3 // Adapted from https://github.com/rust-lang/rust/blob/2a663555ddf36f6b041445894a8c175cd1bc718c/src/librustc_codegen_ssa/base.rs#L159-L307
5 /// Retrieve the information we are losing (making dynamic) in an unsizing
8 /// The `old_info` argument is a bit funny. It is intended for use
9 /// in an upcast, where the new vtable for an object will be derived
11 pub fn unsized_info<'tcx>(
12 fx: &mut FunctionCx<'_, 'tcx, impl Backend>,
15 old_info: Option<Value>,
17 let (source, target) =
18 fx.tcx.struct_lockstep_tails_erasing_lifetimes(source, target, ParamEnv::reveal_all());
19 match (&source.sty, &target.sty) {
20 (&ty::Array(_, len), &ty::Slice(_)) => fx
25 len.eval_usize(fx.tcx, ParamEnv::reveal_all()) as i64,
27 (&ty::Dynamic(..), &ty::Dynamic(..)) => {
28 // For now, upcasts are limited to changes in marker
29 // traits, and hence never actually require an actual
30 // change to the vtable.
31 old_info.expect("unsized_info: missing old info for trait upcast")
33 (_, &ty::Dynamic(ref data, ..)) => crate::vtable::get_vtable(fx, source, data.principal()),
35 "unsized_info: invalid unsizing {:?} -> {:?}",
42 /// Coerce `src` to `dst_ty`. `src_ty` must be a thin pointer.
43 pub fn unsize_thin_ptr<'tcx>(
44 fx: &mut FunctionCx<'_, 'tcx, impl Backend>,
49 match (&src_ty.sty, &dst_ty.sty) {
50 (&ty::Ref(_, a, _), &ty::Ref(_, b, _))
51 | (&ty::Ref(_, a, _), &ty::RawPtr(ty::TypeAndMut { ty: b, .. }))
52 | (&ty::RawPtr(ty::TypeAndMut { ty: a, .. }), &ty::RawPtr(ty::TypeAndMut { ty: b, .. })) => {
53 assert!(!fx.layout_of(a).is_unsized());
54 (src, unsized_info(fx, a, b, None))
56 (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) if def_a.is_box() && def_b.is_box() => {
57 let (a, b) = (src_ty.boxed_ty(), dst_ty.boxed_ty());
58 assert!(!fx.layout_of(a).is_unsized());
59 (src, unsized_info(fx, a, b, None))
61 (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => {
62 assert_eq!(def_a, def_b);
64 let src_layout = fx.layout_of(src_ty);
65 let dst_layout = fx.layout_of(dst_ty);
66 let mut result = None;
67 for i in 0..src_layout.fields.count() {
68 let src_f = src_layout.field(fx, i);
69 assert_eq!(src_layout.fields.offset(i).bytes(), 0);
70 assert_eq!(dst_layout.fields.offset(i).bytes(), 0);
74 assert_eq!(src_layout.size, src_f.size);
76 let dst_f = dst_layout.field(fx, i);
77 assert_ne!(src_f.ty, dst_f.ty);
78 assert_eq!(result, None);
79 result = Some(unsize_thin_ptr(fx, src, src_f.ty, dst_f.ty));
83 _ => bug!("unsize_thin_ptr: called on bad types"),
87 /// Coerce `src`, which is a reference to a value of type `src_ty`,
88 /// to a value of type `dst_ty` and store the result in `dst`
89 pub fn coerce_unsized_into<'tcx>(
90 fx: &mut FunctionCx<'_, 'tcx, impl Backend>,
94 let src_ty = src.layout().ty;
95 let dst_ty = dst.layout().ty;
96 let mut coerce_ptr = || {
97 let (base, info) = if fx
98 .layout_of(src.layout().ty.builtin_deref(true).unwrap().ty)
101 // fat-ptr to fat-ptr unsize preserves the vtable
102 // i.e., &'a fmt::Debug+Send => &'a fmt::Debug
103 src.load_scalar_pair(fx)
105 let base = src.load_scalar(fx);
106 unsize_thin_ptr(fx, base, src_ty, dst_ty)
108 dst.write_cvalue(fx, CValue::by_val_pair(base, info, dst.layout()));
110 match (&src_ty.sty, &dst_ty.sty) {
111 (&ty::Ref(..), &ty::Ref(..))
