[Bug target/86673] [8/9 regression] inline asm sometimes ignores 'register asm("reg")' declarations

[arnd at linaro dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86673

--- Comment #11 from Arnd Bergmann <arnd at linaro dot org> ---
I have checked all instances of 'register const' or 'const register' in the
current linux kernel (4.18-rc), and we never assign a constant expression to
any of them, so I guess none of them are affected:

arch/arm/include/asm/uaccess.h:         register const void __user *__p
asm("r0") = __ptr;      \
arch/h8300/kernel/sim-console.c:        register const char *_ptr
__asm__("er1") = s;
arch/h8300/kernel/sim-console.c:        register const unsigned _len
__asm__("er2") = n;
arch/mips/include/asm/uaccess.h:        register const void __user *__cu_from_r
__asm__("$5");          \
arch/mips/include/asm/uaccess.h:        register const void *__cu_from_r
__asm__("$5");                 \
arch/riscv/kernel/process.c:            const register unsigned long gp __asm__
("gp");
arch/riscv/kernel/stacktrace.c:         const register unsigned long current_sp
__asm__ ("sp");
arch/riscv/kernel/stacktrace.c:         const register unsigned long current_sp
__asm__ ("sp");

Should we drop the 'const' for all of them as a rule? If there is no use case
for ever using a 'const register' variable and it can lead to bugs, should gcc
warn about it in the future?

Should this issue be mentioned in the documentation in
https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html?

I also checked all instances in linux-4.4, and the ARM put_user() helper is the
only one I see that gets a constant expression input, so I suppose that is all
that needs to be fixed in backports, unless someone thinks we should get rid of
all them in backports as well.