[PATCH v2] mips: lib: uncached: fix non-standard usage of variable 'sp'

[Anders Roxell <anders.roxell@linaro.org>]

When building mips tinyconfig with clang the following warning show up:

arch/mips/lib/uncached.c:45:6: warning: variable 'sp' is uninitialized when used here [-Wuninitialized]
        if (sp >= (long)CKSEG0 && sp < (long)CKSEG2)
            ^~
arch/mips/lib/uncached.c:40:18: note: initialize the variable 'sp' to silence this warning
        register long sp __asm__("$sp");
                        ^
                         = 0
1 warning generated.

Rework to make an explicit inline move, instead of the non-standard use
of specifying registers for local variables. This is what's written
from the gcc-10 manual [1] about specifying registers for local
variables:

"6.47.5.2 Specifying Registers for Local Variables
.................................................
[...]

"The only supported use for this feature is to specify registers for
input and output operands when calling Extended 'asm' (*note Extended
Asm::).  [...]".

[1] https://docs.w3cub.com/gcc~10/local-register-variables
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
---
 arch/mips/lib/uncached.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/mips/lib/uncached.c b/arch/mips/lib/uncached.c
index 09d5deea747f..f80a67c092b6 100644
--- a/arch/mips/lib/uncached.c
+++ b/arch/mips/lib/uncached.c
@@ -37,10 +37,12 @@
  */
 unsigned long run_uncached(void *func)
 {
-	register long sp __asm__("$sp");
 	register long ret __asm__("$2");
 	long lfunc = (long)func, ufunc;
 	long usp;
+	long sp;
+
+	__asm__("move %0, $sp" : "=r" (sp));
 
 	if (sp >= (long)CKSEG0 && sp < (long)CKSEG2)
 		usp = CKSEG1ADDR(sp);
-- 
2.29.2