How to get operand info of ZydisEncodableInstruction?

[loveandkindess]

ZydisEncodableInstruction_ has this member:
ZyanU16 operand_mask;

How do I get the operand count?
How do I get the exact info of each operand, like register type (or memory type), count, size, and addressing mode?

I see you can only get it via decoding an instruction, how do i get it while encoding?

edit: in other words, how can I decode the operand mask

[mappzor]

ZydisEncodableInstruction is an internal structure. It's not exposed by any API. Encodes uses operand_mask for optimization to quickly reject instruction variants that don't have a chance to match encoder request.

I see you can only get it via decoding an instruction, how do i get it while encoding?

What are you trying to achieve exactly?

[loveandkindess]

@mappzor I am trying to decode the operand mask..

[mappzor]

You are not meant to access it at all. What's your use case?

[loveandkindess]

To generate random instructions with more constraints. example:
generate random instruction with 64 bit memory or register operands

[mappzor]

Using internal structures for this is possible to an extent but you might want to just parse zydis-db in a way that's more suitable for you. Operand masks are just operand counts + basic operand types (register, memory, immediate or pointer). Accessing more detailed information about additional constraints can be rather convoluted.

Table generation scripts for encoder are open-sourced here.

[ZehMatt]

Perhaps this can help you a little bit understand it, I also use the internal structures https://github.com/ZehMatt/x86Tester/blob/36532119a757e596256b99b29e22aeb01095d854/src/generator/generator.cpp#L519-L566