typeof and operands in named address spaces

Uros Bizjak ubizjak@gmail.com
Wed Nov 4 18:31:42 GMT 2020


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__;                    \
    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 addr;

  asm volatile ("lea %1, %0" : "=r"(addr) : "m"(*p));

  return addr;
}
--cut here--

The gcc documentation advises explicit casts:

--q--
This means that explicit casts are required to convert pointers
between these address spaces and the generic address space.  In
practice the application should cast to 'uintptr_t' and apply the
segment base offset that it installed previously.
--/q--

However, a warning is emitted when compiling the above example:

pcpu1.c: In function ‘test’:
pcpu1.c:7:18: warning: cast to generic address space pointer from
disjoint __seg_fs address space pointer

but the desired result is obtained nevertheless.

       lea x(%rip), %rax

As shown in the referred patchset, named address spaces have quite
some optimization potential, please see [1] for the list.

[1] https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg2053461.html
[2] https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg2053462.html

Uros.


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