From: Uros Bizjak <ubizjak@gmail.com>
To: Alexander Monakov <amonakov@ispras.ru>
Cc: Richard Biener <richard.guenther@gmail.com>,
Jakub Jelinek <jakub@redhat.com>,
GCC Development <gcc@gcc.gnu.org>, X86 ML <x86@kernel.org>,
Andy Lutomirski <luto@amacapital.net>
Subject: Re: typeof and operands in named address spaces
Date: Thu, 5 Nov 2020 11:33:26 +0100 [thread overview]
Message-ID: <CAFULd4aa1-+KP5A2Zfyjh8b+cnZoJmU8GKDb9auTuMOwt0L2oA@mail.gmail.com> (raw)
In-Reply-To: <alpine.LNX.2.20.13.2011051224220.9902@monopod.intra.ispras.ru>
On Thu, Nov 5, 2020 at 10:36 AM Alexander Monakov <amonakov@ispras.ru> wrote:
>
> On Thu, 5 Nov 2020, Uros Bizjak via Gcc wrote:
>
> > > Looks like writing
> > >
> > > typeof((typeof(_var))0) tmp__;
> > >
> > > makes it work. Assumes there's a literal zero for the type of course.
> >
> > This is very limiting assumption, which already breaks for the following test:
>
> To elaborate Richard's idea, you need a way to decay lvalue to rvalue inside
> the typeof to strip the address space; if you need the macro to work for
> more types than just scalar types, the following expression may be useful:
>
> typeof(0?(_var):(_var))
>
> (though there's a bug: +(_var) should also suffice for scalar types, but
> somehow GCC keeps the address space on the resulting rvalue)
>
> But I wonder if you actually need this at all:
>
> > > works around the warning. I think the wording you cite
> > > suggests (uintptr_t) &y here, not sure if there's a reliable
> > > way to get the lea with just a uintptr_t operand though.
> >
> > No, because we have to use the "m" constraint for the LEA. We get the
> > following error:
>
> What is the usecase for stripping the address space for asm operands?
Please see the end of [2], where the offset to <mem> is passed in %rsi
to the call to this_cpu_cmpxchg16b_emu. this_cpu_cmpxchg16b_emu
implements access with PER_CPU_VAR((%rsi)), which expands to
%gs:(%rsi), so it is the same as %gs:<mem> in cmpxchg16b alternative.
The offset is loaded by lea <mem>, %rsi to %rsi reg.
> From reading the patch I understand the kernel wants to pass qualified
> lvalues to inline assembly to get
>
> lea <reg>, %fs:<mem>
No, this will emit an assembler warning that "segment override on
'lea' is ineffectual".
Uros.
> LEA without the %fs will produce the offset within the segment, which
> you can obtain simply by casting the pointer to intptr_t in the first place.
> Alexander
next prev parent reply other threads:[~2020-11-05 10:33 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-04 18:31 Uros Bizjak
2020-11-05 7:26 ` Richard Biener
2020-11-05 8:56 ` Uros Bizjak
2020-11-05 9:36 ` Alexander Monakov
2020-11-05 10:33 ` Uros Bizjak [this message]
2020-11-05 11:38 ` Alexander Monakov
2020-11-05 12:00 ` Uros Bizjak
2020-11-05 12:14 ` Alexander Monakov
2020-11-05 12:24 ` Richard Biener
2020-11-05 12:32 ` Uros Bizjak
2020-11-05 12:35 ` Uros Bizjak
2020-11-05 13:22 ` Alexander Monakov
2020-11-05 13:39 ` Alexander Monakov
2020-11-05 13:46 ` Uros Bizjak
2020-11-05 12:26 ` Uros Bizjak
2020-11-05 15:27 ` Andy Lutomirski
2020-11-05 11:03 ` Uros Bizjak
2020-11-05 9:45 ` Richard Biener
2020-11-05 9:51 ` Jakub Jelinek
2020-11-09 12:47 ` Peter Zijlstra
2020-11-09 19:38 ` Segher Boessenkool
2020-11-09 19:50 ` Nick Desaulniers
2020-11-10 7:57 ` Peter Zijlstra
2020-11-10 18:42 ` Nick Desaulniers
2020-11-10 20:11 ` Peter Zijlstra
2020-11-12 0:40 ` Segher Boessenkool
2020-11-12 0:47 ` Segher Boessenkool
2020-11-10 7:52 ` Peter Zijlstra
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=CAFULd4aa1-+KP5A2Zfyjh8b+cnZoJmU8GKDb9auTuMOwt0L2oA@mail.gmail.com \
--to=ubizjak@gmail.com \
--cc=amonakov@ispras.ru \
--cc=gcc@gcc.gnu.org \
--cc=jakub@redhat.com \
--cc=luto@amacapital.net \
--cc=richard.guenther@gmail.com \
--cc=x86@kernel.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
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).