error: couldn't allocate input reg for constraint 'r' on MIPS when building the Linux kernel

[nathanchance]

When compiling the Linux kernel for ARCH=mips, there is an error in drivers/misc/xilinx_sdfec.c:

drivers/misc/xilinx_sdfec.c:787:8: error: couldn't allocate input reg for constraint 'r'
        err = put_user(is_active, arg);
              ^
./arch/mips/include/asm/uaccess.h:112:33: note: expanded from macro 'put_user'
        access_ok(__p, sizeof(*__p)) ? __put_user((x), __p) : -EFAULT;  \
                                       ^
./arch/mips/include/asm/uaccess.h:171:3: note: expanded from macro '__put_user'
                __put_data_asm(user_sb, __pu_ptr);                      \
                ^
./arch/mips/include/asm/uaccess.h:344:2: note: expanded from macro '__put_data_asm'
        "1:     "insn("%z2", "%3")"     # __put_data_asm        \n"     \
        ^
1 error generated.

Reduced C reproducer (thanks to @telans):

_Bool a;
void b() {
  __asm__("" : : "Jr"(a));
}

$ clang --target=mips64el-linux-gnuabi64 -c test.c
test.c:3:11: error: couldn't allocate input reg for constraint 'r'
  __asm__("" : : "Jr"(a));
          ^
1 error generated.

Reduced LLVM IR reproducer:

target triple = "mips64el-unknown-linux-gnuabi64"

define void @b() {
entry:
  tail call void asm sideeffect "", "Jr,~{$1}"(i1 true)
  ret void
}

$ llc test.ll
error: couldn't allocate input reg for constraint 'r'

[nathanchance]

cc @atanasyan (apologies if you were already aware, I know Bugzilla would have auto CC'd you but I don't think that works on GitHub just yet).

[nathanchance]

Thanks Simon!

2de9338