Does ARM64 have intentionally undefined opcodes?

Question

I'd like to trigger an intentional undefined opcode exception. Does ARM64 have an instruction explicitly reserved as undefined, like x86's ud2 and ARM32's udf?

Answer 1

Yes, it's also called udf. It appears in the ARMv8 Architecture Reference Manual on page C6-1380. It takes an immediate operand which can be any 16-bit value and is ignored, e.g. udf #0xdead.

The GNU assembler added support for the udf mnemonic fairly recently (I believe in version 2.35, released July 2020), so if you have an older version, it may not be supported. However, the encoding of udf #0 is conveniently 0x00000000, so you can just write .inst 0 instead, or define a macro if you plan to use it a lot.

In fact, any instruction whose high 16 bits are 0 is a guaranteed udf, and the low 16 bits are the immediate value. You could use different ones to mark different parts of your code, for easy spotting in a memory dump. So you can also do .inst 0x0000dead if you want.