:45                       ` Al Viro
2021-04-16 23:46                       ` Miguel Ojeda
2021-04-17  4:24                         ` Willy Tarreau
2021-04-17 15:38                           ` Miguel Ojeda
2021-04-16 21:19               ` Miguel Ojeda
2021-04-16 17:34     ` Miguel Ojeda
2021-04-19 19:58       ` David Sterba
2021-04-19 20:17         ` Matthew Wilcox
2021-04-19 21:03           ` Miguel Ojeda
2021-04-19 20:54         ` Miguel Ojeda
2021-04-14 18:45 ` [PATCH 05/13] Rust: Compiler builtins crate ojeda
2021-04-14 19:19   ` Linus Torvalds
2021-04-14 19:34     ` Miguel Ojeda
2021-04-14 18:45 ` [PATCH 06/13] Rust: Module crate ojeda
2021-04-14 18:45 ` [PATCH 07/13] Rust: Kernel crate ojeda
2021-04-14 19:31   ` Linus Torvalds
2021-04-14 19:50     ` Miguel Ojeda
2021-04-14 18:45 ` [PATCH 08/13] Rust: Export generated symbols ojeda
2021-04-14 18:46 ` [PATCH 09/13] Samples: Rust examples ojeda
2021-04-14 19:34   ` Linus Torvalds
2021-04-14 19:42     ` Miguel Ojeda
2021-04-14 19:49       ` Matthew Wilcox
2021-04-16 11:46       ` Andrej Shadura
2021-04-14 23:24     ` Nick Desaulniers
2021-04-15  7:10       ` Greg Kroah-Hartman
2021-04-15  7:39         ` Nick Desaulniers
2021-04-15 12:42         ` Miguel Ojeda
2021-04-16 13:07         ` Sven Van Asbroeck
2021-04-16 13:20           ` Greg Kroah-Hartman
2021-04-14 18:46 ` [PATCH 10/13] Documentation: Rust general information ojeda
2021-04-14 22:17   ` Nick Desaulniers
2021-04-14 23:34     ` Miguel Ojeda
2021-04-14 18:46 ` [PATCH 11/13] MAINTAINERS: Rust ojeda
2021-04-14 21:55   ` Nick Desaulniers
2021-04-14 22:02     ` Miguel Ojeda
2021-04-14 22:36   ` Nick Desaulniers
2021-04-14 18:46 ` [PATCH 12/13] Rust: add abstractions for Binder (WIP) ojeda
2021-04-14 18:46 ` [PATCH 13/13] Android: Binder IPC in Rust (WIP) ojeda
2021-04-14 19:44 ` [PATCH 00/13] [RFC] Rust support Linus Torvalds
2021-04-14 20:20   ` Miguel Ojeda
2021-04-15  1:38     ` Kees Cook
2021-04-15  8:26       ` David Laight
2021-04-15 18:08         ` Kees Cook
2021-04-15 12:39       ` Miguel Ojeda
2021-04-14 20:09 ` Matthew Wilcox
2021-04-14 20:21   ` Linus Torvalds
2021-04-14 20:35     ` Josh Triplett
2021-04-14 22:08     ` David Laight
2021-04-14 20:29   ` Miguel Ojeda
2021-04-18 15:31   ` Wedson Almeida Filho
2021-04-15  0:22 ` Nick Desaulniers
2021-04-15 10:05   ` Miguel Ojeda
2021-04-15 18:58 ` Peter Zijlstra
2021-04-16  2:22   ` Wedson Almeida Filho
2021-04-16  4:25     ` Al Viro
2021-04-16  5:02       ` Wedson Almeida Filho
2021-04-16  5:39         ` Paul Zimmerman
2021-04-16  7:46         ` Peter Zijlstra
2021-04-16  7:09     ` Peter Zijlstra
2021-04-17  5:23       ` comex
2021-04-17 12:46       ` David Laight
2021-04-17 14:51       ` Paolo Bonzini
2021-04-19  7:32         ` Peter Zijlstra
2021-04-19  7:53           ` Paolo Bonzini
2021-04-19  8:26             ` Peter Zijlstra
2021-04-19  8:35               ` Peter Zijlstra
2021-04-19  9:02               ` Paolo Bonzini
2021-04-19  9:36                 ` Peter Zijlstra
2021-04-19  9:40                   ` Paolo Bonzini
2021-04-19 11:01                     ` Will Deacon
2021-04-19 17:14                   ` Linus Torvalds
2021-04-19 18:38                     ` Paolo Bonzini
2021-04-19 18:50                       ` Linus Torvalds
2021-04-22 10:03     ` Linus Walleij
2021-04-22 14:09       ` David Laight
2021-04-22 15:24       ` Wedson Almeida Filho
2021-04-26  0:18         ` Linus Walleij
2021-04-26 14:26           ` Miguel Ojeda
2021-04-26 14:40           ` Wedson Almeida Filho
2021-04-26 16:03             ` Miguel Ojeda
2021-04-27 10:54             ` Linus Walleij
2021-04-27 11:13               ` Robin Randhawa
2021-04-29  1:52               ` Wedson Almeida Filho
2021-04-26 18:01           ` Miguel Ojeda
2021-04-22 21:28       ` Miguel Ojeda
2021-04-26  0:31         ` Linus Walleij
2021-04-26 18:18           ` Miguel Ojeda
2021-04-27 11:13             ` Linus Walleij
2021-04-28  2:51               ` Kyle Strand
2021-04-28  3:10               ` Miguel Ojeda
2021-05-04 21:21                 ` Linus Walleij
2021-05-04 23:30                   ` Miguel Ojeda
2021-05-05 11:34                     ` Linus Walleij
2021-05-05 14:17                       ` Miguel Ojeda
2021-05-05 15:13                         ` Enrico Weigelt, metux IT consult
