1 //! Codegen of the [`PointerCast::Unsize`] operation.
3 //! [`PointerCast::Unsize`]: `rustc_middle::ty::adjustment::PointerCast::Unsize`
7 // Adapted from https://github.com/rust-lang/rust/blob/2a663555ddf36f6b041445894a8c175cd1bc718c/src/librustc_codegen_ssa/base.rs#L159-L307
9 /// Retrieve the information we are losing (making dynamic) in an unsizing
12 /// The `old_info` argument is a bit funny. It is intended for use
13 /// in an upcast, where the new vtable for an object will be derived
15 pub(crate) fn unsized_info<'tcx>(
16 fx: &mut FunctionCx<'_, '_, 'tcx>,
19 old_info: Option<Value>,
21 let (source, target) =
22 fx.tcx.struct_lockstep_tails_erasing_lifetimes(source, target, ParamEnv::reveal_all());
23 match (&source.kind(), &target.kind()) {
24 (&ty::Array(_, len), &ty::Slice(_)) => fx
27 .iconst(fx.pointer_type, len.eval_usize(fx.tcx, ParamEnv::reveal_all()) as i64),
28 (&ty::Dynamic(ref data_a, ..), &ty::Dynamic(ref data_b, ..)) => {
30 old_info.expect("unsized_info: missing old info for trait upcasting coercion");
31 if data_a.principal_def_id() == data_b.principal_def_id() {
32 // A NOP cast that doesn't actually change anything, should be allowed even with invalid vtables.
36 // trait upcasting coercion
38 fx.tcx.vtable_trait_upcasting_coercion_new_vptr_slot((source, target));
40 if let Some(entry_idx) = vptr_entry_idx {
41 let entry_idx = u32::try_from(entry_idx).unwrap();
42 let entry_offset = entry_idx * fx.pointer_type.bytes();
43 let vptr_ptr = Pointer::new(old_info).offset_i64(fx, entry_offset.into()).load(
46 crate::vtable::vtable_memflags(),
53 (_, &ty::Dynamic(ref data, ..)) => crate::vtable::get_vtable(fx, source, data.principal()),
54 _ => bug!("unsized_info: invalid unsizing {:?} -> {:?}", source, target),
58 /// Coerce `src` to `dst_ty`.
60 fx: &mut FunctionCx<'_, '_, 'tcx>,
62 src_layout: TyAndLayout<'tcx>,
63 dst_layout: TyAndLayout<'tcx>,
64 old_info: Option<Value>,
66 match (&src_layout.ty.kind(), &dst_layout.ty.kind()) {
67 (&ty::Ref(_, a, _), &ty::Ref(_, b, _))
68 | (&ty::Ref(_, a, _), &ty::RawPtr(ty::TypeAndMut { ty: b, .. }))
69 | (&ty::RawPtr(ty::TypeAndMut { ty: a, .. }), &ty::RawPtr(ty::TypeAndMut { ty: b, .. })) => {
70 (src, unsized_info(fx, *a, *b, old_info))
72 (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) if def_a.is_box() && def_b.is_box() => {
73 let (a, b) = (src_layout.ty.boxed_ty(), dst_layout.ty.boxed_ty());
74 (src, unsized_info(fx, a, b, old_info))
76 (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => {
77 assert_eq!(def_a, def_b);
79 if src_layout == dst_layout {
80 return (src, old_info.unwrap());
83 let mut result = None;
84 for i in 0..src_layout.fields.count() {
85 let src_f = src_layout.field(fx, i);
86 assert_eq!(src_layout.fields.offset(i).bytes(), 0);
87 assert_eq!(dst_layout.fields.offset(i).bytes(), 0);
91 assert_eq!(src_layout.size, src_f.size);
93 let dst_f = dst_layout.field(fx, i);
94 assert_ne!(src_f.ty, dst_f.ty);
95 assert_eq!(result, None);
96 result = Some(unsize_ptr(fx, src, src_f, dst_f, old_info));
100 _ => bug!("unsize_ptr: called on bad types"),
104 /// Coerce `src`, which is a reference to a value of type `src_ty`,
105 /// to a value of type `dst_ty` and store the result in `dst`
106 pub(crate) fn coerce_unsized_into<'tcx>(
107 fx: &mut FunctionCx<'_, '_, 'tcx>,
111 let src_ty = src.layout().ty;
112 let dst_ty = dst.layout().ty;
113 let mut coerce_ptr = || {
115 if fx.layout_of(src.layout().ty.builtin_deref(true).unwrap().ty).is_unsized() {
116 let (old_base, old_info) = src.load_scalar_pair(fx);
117 unsize_ptr(fx, old_base, src.layout(), dst.layout(), Some(old_info))
119 let base = src.load_scalar(fx);
120 unsize_ptr(fx, base, src.layout(), dst.layout(), None)
122 dst.write_cvalue(fx, CValue::by_val_pair(base, info, dst.layout()));
124 match (&src_ty.kind(), &dst_ty.kind()) {
125 (&ty::Ref(..), &ty::Ref(..))
