New opcode 'PACKSTRUCT'

[dicarlo2]

Like PACK but for structs.

[erikzhang]

Is there a specific scenario that you need this for?

[dicarlo2]

Without going into too much detail, we effectively store every value as an array of [<number>, <value>] in case we need to know the type at runtime[1]. Currently we store this as an array because it's relatively efficient to create:

// assume <value> is on stack
PUSH<number> // free
PUSH2 // free
PACK // 1 droplet

An equivalent struct is much more expensive to create, to the point that it's not feasible to use structs at all.

If we were able to use structs, then we could make other parts more efficient, for example, === becomes a simple EQUAL op, rather than needing to unwrap the value:

// assume two wrapped values are on the stack
PUSH1 // free
PICKITEM // 1 droplet
SWAP // 1 droplet
PUSH1 // free
PICKITEM // 1 droplet
EQUAL // 1 droplet

4x more efficient if we can use a struct and a single EQUAL. Another example scenario is using these wrapped values as keys in maps, this would preserve the semantics of JavaScript along with #50.

[1] Note, later iterations of neo-one will optimize this away when possible, but still in some cases we will need to continue wrapping.

[erikzhang]

Creating struct with PACKSTRUCT

PUSH value3
PUSH value2
PUSH value1
PUSH 3
PACKSTRUCT

Creating struct without PACKSTRUCT

PUSH 0
NEWSTRUCT
DUP
PUSH value1
APPEND
DUP
PUSH value2
APPEND
DUP
PUSH value3
APPEND

[anthdm]

@dicarlo2 Not sure why you should need that? Structs are treated the same as arrays. Their values are just an ordered union on the stack. The compiler should track all additional information about the underlying type and index of a field.

[dicarlo2]

@anthdm Structs do not work the same as arrays - I invite you to take a look at the source code to see the differences.

[dicarlo2]

@erikzhang do you have objections to adding PACKSTRUCT? This is blocking for NEO-ONE TypeScript smart contracts.