I am trying to learn the 'decoding decision tree' for aarch64 in binutils by trying to add a new assembly instruction 'addvp'.

For example: addvp x0, x0, 9

For this, I added a entry in struct aarch64_opcode aarch64_opcode_table[] (file opcodes/aarch64-tbl.h) as below:

 {"addvp", 0x01000000, 0x7f000000, addsub_imm, 0, CORE, OP3 (Rd_SP, Rn_SP, AIMM), QL_R2NIL, F_SF},

ARM manual say, bit 27 & bit 28 are unallocated. Thus for addvp, I am giving opcode 01000000 (with bit 27 & 28 as 0).

With this, generating object file from assembly file is successful (test.s --> test.o); but while disassembling using objdump, it say undefined instruction.

From objdump log:  

81002400        .inst   0x81002400 ; undefined 

(but instruction was generated correct i.e. 81002400 !!!).

I know since addvp is a hack instruction, it won't execute on cpu. But still disassembly should succeed.

1. Please help me in knowing what I am doing wrong here ? What else I should do to add a new instruction in binutils ?

2. I also saw some printf in opcodes/aarch64-gen.c which I guess create decoding tree (initialize_decoder_tree()). How to print them ? I made debug =1 but still print is not coming.

3. There are some auto-generated files like aarch64-asm-2.c, aarch64-dis-2.c. How to re-generate them ?

Thanks.