From: Segher Boessenkool <segher@kernel.crashing.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
Uros Bizjak <ubizjak@gmail.com>,
GCC Development <gcc@gcc.gnu.org>, X86 ML <x86@kernel.org>,
Jakub Jelinek <jakub@redhat.com>,
Andy Lutomirski <luto@amacapital.net>,
linux-toolchains@vger.kernel.org,
Christian Borntraeger <borntraeger@de.ibm.com>,
Will Deacon <will@kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Michael Ellerman <mpe@ellerman.id.au>
Subject: Re: typeof and operands in named address spaces
Date: Wed, 11 Nov 2020 18:40:57 -0600 [thread overview]
Message-ID: <20201112004057.GN2672@gate.crashing.org> (raw)
In-Reply-To: <20201110201108.GQ2611@hirez.programming.kicks-ass.net>
On Tue, Nov 10, 2020 at 09:11:08PM +0100, Peter Zijlstra wrote:
> On Tue, Nov 10, 2020 at 10:42:58AM -0800, Nick Desaulniers wrote:
> > When I think of qualifiers, I think of const and volatile. I'm not
> > sure why the first post I'm cc'ed on talks about "segment" qualifiers.
> > Maybe it's in reference to a variable attribute that the kernel
> > defines? Looking at Clang's Qualifier class, I see const, volatile,
> > restrict (ah, right), some Objective-C stuff, and address space
> > (TR18037 is referenced, I haven't looked up what that is) though maybe
> > "segment" pseudo qualifiers the kernel defines expand to address space
> > variable attributes?
>
> Right, x86 Named Address Space:
>
> https://gcc.gnu.org/onlinedocs/gcc-10.2.0/gcc/Named-Address-Spaces.html#Named-Address-Spaces
>
> Also, Google found me this:
>
> https://reviews.llvm.org/D64676
>
> The basic problem seems to be they act exactly like qualifiers in that
> typeof() preserves them, so if you have:
GCC has the four standard type qualifiers (const, volatile, restrict,
and _Atomic), but also the address space things yes.
> > Maybe stripping all qualifiers is fine since you can add them back in
> > if necessary?
>
> So far that seems sufficient. Although the Devil's advocate in me is
> trying to construct a case where we need to preserve const but strip
> volatile and that's then means we need to detect if the original has
> const or not, because unconditionally adding it will be wrong.
If you want to drop all qualifiers, you only need a way to convert
something to an rvalue (which always has an unqualified type). So maybe
make syntax for just *that*? __builtin_unqualified() perhaps? Which
could be useful in more places than just doing an unqualified_typeof.
Segher
next prev parent reply other threads:[~2020-11-12 0:44 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
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 [this message]
2020-11-12 0:47 ` Segher Boessenkool
2020-11-10 7:52 ` Peter Zijlstra
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