Re: typeof and operands in named address spaces

[Peter Zijlstra <peterz@infradead.org>]

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:

( and now I realize the parent isn't Cc'd to LKML, find here:
  https://gcc.gnu.org/pipermail/gcc/2020-November/234119.html )

> --cut here--
> #define foo(_var)                    \
> ({                            \
> typeof(_var) tmp__;                    \
> asm ("mov %1, %0" : "=r"(tmp__) : "m"(_var));    \
> tmp__;                        \
> })
>
> __seg_fs int x;
>
> int test (void)
> {
> int y;
>
> y = foo (x);
> return y;
> }
> --cut here--

> when compiled with -O2 for x86 target, the compiler reports:
>
> pcpu.c: In function ‘test’:
> pcpu.c:14:3: error: ‘__seg_fs’ specified for auto variable ‘tmp__’


> 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.