Backward Compatibility

[martin-fink]

This is a draft PR. Make sure to also check out the custom_sections branch in the wasm-tools repo.

Parse a custom section that replaces instructions with other instructions.
This is a bit hacky, but it's a quick way to prototype backward compatibility.

The custom section has the name mem-safety and is structured in the following way:

(pos len instruction)*

where pos is 64 bits (little endian) and len is a 64 bit leb128 encoded integer, and instruction the encoded instruction (variable length).

• pos is the offset of the instruction(s) that should be replaced
• len is the number of instructions that should be replaced

See the following example:

  (func (export "foo") (result i32)
        (local $ptr i64)
        (i64.const 0)
        (i64.const 16)
        ;; pos = 0x3D
        drop
        (local.tee $ptr)
        (i32.const 42)
        (i32.store)

        (local.get $ptr)
        (i32.load)

        (local.get $ptr)
        (i64.const 0)
        (i64.const 16)
        ;; pos = 0x50
        drop
        drop
        drop
        (return)
  )

Hexdump of the custom section:

;; pos                       len   inst
    3D 00 00 00 00 00 00 00  01    FA 02 00 00
    50 00 00 00 00 00 00 00  03    FA 03 00 00