Struct HeapData

pub struct HeapData {
    pub base: GlobalValue,
    pub min_size: u64,
    pub max_size: Option<u64>,
    pub memory_type: Option<MemoryType>,
    pub offset_guard_size: u64,
    pub style: HeapStyle,
    pub index_type: Type,
    pub page_size_log2: u8,
}

A heap implementing a WebAssembly linear memory.

Code compiled from WebAssembly runs in a sandbox where it can’t access all process memory. Instead, it is given a small set of memory areas to work in, and all accesses are bounds checked. cranelift-wasm models this through the concept of heaps.

Heap addresses can be smaller than the native pointer size, for example unsigned i32 offsets on a 64-bit architecture.

A heap appears as three consecutive ranges of address space:

1. The mapped pages are the accessible memory range in the heap. A heap may have a minimum guaranteed size which means that some mapped pages are always present.

2. The unmapped pages is a possibly empty range of address space that may be mapped in the future when the heap is grown. They are addressable but not accessible.

3. The offset-guard pages is a range of address space that is guaranteed to always cause a trap when accessed. It is used to optimize bounds checking for heap accesses with a shared base pointer. They are addressable but not accessible.

The heap bound is the total size of the mapped and unmapped pages. This is the bound that heap_addr checks against. Memory accesses inside the heap bounds can trap if they hit an unmapped page (which is not accessible).

Two styles of heaps are supported, static and dynamic. They behave differently when resized.

Static heaps

A static heap starts out with all the address space it will ever need, so it never moves to a different address. At the base address is a number of mapped pages corresponding to the heap’s current size. Then follows a number of unmapped pages where the heap can grow up to its maximum size. After the unmapped pages follow the offset-guard pages which are also guaranteed to generate a trap when accessed.

Dynamic heaps

A dynamic heap can be relocated to a different base address when it is resized, and its bound can move dynamically. The offset-guard pages move when the heap is resized. The bound of a dynamic heap is stored in a global value.

Fields

base: GlobalValue

The address of the start of the heap’s storage.

min_size: u64

Guaranteed minimum heap size in bytes. Heap accesses before min_size don’t need bounds checking.

max_size: Option<u64>

The maximum heap size in bytes.

Heap accesses larger than this will always trap.

memory_type: Option<MemoryType>

The memory type for the pointed-to memory, if using proof-carrying code.

offset_guard_size: u64

Size in bytes of the offset-guard pages following the heap.

style: HeapStyle

Heap style, with additional style-specific info.

index_type: Type

The index type for the heap.

page_size_log2: u8

The log2 of this memory’s page size.

Trait Implementations

impl Clone for HeapData

fn clone(&self) -> HeapData

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Hash for HeapData

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl PartialEq for HeapData

fn eq(&self, other: &HeapData) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl StructuralPartialEq for HeapData