2021-05-06 12:47                         ` Linus Walleij
2021-05-07 18:23                           ` Miguel Ojeda
2021-04-16  4:27   ` Boqun Feng
2021-04-16  6:04     ` Nick Desaulniers
2021-04-16 18:47       ` Paul E. McKenney
2021-04-19 20:35         ` Nick Desaulniers
2021-04-19 21:37           ` Paul E. McKenney
2021-04-19 22:03           ` Miguel Ojeda
2021-04-16 20:48     ` Josh Triplett
2021-04-16  8:16   ` Michal Kubecek
2021-04-16  9:29     ` Willy Tarreau
2021-04-16 11:24 ` Peter Zijlstra
2021-04-16 13:07   ` Wedson Almeida Filho
2021-04-16 14:19     ` Peter Zijlstra
2021-04-16 15:04       ` Miguel Ojeda
2021-04-16 15:43         ` Peter Zijlstra
2021-04-16 16:21           ` Miguel Ojeda
2021-04-16 15:33       ` Wedson Almeida Filho
2021-04-16 16:14         ` Willy Tarreau
2021-04-16 17:10           ` Miguel Ojeda
2021-04-16 17:18             ` Peter Zijlstra
2021-04-16 18:08               ` Matthew Wilcox
2021-04-17 11:17                 ` Peter Zijlstra
2021-04-17 11:46                   ` Willy Tarreau [this message]
2021-04-17 14:24                     ` Peter Zijlstra
2021-04-17 14:36                       ` Willy Tarreau
2021-04-17 13:46                   ` David Laight
2021-04-16 17:37             ` Willy Tarreau
2021-04-16 17:46               ` Connor Kuehl
2021-04-20  0:24               ` Nick Desaulniers
2021-04-20  3:47                 ` Willy Tarreau
2021-04-20  5:56                 ` Greg Kroah-Hartman
2021-04-20  6:16                   ` Willy Tarreau
2021-04-29 15:38                     ` peter enderborg
2021-04-17 13:53           ` Wedson Almeida Filho
2021-04-17 14:21             ` Willy Tarreau
2021-04-17 15:23               ` Miguel Ojeda
2021-04-18 15:51               ` Wedson Almeida Filho
2021-04-17 12:41       ` David Laight
2021-04-17 13:01         ` Wedson Almeida Filho
2021-04-16 15:03     ` Matthew Wilcox
2021-04-17 13:29       ` Wedson Almeida Filho
2021-04-16 15:58     ` Theodore Ts'o
2021-04-16 16:21       ` Wedson Almeida Filho
2021-04-17 15:11       ` Paolo Bonzini
2021-04-16 14:21   ` Miguel Ojeda
2021-04-17 20:42 ` Richard Weinberger
2021-04-28 18:34 ` Mariusz Ceier
2021-04-28 20:25   ` Nick Desaulniers
2021-04-28 21:21   ` David Laight
2021-04-29 11:14     ` Kajetan Puchalski
2021-04-29 11:25   ` Kajetan Puchalski
2021-04-29 14:06     ` Mariusz Ceier
2021-04-29 14:13       ` Sven Van Asbroeck
2021-04-29 14:26         ` Willy Tarreau
2021-04-29 15:06       ` Al Viro
2021-04-29 16:09         ` Mariusz Ceier
2021-04-30  6:39     ` Thomas Schoebel-Theuer
2021-04-30  8:30       ` David Laight
2021-05-05 13:58       ` Enrico Weigelt, metux IT consult
2021-05-05 14:41         ` Miguel Ojeda
2021-04-29  5:20 Mariusz Ceier
2021-04-29  5:21 Mariusz Ceier
2021-04-29  8:18 ` David Laight

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20210417114623.GA15120@1wt.eu \
    --to=w@1wt.eu \
    --cc=gregkh@linuxfoundation.org \
    --cc=linux-doc@vger.kernel.org \
    --cc=linux-kbuild@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=miguel.ojeda.sandonis@gmail.com \
    --cc=ojeda@kernel.org \
    --cc=peterz@infradead.org \
    --cc=rust-for-linux@vger.kernel.org \
    --cc=torvalds@linux-foundation.org \
    --cc=wedsonaf@google.com \
    --cc=willy@infradead.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/lkml/0 lkml/git/0.git
	git clone --mirror https://lore.kernel.org/lkml/1 lkml/git/1.git
	git clone --mirror https://lore.kernel.org/lkml/2 lkml/git/2.git
	git clone --mirror https://lore.kernel.org/lkml/3 lkml/git/3.git
	git clone --mirror https://lore.kernel.org/lkml/4 lkml/git/4.git
	git clone --mirror https://lore.kernel.org/lkml/5 lkml/git/5.git
	git clone --mirror https://lore.kernel.org/lkml/6 lkml/git/6.git
	git clone --mirror https://lore.kernel.org/lkml/7 lkml/git/7.git
	git clone --mirror https://lore.kernel.org/lkml/8 lkml/git/8.git
	git clone --mirror https://lore.kernel.org/lkml/9 lkml/git/9.git
	git clone --mirror https://lore.kernel.org/lkml/10 lkml/git/10.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ https://lore.kernel.org/lkml \
		linux-kernel@vger.kernel.org
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kernel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git