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.
2. Space for signal stacks and red zones.
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).

Address of region 3 must be aligned so that the threshold between regions 5 and 6 (the stack limit) is a multiple of 256.

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).