From: Uros Bizjak <ubizjak@gmail.com>
To: Alexander Monakov <amonakov@ispras.ru>
Cc: Jakub Jelinek <jakub@redhat.com>, X86 ML <x86@kernel.org>,
Andy Lutomirski <luto@amacapital.net>,
GCC Development <gcc@gcc.gnu.org>
Subject: Re: typeof and operands in named address spaces
Date: Thu, 5 Nov 2020 13:26:13 +0100 [thread overview]
Message-ID: <CAFULd4b_kBKf0i7f1ytyyJpGwdoWfiGjJ+aY4PLy5cyq5JWxVA@mail.gmail.com> (raw)
In-Reply-To: <alpine.LNX.2.20.13.2011051506090.9902@monopod.intra.ispras.ru>
On Thu, Nov 5, 2020 at 1:14 PM Alexander Monakov <amonakov@ispras.ru> wrote:
> > I was also thinking of introducing of operand modifier, but Richi
> > advises the following:
> >
> > --cut here--
> > typedef __UINTPTR_TYPE__ uintptr_t;
> >
> > __seg_fs int x;
> >
> > uintptr_t test (void)
> > {
> > uintptr_t *p = (uintptr_t *)(uintptr_t) &x;
> > uintptr_t addr;
> >
> > asm volatile ("lea %1, %0" : "=r"(addr) : "m"(*p));
> >
> > return addr;
> > }
>
> This is even worse undefined behavior compared to my solution above:
> this code references memory in uintptr_t type, while mine preserves the
> original type via __typeof. So this can visibly break with TBAA (though
> the kernel uses -fno-strict-aliasing, so this particular concern wouldn't
> apply there).
Agreed, but I was trying to solve this lone use case in the kernel. It
fits this particular usage, so I found a bit of overkill to implement
the otherwise useless operand modifier in gcc. As discussed
previously, these hacks are needed exclusively in asm templates, they
are not needed in "normal" C code.
>
> If you don't care about preserving sizeof and type you can use a cast to char:
>
> #define strip_as(mem) (*(char *)(intptr_t)&(mem))
I hope that a developer from kernel can chime in and express their
opinion on the proposed approaches.
Uros.
next prev parent reply other threads:[~2020-11-05 12:26 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
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 [this message]
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
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