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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
View File

@ -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);

45
src/test/codegen/dst-vtable-align-nonzero.rs Normal file
View File

@ -0,0 +1,45 @@
// compile-flags: -O
#![crate_type = "lib"]
// This test checks that we annotate alignment loads from vtables with nonzero range metadata,
// and that this allows LLVM to eliminate redundant `align >= 1` checks.
pub trait Trait {
fn f(&self);
}
pub struct WrapperWithAlign1<T: ?Sized> { x: u8, y: T }
pub struct WrapperWithAlign2<T: ?Sized> { x: u16, y: T }
pub struct Struct<W: ?Sized> {
_field: i8,
dst: W,
}
// CHECK-LABEL: @eliminates_runtime_check_when_align_1
#[no_mangle]
pub fn eliminates_runtime_check_when_align_1(
x: &Struct<WrapperWithAlign1<dyn Trait>>
) -> &WrapperWithAlign1<dyn Trait> {
// CHECK: load [[USIZE:i[0-9]+]], {{.+}} !range [[RANGE_META:![0-9]+]]
// CHECK-NOT: icmp
// CHECK-NOT: select
// CHECK: ret
&x.dst
}
// CHECK-LABEL: @does_not_eliminate_runtime_check_when_align_2
#[no_mangle]
pub fn does_not_eliminate_runtime_check_when_align_2(
x: &Struct<WrapperWithAlign2<dyn Trait>>
) -> &WrapperWithAlign2<dyn Trait> {
// CHECK: [[X0:%[0-9]+]] = load [[USIZE]], {{.+}} !range [[RANGE_META]]
// CHECK: [[X1:%[0-9]+]] = icmp {{.+}} [[X0]]
// CHECK: [[X2:%[0-9]+]] = select {{.+}} [[X1]]
// CHECK: ret
&x.dst
}
// CHECK: [[RANGE_META]] = !{[[USIZE]] 1, [[USIZE]] 0}