bjoernager/rust
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Auto merge of #122662 - Mark-Simulacrum:optional-drop, r=bjorn3

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Omit non-needs_drop drop_in_place in vtables

This replaces the drop_in_place reference with null in vtables. On librustc_driver.so, this drops about ~17k (11%) dynamic relocations from the output, since many vtables can now be placed in read-only memory, rather than having a relocated pointer included.

This makes a tradeoff by adding a null check at vtable call sites. I'm not sure that's readily avoidable without changing the vtable format (e.g., so that we can use a pc-relative relocation instead of an absolute address, and avoid the dynamic relocation that way). But it seems likely that the check is cheap at runtime.

Accepted MCP: https://github.com/rust-lang/compiler-team/issues/730
This commit is contained in:
bors 2024-05-28 16:04:14 +00:00
commit 8c4db851a7
7 changed files with 203 additions and 116 deletions
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12
compiler/rustc_middle/src/ty/vtable.rs
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@ -84,10 +84,14 @@ pub(super) fn vtable_allocation_provider<'tcx>(
let idx: u64 = u64::try_from(idx).unwrap();
let scalar = match entry {
VtblEntry::MetadataDropInPlace => {
let instance = ty::Instance::resolve_drop_in_place(tcx, ty);
let fn_alloc_id = tcx.reserve_and_set_fn_alloc(instance);
let fn_ptr = Pointer::from(fn_alloc_id);
Scalar::from_pointer(fn_ptr, &tcx)
if ty.needs_drop(tcx, ty::ParamEnv::reveal_all()) {
let instance = ty::Instance::resolve_drop_in_place(tcx, ty);
let fn_alloc_id = tcx.reserve_and_set_fn_alloc(instance);
let fn_ptr = Pointer::from(fn_alloc_id);
Scalar::from_pointer(fn_ptr, &tcx)
} else {
Scalar::from_maybe_pointer(Pointer::null(), &tcx)
}
}
VtblEntry::MetadataSize => Scalar::from_uint(size, ptr_size),
VtblEntry::MetadataAlign => Scalar::from_uint(align, ptr_size),