MIPS: Don't use bc_false uninitialized in __mm_isBranchInstr

MIPS: Don't use bc_false uninitialized in __mm_isBranchInstr

clang warns:

arch/mips/kernel/branch.c:148:8: error: variable 'bc_false' is used
uninitialized whenever switch case is taken
[-Werror,-Wsometimes-uninitialized]
                case mm_bc2t_op:
                     ^~~~~~~~~~
arch/mips/kernel/branch.c:157:8: note: uninitialized use occurs here
                        if (bc_false)
                            ^~~~~~~~
arch/mips/kernel/branch.c:149:8: error: variable 'bc_false' is used
uninitialized whenever switch case is taken
[-Werror,-Wsometimes-uninitialized]
                case mm_bc1t_op:
                     ^~~~~~~~~~
arch/mips/kernel/branch.c:157:8: note: uninitialized use occurs here
                        if (bc_false)
                            ^~~~~~~~
arch/mips/kernel/branch.c:142:4: note: variable 'bc_false' is declared
here
                        int bc_false = 0;
                        ^
2 errors generated.

When mm_bc1t_op and mm_bc2t_op are taken, the bc_false initialization
does not happen, which leads to a garbage value upon use, as illustrated
below with a small sample program.

$ mipsel-linux-gnu-gcc --version | head -n1
mipsel-linux-gnu-gcc (Debian 8.3.0-2) 8.3.0

$ clang --version | head -n1
ClangBuiltLinux clang version 9.0.0 (git://github.com/llvm/llvm-project
544315b4197034a3be8acd12cba56a75fb1f08dc) (based on LLVM 9.0.0svn)

$ cat test.c
 #include <stdio.h>

 static void switch_scoped(int opcode)
 {
	 switch (opcode) {
	 case 1:
	 case 2: {
		 int bc_false = 0;

		 bc_false = 4;
	 case 3:
	 case 4:
		 printf("\t* switch scoped bc_false = %d\n", bc_false);
	 }
	 }
 }

 static void function_scoped(int opcode)
 {
	 int bc_false = 0;

	 switch (opcode) {
	 case 1:
	 case 2: {
		 bc_false = 4;
	 case 3:
	 case 4:
		 printf("\t* function scoped bc_false = %d\n", bc_false);
	 }
	 }
 }

 int main(void)
 {
	 int opcode;

	 for (opcode = 1; opcode < 5; opcode++) {
		 printf("opcode = %d:\n", opcode);
		 switch_scoped(opcode);
		 function_scoped(opcode);
		 printf("\n");
	 }

	 return 0;
 }

$ mipsel-linux-gnu-gcc -std=gnu89 -static test.c && \
  qemu-mipsel a.out
opcode = 1:
        * switch scoped bc_false = 4
        * function scoped bc_false = 4

opcode = 2:
        * switch scoped bc_false = 4
        * function scoped bc_false = 4

opcode = 3:
        * switch scoped bc_false = 2147483004
        * function scoped bc_false = 0

opcode = 4:
        * switch scoped bc_false = 2147483004
        * function scoped bc_false = 0

$ clang -std=gnu89 --target=mipsel-linux-gnu -m32 -static test.c && \
  qemu-mipsel a.out
opcode = 1:
        * switch scoped bc_false = 4
        * function scoped bc_false = 4

opcode = 2:
        * switch scoped bc_false = 4
        * function scoped bc_false = 4

opcode = 3:
        * switch scoped bc_false = 2147483004
        * function scoped bc_false = 0

opcode = 4:
        * switch scoped bc_false = 2147483004
        * function scoped bc_false = 0

Move the definition up so that we get the right behavior and mark it
__maybe_unused as it will not be used when CONFIG_MIPS_FP_SUPPORT
isn't enabled.

Fixes: 6a1cc218b9cc ("MIPS: branch: Remove FP branch handling when CONFIG_MIPS_FP_SUPPORT=n")
Link: https://github.com/ClangBuiltLinux/linux/issues/603
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: clang-built-linux@googlegroups.com