objtool/LoongArch: Mark special atomic instruction as INSN_BUG type

When compiling with LLVM and CONFIG_RUST is set, there exists the
following objtool warning:

rust/compiler_builtins.o: warning: objtool: __rust__unordsf2(): unexpected end of section .text.unlikely.

objdump shows that the end of section .text.unlikely is an atomic
instruction:

amswap.w $zero, $ra, $zero

According to the LoongArch Reference Manual, if the amswap.w atomic
memory access instruction has the same register number as rd and rj,
the execution will trigger an Instruction Non-defined Exception, so
mark the above instruction as INSN_BUG type to fix the warning.

Cc: [e-mail]
Signed-off-by: Tiezhu Yang <[e-mail]>
Signed-off-by: Huacai Chen <[e-mail]>

tools/arch/loongarch/include/asm/inst.h

···
 	bgeu_op		= 0x1b,
 };
 
+enum reg3_op {
+	amswapw_op	= 0x70c0,
+};
+
 struct reg0i15_format {
 	unsigned int immediate : 15;
 	unsigned int opcode : 17;
···
 	unsigned int opcode : 6;
 };
 
+struct reg3_format {
+	unsigned int rd : 5;
+	unsigned int rj : 5;
+	unsigned int rk : 5;
+	unsigned int opcode : 17;
+};
+
 union loongarch_instruction {
 	unsigned int word;
 	struct reg0i15_format	reg0i15_format;
···
 	struct reg2i12_format	reg2i12_format;
 	struct reg2i14_format	reg2i14_format;
 	struct reg2i16_format	reg2i16_format;
+	struct reg3_format	reg3_format;
 };
 
 #define LOONGARCH_INSN_SIZE	sizeof(union loongarch_instruction)

tools/objtool/arch/loongarch/decode.c

···
 	return true;
 }
 
+static bool decode_insn_reg3_fomat(union loongarch_instruction inst,
+				   struct instruction *insn)
+{
+	switch (inst.reg3_format.opcode) {
+	case amswapw_op:
+		if (inst.reg3_format.rd == LOONGARCH_GPR_ZERO &&
+		    inst.reg3_format.rk == LOONGARCH_GPR_RA &&
+		    inst.reg3_format.rj == LOONGARCH_GPR_ZERO) {
+			/* amswap.w $zero, $ra, $zero */
+			insn->type = INSN_BUG;
+		}
+		break;
+	default:
+		return false;
+	}
+
+	return true;
+}
+
 int arch_decode_instruction(struct objtool_file *file, const struct section *sec,
 			    unsigned long offset, unsigned int maxlen,
 			    struct instruction *insn)
···
 	if (decode_insn_reg2i14_fomat(inst, insn, ops_list, op))
 		return 0;
 	if (decode_insn_reg2i16_fomat(inst, insn))
+		return 0;
+	if (decode_insn_reg3_fomat(inst, insn))
 		return 0;
 
 	if (inst.word == 0) {