Branches Proof Of Concept

[damnMeddlingKid]

What

This PR implements a proof of concept for handling branches in the Liquid VM.

How

OP Codes

To implement branches i've added 3 OP codes:

• OP_EVAL_CONDITION: This operation has a constant which is a Liquid Condition object. When executed it evaluates the Condition and pushes the RTEST of the result onto the stack
• OP_BRANCH_UNLESS: Pops a value off the stack, if its not true then jump to the offset located in the lower 16bits of the instruction.
• OP_BRANCH: This is an unconditional branch, it's used to exit out of a block to the "endif".

Process

When an if tag is encountered we add an OP_EVAL_CONDITION for the if statement and add an OP_BRANCH_UNLESS operation that jumps to the next clause (or endif)

Parsing then resumes as normal by calling internal_block_body_parse, when internal_block_body_parse returns with an unknown tag we check if its one of the clauses we can handle like else if so we add an unconditional OP_BRANCH which exits to the next endif and continue parsing the tokens for the else block.

Nesting is handled through recursion.

Heres an example of a liquid if block and its corresponding bytecode.

Theres a more complete example in test.rb with elsif and nesting.

Limitations of the POC

I'm sure theres a lot thats missing, heres the stuff im aware of:

• Currently i've only parsed conditions with a single binary comparison, this needs to be extended to handle more comparisons
• In order to support multiple elsif clauses i've added a small array that can handle 10 clauses, this needs to be dynamically sized.
• Theres no validation of the structure of the tag. right now its possible to have an else before an elsif
• No restriction on the nesting depth

[damnMeddlingKid]

This array is used to keep track of all the branches that need to jump to "endif". This needs to use a dynamically sized list here, something like cbuffer.

[damnMeddlingKid]

The offsets get stored with the instruction in 16 bits. assuming this is ok for a jump offset, otherwise need to store them somewhere else.

[damnMeddlingKid]

Only supports parsing 1 binary comparison, need to extend this to support more conditions.

[damnMeddlingKid]

Calculating indices since cbuffer can realloc.

[damnMeddlingKid]

using -1 as a sentinel value to indicate no open branch.

[damnMeddlingKid]

This is kind of scrappy, need a better abstraction for this.

[damnMeddlingKid]

I was thinking of something along the lines of #96 so we could pick the right parsing handler from ruby land.

[damnMeddlingKid]

not sure what these two do, might be updating them incorrectly.