Attach range metadata to alignment loads from vtables

...because alignment is always nonzero.

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

Author: erikdesjardins

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);

src/test/codegen/dst-vtable-align-nonzero.rs

@@ -0,0 +1,43 @@
+#![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}