126 | (&ty::Ref(..), &ty::RawPtr(..))
127 | (&ty::RawPtr(..), &ty::RawPtr(..)) => coerce_ptr(),
128 (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => {
129 assert_eq!(def_a, def_b);
131 for i in 0..def_a.variant(VariantIdx::new(0)).fields.len() {
132 let src_f = src.value_field(fx, mir::Field::new(i));
133 let dst_f = dst.place_field(fx, mir::Field::new(i));
135 if dst_f.layout().is_zst() {
139 if src_f.layout().ty == dst_f.layout().ty {
140 dst_f.write_cvalue(fx, src_f);
142 coerce_unsized_into(fx, src_f, dst_f);
146 _ => bug!("coerce_unsized_into: invalid coercion {:?} -> {:?}", src_ty, dst_ty),
150 // Adapted from https://github.com/rust-lang/rust/blob/2a663555ddf36f6b041445894a8c175cd1bc718c/src/librustc_codegen_ssa/glue.rs
152 pub(crate) fn size_and_align_of_dst<'tcx>(
153 fx: &mut FunctionCx<'_, '_, 'tcx>,
154 layout: TyAndLayout<'tcx>,
156 ) -> (Value, Value) {
157 assert!(layout.is_unsized() || layout.abi == Abi::Uninhabited);
158 match layout.ty.kind() {
160 // load size/align from vtable
161 (crate::vtable::size_of_obj(fx, info), crate::vtable::min_align_of_obj(fx, info))
163 ty::Slice(_) | ty::Str => {
164 let unit = layout.field(fx, 0);
165 // The info in this case is the length of the str, so the size is that
166 // times the unit size.
168 fx.bcx.ins().imul_imm(info, unit.size.bytes() as i64),
169 fx.bcx.ins().iconst(fx.pointer_type, unit.align.abi.bytes() as i64),
173 // First get the size of all statically known fields.
174 // Don't use size_of because it also rounds up to alignment, which we
175 // want to avoid, as the unsized field's alignment could be smaller.
176 assert!(!layout.ty.is_simd());
178 let i = layout.fields.count() - 1;
179 let sized_size = layout.fields.offset(i).bytes();
180 let sized_align = layout.align.abi.bytes();
181 let sized_align = fx.bcx.ins().iconst(fx.pointer_type, sized_align as i64);
183 // Recurse to get the size of the dynamically sized field (must be
185 let field_layout = layout.field(fx, i);
186 let (unsized_size, mut unsized_align) = size_and_align_of_dst(fx, field_layout, info);
188 // FIXME (#26403, #27023): We should be adding padding
189 // to `sized_size` (to accommodate the `unsized_align`
190 // required of the unsized field that follows) before
191 // summing it with `sized_size`. (Note that since #26403
192 // is unfixed, we do not yet add the necessary padding
193 // here. But this is where the add would go.)
195 // Return the sum of sizes and max of aligns.
196 let size = fx.bcx.ins().iadd_imm(unsized_size, sized_size as i64);
198 // Packed types ignore the alignment of their fields.
199 if let ty::Adt(def, _) = layout.ty.kind() {
200 if def.repr().packed() {
201 unsized_align = sized_align;
205 // Choose max of two known alignments (combined value must
206 // be aligned according to more restrictive of the two).
207 let cmp = fx.bcx.ins().icmp(IntCC::UnsignedGreaterThan, sized_align, unsized_align);
208 let align = fx.bcx.ins().select(cmp, sized_align, unsized_align);
210 // Issue #27023: must add any necessary padding to `size`
211 // (to make it a multiple of `align`) before returning it.
213 // Namely, the returned size should be, in C notation:
215 // `size + ((size & (align-1)) ? align : 0)`
217 // emulated via the semi-standard fast bit trick:
219 // `(size + (align-1)) & -align`
220 let addend = fx.bcx.ins().iadd_imm(align, -1);
221 let add = fx.bcx.ins().iadd(size, addend);
222 let neg = fx.bcx.ins().ineg(align);
223 let size = fx.bcx.ins().band(add, neg);