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style: Allow to use ThinVec/nsTArray in the style crate

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This allows to clean-up the previous patches by using a single ThinVec
(which stores length / capacity along with the allocation).

Differential Revision: https://phabricator.services.mozilla.com/D175029
This commit is contained in:
Emilio Cobos Álvarez 2023-04-12 21:58:29 +00:00 • committed by Martin Robinson
parent a2df8f7ea5
commit 0709e13446
9 changed files with 110 additions and 48 deletions
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55
components/to_shmem/lib.rs
View file

@ -12,6 +12,8 @@
#![crate_name = "to_shmem"]
#![crate_type = "rlib"]
extern crate thin_vec;
use std::alloc::Layout;
#[cfg(debug_assertions)]
use std::any::TypeId;
@ -31,6 +33,7 @@ use std::{isize, slice, str};
use servo_arc::{Arc, ThinArc};
use smallbitvec::{InternalStorage, SmallBitVec};
use smallvec::{Array, SmallVec};
use thin_vec::ThinVec;
/// Result type for ToShmem::to_shmem.
///
@ -498,6 +501,58 @@ impl<H: 'static + ToShmem, T: 'static + ToShmem> ToShmem for ThinArc<H, T> {
}
}
impl<T: ToShmem> ToShmem for ThinVec<T> {
fn to_shmem(&self, builder: &mut SharedMemoryBuilder) -> Result<Self> {
let len = self.len();
if len == 0 {
return Ok(ManuallyDrop::new(Self::new()))
}
assert_eq!(mem::size_of::<Self>(), mem::size_of::<*const ()>());
// nsTArrayHeader size.
// FIXME: Would be nice not to hard-code this, but in practice thin-vec crate also relies
// on this.
let header_size = 2 * mem::size_of::<u32>();
let header_align = mem::size_of::<u32>();
let item_size = mem::size_of::<T>();
let item_align = mem::align_of::<T>();
// We don't need to support underalignment for now, this could be supported if needed.
assert!(item_align >= header_align);
// This is explicitly unsupported by ThinVec, see:
// https://searchfox.org/mozilla-central/rev/ad732108b073742d7324f998c085f459674a6846/third_party/rust/thin-vec/src/lib.rs#375-386
assert!(item_align <= header_size);
let header_padding = 0;
let layout = Layout::from_size_align(header_size + header_padding + padded_size(item_size, item_align) * len, item_align).unwrap();
let shmem_header_ptr = builder.alloc::<u8>(layout);
let shmem_data_ptr = unsafe { shmem_header_ptr.add(header_size + header_padding) };
let data_ptr = self.as_ptr() as *const T as *const u8;
let header_ptr = unsafe { data_ptr.sub(header_size + header_padding) };
unsafe {
// Copy the header. Note this might copy a wrong capacity, but it doesn't matter,
// because shared memory ptrs are immutable anyways, and we can't relocate.
ptr::copy(header_ptr, shmem_header_ptr, header_size);
// ToShmem + copy the contents into the shared buffer.
to_shmem_slice_ptr(self.iter(), shmem_data_ptr as *mut T, builder)?;
// Return the new ThinVec, which is just a pointer to the shared memory buffer.
let shmem_thinvec: Self = mem::transmute(shmem_header_ptr);
// Sanity-check that the ptr and length match.
debug_assert_eq!(shmem_thinvec.as_ptr(), shmem_data_ptr as *const T);
debug_assert_eq!(shmem_thinvec.len(), len);
Ok(ManuallyDrop::new(shmem_thinvec))
}
}
}
impl ToShmem for SmallBitVec {
fn to_shmem(&self, builder: &mut SharedMemoryBuilder) -> Result<Self> {
let storage = match self.clone().into_storage() {