3 // Code relating to drop glue.
5 use crate::common::IntPredicate;
8 use rustc_middle::ty::{self, Ty};
10 pub fn size_and_align_of_dst<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
13 info: Option<Bx::Value>,
14 ) -> (Bx::Value, Bx::Value) {
15 let layout = bx.layout_of(t);
16 debug!("size_and_align_of_dst(ty={}, info={:?}): layout: {:?}", t, info, layout);
17 if !layout.is_unsized() {
18 let size = bx.const_usize(layout.size.bytes());
19 let align = bx.const_usize(layout.align.abi.bytes());
24 // load size/align from vtable
25 let vtable = info.unwrap();
27 meth::VirtualIndex::from_index(ty::COMMON_VTABLE_ENTRIES_SIZE)
28 .get_usize(bx, vtable),
29 meth::VirtualIndex::from_index(ty::COMMON_VTABLE_ENTRIES_ALIGN)
30 .get_usize(bx, vtable),
33 ty::Slice(_) | ty::Str => {
34 let unit = layout.field(bx, 0);
35 // The info in this case is the length of the str, so the size is that
36 // times the unit size.
38 bx.mul(info.unwrap(), bx.const_usize(unit.size.bytes())),
39 bx.const_usize(unit.align.abi.bytes()),
43 // First get the size of all statically known fields.
44 // Don't use size_of because it also rounds up to alignment, which we
45 // want to avoid, as the unsized field's alignment could be smaller.
46 assert!(!t.is_simd());
47 debug!("DST {} layout: {:?}", t, layout);
49 let i = layout.fields.count() - 1;
50 let sized_size = layout.fields.offset(i).bytes();
51 let sized_align = layout.align.abi.bytes();
52 debug!("DST {} statically sized prefix size: {} align: {}", t, sized_size, sized_align);
53 let sized_size = bx.const_usize(sized_size);
54 let sized_align = bx.const_usize(sized_align);
56 // Recurse to get the size of the dynamically sized field (must be
58 let field_ty = layout.field(bx, i).ty;
59 let (unsized_size, mut unsized_align) = size_and_align_of_dst(bx, field_ty, info);
61 // FIXME (#26403, #27023): We should be adding padding
62 // to `sized_size` (to accommodate the `unsized_align`
63 // required of the unsized field that follows) before
64 // summing it with `sized_size`. (Note that since #26403
65 // is unfixed, we do not yet add the necessary padding
66 // here. But this is where the add would go.)
68 // Return the sum of sizes and max of aligns.
69 let size = bx.add(sized_size, unsized_size);
71 // Packed types ignore the alignment of their fields.
72 if let ty::Adt(def, _) = t.kind() {
73 if def.repr.packed() {
74 unsized_align = sized_align;
78 // Choose max of two known alignments (combined value must
79 // be aligned according to more restrictive of the two).
81 bx.const_to_opt_u128(sized_align, false),
82 bx.const_to_opt_u128(unsized_align, false),
84 (Some(sized_align), Some(unsized_align)) => {
85 // If both alignments are constant, (the sized_align should always be), then
86 // pick the correct alignment statically.
87 bx.const_usize(std::cmp::max(sized_align, unsized_align) as u64)
90 let cmp = bx.icmp(IntPredicate::IntUGT, sized_align, unsized_align);
91 bx.select(cmp, sized_align, unsized_align)
95 // Issue #27023: must add any necessary padding to `size`
96 // (to make it a multiple of `align`) before returning it.
98 // Namely, the returned size should be, in C notation:
100 // `size + ((size & (align-1)) ? align : 0)`
102 // emulated via the semi-standard fast bit trick:
104 // `(size + (align-1)) & -align`
105 let one = bx.const_usize(1);
106 let addend = bx.sub(align, one);
107 let add = bx.add(size, addend);
108 let neg = bx.neg(align);
109 let size = bx.and(add, neg);