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

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: https://github.com/rust-lang/rust/blob/772d51f887fa407216860bf8ecf3f1a32fb795b4/compiler/rustc_middle/src/ty/vtable.rs#L68-L90 and is nonzero because [`Align::bytes()`](https://github.com/rust-lang/rust/blob/772d51f887fa407216860bf8ecf3f1a32fb795b4/compiler/rustc_target/src/abi/mod.rs#L547-L549) is always nonzero.

1 files changed, 11 insertions, 7 deletions

diff --git a/compiler/rustc_codegen_ssa/src/glue.rs b/compiler/rustc_codegen_ssa/src/glue.rs
index cf217b52c86..2c4e6bbe9a5 100644
--- a/compiler/rustc_codegen_ssa/src/glue.rs
+++ b/compiler/rustc_codegen_ssa/src/glue.rs
@@ -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);