Re: typeof and operands in named address spaces

[Uros Bizjak <ubizjak@gmail.com>]

On Thu, Nov 5, 2020 at 8:26 AM Richard Biener
<richard.guenther@gmail.com> wrote:
>
> On Wed, Nov 4, 2020 at 7:33 PM Uros Bizjak via Gcc <gcc@gcc.gnu.org> wrote:
> >
> > Hello!
> >
> > I was looking at the recent linux patch series [1] where segment
> > qualifiers (named address spaces) were introduced to handle percpu
> > variables. In the patch [2], the author mentions that:
> >
> > --q--
> > Unfortunately, gcc does not provide a way to remove segment
> > qualifiers, which is needed to use typeof() to create local instances
> > of the per-cpu variable. For this reason, do not use the segment
> > qualifier for per-cpu variables, and do casting using the segment
> > qualifier instead.
> > --/q--
> >
> > The core of the problem can be seen with the following testcase:
> >
> > --cut here--
> > #define foo(_var)                    \
> >   ({                            \
> >     typeof(_var) tmp__;                    \
>
> 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:

--cut here--
typedef struct { short a; short b; } pair_t;

#define foo(_var)                     \
  ({                             \
    typeof((typeof(_var))0) tmp__;             \
    asm ("mov %1, %0" : "=r"(tmp__) : "m"(_var));    \
    tmp__;                         \
  })

__seg_fs pair_t x;

pair_t
test (void)
{
  pair_t y;

  y = foo (x);
  return y;
}
--cut here--

So, what about introducing e.g. typeof_noas (not sure about the name)
that would simply strip the address space from typeof?

> Basically I try to get at a rvalue for the typeof.
>
> Is there a way to query the address space of an object so I can
> put another variable in the same address space?

I think that would go hand in hand with the above typeof_noas. Perhaps
typeof_as, that would return the address space of the variable?

> >     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__’
> >
> > It looks to me that the compiler should remove address space
> > information when typeof is used, otherwise, there is no way to use
> > typeof as intended in the above example.
> >
> > A related problem is exposed when we want to cast address from the
> > named address space to a generic address space (e.g. to use it with
> > LEA):
> >
> > --cut here--
> > typedef __UINTPTR_TYPE__ uintptr_t;
> >
> > __seg_fs int x;
> >
> > uintptr_t test (void)
> > {
> >   uintptr_t *p = (uintptr_t *) &y;
>
>    uintptr_t *p = (uintptr_t *)(uintptr_t) &y;

Indeed, this works as expected.

> 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:

as1.c:10:49: error: memory input 1 is not directly addressable

Uros.