compile

Overview ¶

Package compile implements a WebAssembly compiler.

Text ¶

Module sections (wasm v1) which affect the immutable text (machine code):

Type
Import
Function
Table
Memory (*)
Global (*)
Element
Code

(*) Memory sizes and global values do not affect text, only their counts and types do.

Sections which have no effect on text:

Export
Start
Data
Custom sections

Vector-based import function indexes also affect text, but their addresses are configured at run-time by mapping the vector immediately before text.

Globals and memory ¶

The memory, global and data sections comprise a single mutable buffer.

Stack ¶

Any effect the export and start sections have happens via the run-time stack buffer, which must be initialized with optional start and entry function addresses.

Stack regions from low to high address:

1. Space for runtime-specific variables (size must be a multiple of 32).
2. Space for signal stacks and red zones (size must be a multiple of 32).
3. 240 bytes for use by trap handlers and vector-based import functions.
4. 8 bytes for trap handler return address (included in call stack).
5. 8 bytes for an extra function call (included in call stack).
6. The rest of the call stack (size must be a multiple of 8).
7. Start function address (4 bytes padded to 8).
8. Entry function address (4 bytes padded to 8).

Function addresses are relative to the start of text. Zero address causes the function to be skipped.

Stack pointer is initially positioned between regions 6 and 7. Function prologue compares the stack pointer against the threshold between regions 5 and 6 (the stack limit).

(Size requirements for regions 1-5 cause their total size to be a multiple of 32, so that the stack limit divided by 16 is an even number. It's necessary for the ARM64 backend's suspension logic.)