Use of register keyword for Globals

[Joel Sherrill]

Hi

In the SPARC port of RTEMS, there is a global variable assigned to a
register for performance reasons. This is the near decade old line of code:

register struct Per_CPU_Control *_SPARC_Per_CPU_current __asm__( "g6" );

Per https://gcc.gnu.org/onlinedocs/gcc/Global-Register-Variables.html, this
is still the correct way to accomplish this. But -pedantic and specifying
C17 result in this warning:

../../../cpukit/score/cpu/sparc/include/rtems/score/cpuimpl.h:230:17:
warning: file-scope declaration of '_SPARC_Per_CPU_current' specifies
'register' [-Wpedantic]
  230 | register struct Per_CPU_Control *_SPARC_Per_CPU_current __asm__(
"g6" );
      |

We do not want to move this pointer out of a register and the GCC docs
section cited above doesn't mention anything about the register keyword
being moved to reserved keyword in C17 and C++17 with undefined meaning.
The example in the GCC manual is similar to our use.

Is there more current guidance on having a global register variable? Other
than disabling the pedantic warning around that one line, how should the
warning be addressed?

Thanks.

--Joel Sherrill