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73 lines
2.8 KiB
Rust
73 lines
2.8 KiB
Rust
//! This module contains a canonical definition of the `cabi_realloc` function
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//! for the component model.
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//!
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//! The component model's canonical ABI for representing datatypes in memory
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//! makes use of this function when transferring lists and strings, for example.
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//! This function behaves like C's `realloc` but also takes alignment into
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//! account.
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//!
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//! Components are notably not required to export this function, but nearly
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//! all components end up doing so currently. This definition in the standard
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//! library removes the need for all compilations to define this themselves.
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//!
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//! More information about the canonical ABI can be found at
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//! <https://github.com/WebAssembly/component-model/blob/main/design/mvp/CanonicalABI.md>
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//!
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//! Note that the name of this function is not standardized in the canonical ABI
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//! at this time. Instead it's a convention of the "componentization process"
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//! where a core wasm module is converted to a component to use this name.
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//! Additionally this is not the only possible definition of this function, so
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//! this is defined as a "weak" symbol. This means that other definitions are
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//! allowed to overwrite it if they are present in a compilation.
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use alloc::{Layout, alloc};
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use core::ptr;
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#[used]
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static FORCE_CODEGEN_OF_CABI_REALLOC: unsafe extern "C" fn(
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*mut u8,
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usize,
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usize,
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usize,
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) -> *mut u8 = cabi_realloc;
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#[unsafe(no_mangle)]
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pub unsafe extern "C" fn cabi_realloc(
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old_ptr: *mut u8,
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old_len: usize,
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mut align: usize,
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new_len: usize,
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) -> *mut u8 {
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// HACK: The object crate requires the data alignment for 64-bit objects to be 8,
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// but in wasm32, our allocator will have a minimum alignment of 4. We can't specify
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// the alignment of `list<u8>` in the component model, so we work around this here.
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// https://github.com/WebAssembly/component-model/issues/258
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#[cfg(target_pointer_width = "32")]
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if align == 1 {
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align = 8;
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}
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let layout;
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let ptr = if old_len == 0 {
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if new_len == 0 {
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return ptr::without_provenance_mut(align);
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}
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layout = unsafe { Layout::from_size_align_unchecked(new_len, align) };
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unsafe { alloc::alloc(layout) }
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} else {
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debug_assert_ne!(new_len, 0, "non-zero old_len requires non-zero new_len!");
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layout = unsafe { Layout::from_size_align_unchecked(old_len, align) };
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unsafe { alloc::realloc(old_ptr, layout, new_len) }
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};
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if ptr.is_null() {
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// Print a nice message in debug mode, but in release mode don't
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// pull in so many dependencies related to printing so just emit an
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// `unreachable` instruction.
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if cfg!(debug_assertions) {
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alloc::handle_alloc_error(layout);
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} else {
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core::unreachable!("allocation failed")
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}
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}
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ptr
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}
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