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Auto merge of #91569 - erikdesjardins:vt-align, r=nikic

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Attach range metadata to alignment loads from vtables

...because alignment is always nonzero[0].

This helps eliminate redundant runtime alignment checks, when a DST
is a field of a struct whose remaining fields have alignment 1.

Fixes #91438.

---
[0]:

The [reference](https://doc.rust-lang.org/reference/type-layout.html) says that alignment must be at least 1.

And in practice, the alignment field for all vtables is generated here: 772d51f887/compiler/rustc_middle/src/ty/vtable.rs (L68-L90) and is nonzero because [`Align::bytes()`](772d51f887/compiler/rustc_target/src/abi/mod.rs (L547-L549)) is always nonzero.
This commit is contained in:
bors 2021-12-13 04:29:20 +00:00
commit 4a7fb971c9
2 changed files with 56 additions and 7 deletions
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18
compiler/rustc_codegen_ssa/src/glue.rs
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@ -6,6 +6,7 @@ use crate::common::IntPredicate;
use crate::meth;
use crate::traits::*;
use rustc_middle::ty::{self, Ty};
use rustc_target::abi::WrappingRange;
pub fn size_and_align_of_dst<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
bx: &mut Bx,
@ -21,14 +22,17 @@ pub fn size_and_align_of_dst<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
}
match t.kind() {
ty::Dynamic(..) => {
// load size/align from vtable
// Load size/align from vtable.
let vtable = info.unwrap();
(
meth::VirtualIndex::from_index(ty::COMMON_VTABLE_ENTRIES_SIZE)
.get_usize(bx, vtable),
meth::VirtualIndex::from_index(ty::COMMON_VTABLE_ENTRIES_ALIGN)
.get_usize(bx, vtable),
)
let size = meth::VirtualIndex::from_index(ty::COMMON_VTABLE_ENTRIES_SIZE)
.get_usize(bx, vtable);
let align = meth::VirtualIndex::from_index(ty::COMMON_VTABLE_ENTRIES_ALIGN)
.get_usize(bx, vtable);
// Alignment is always nonzero.
bx.range_metadata(align, WrappingRange { start: 1, end: !0 });
(size, align)
}
ty::Slice(_) | ty::Str => {
let unit = layout.field(bx, 0);