From: Alexander Monakov <amonakov@ispras.ru>
To: Uros Bizjak <ubizjak@gmail.com>
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 14:38:28 +0300 (MSK) [thread overview]
Message-ID: <alpine.LNX.2.20.13.2011051353350.9902@monopod.intra.ispras.ru> (raw)
In-Reply-To: <CAFULd4aa1-+KP5A2Zfyjh8b+cnZoJmU8GKDb9auTuMOwt0L2oA@mail.gmail.com>
On Thu, 5 Nov 2020, Uros Bizjak via Gcc wrote:
> > 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.
I see, thanks. But then with the typeof-stripping-address-space solution
you'd be making a very evil cast (producing address of an object that
does not actually exist in the generic address space). I can write such
a solution, but it is clearly Undefined Behavior:
#define strip_as(mem) (*(__typeof(0?(mem):(mem))*)(intptr_t)&(mem))
void foo(__seg_fs int *x)
{
asm("# %0" :: "m"(x[1]));
asm("# %0" :: "m"(strip_as(x[1])));
}
yields
foo:
# %fs:4(%rdi)
# 4(%rdi)
ret
I think a clean future solution is adding a operand modifier that would
print the memory operand without the segment prefix.
Alexander
next prev parent reply other threads:[~2020-11-05 11:38 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 [this message]
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
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