112 | (&ty::Ref(..), &ty::RawPtr(..))
113 | (&ty::RawPtr(..), &ty::RawPtr(..)) => coerce_ptr(),
114 (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => {
115 assert_eq!(def_a, def_b);
117 for i in 0..def_a.variants[VariantIdx::new(0)].fields.len() {
118 let src_f = src.value_field(fx, mir::Field::new(i));
119 let dst_f = dst.place_field(fx, mir::Field::new(i));
121 if dst_f.layout().is_zst() {
125 if src_f.layout().ty == dst_f.layout().ty {
126 dst_f.write_cvalue(fx, src_f);
128 coerce_unsized_into(fx, src_f, dst_f);
133 "coerce_unsized_into: invalid coercion {:?} -> {:?}",
140 // Adapted from https://github.com/rust-lang/rust/blob/2a663555ddf36f6b041445894a8c175cd1bc718c/src/librustc_codegen_ssa/glue.rs
142 pub fn size_and_align_of_dst<'tcx>(
143 fx: &mut FunctionCx<'_, 'tcx, impl Backend>,
146 ) -> (Value, Value) {
147 let layout = fx.layout_of(ty);
148 if !layout.is_unsized() {
152 .iconst(fx.pointer_type, layout.size.bytes() as i64);
156 .iconst(fx.pointer_type, layout.align.abi.bytes() as i64);
157 return (size, align);
161 // load size/align from vtable
163 crate::vtable::size_of_obj(fx, info),
164 crate::vtable::min_align_of_obj(fx, info),
167 ty::Slice(_) | ty::Str => {
168 let unit = layout.field(fx, 0);
169 // The info in this case is the length of the str, so the size is that
170 // times the unit size.
172 fx.bcx.ins().imul_imm(info, unit.size.bytes() as i64),
175 .iconst(fx.pointer_type, unit.align.abi.bytes() as i64),
179 // First get the size of all statically known fields.
180 // Don't use size_of because it also rounds up to alignment, which we
181 // want to avoid, as the unsized field's alignment could be smaller.
182 assert!(!ty.is_simd());
184 let i = layout.fields.count() - 1;
185 let sized_size = layout.fields.offset(i).bytes();
186 let sized_align = layout.align.abi.bytes();
187 let sized_align = fx.bcx.ins().iconst(fx.pointer_type, sized_align as i64);
189 // Recurse to get the size of the dynamically sized field (must be
191 let field_ty = layout.field(fx, i).ty;
192 let (unsized_size, mut unsized_align) = size_and_align_of_dst(fx, field_ty, info);
194 // FIXME (#26403, #27023): We should be adding padding
195 // to `sized_size` (to accommodate the `unsized_align`
196 // required of the unsized field that follows) before
197 // summing it with `sized_size`. (Note that since #26403
198 // is unfixed, we do not yet add the necessary padding
199 // here. But this is where the add would go.)
201 // Return the sum of sizes and max of aligns.
202 let size = fx.bcx.ins().iadd_imm(unsized_size, sized_size as i64);
204 // Packed types ignore the alignment of their fields.
205 if let ty::Adt(def, _) = ty.sty {
206 if def.repr.packed() {
207 unsized_align = sized_align;
211 // Choose max of two known alignments (combined value must
212 // be aligned according to more restrictive of the two).
216 .icmp(IntCC::UnsignedGreaterThan, sized_align, unsized_align);
217 let align = fx.bcx.ins().select(cmp, sized_align, unsized_align);
219 // Issue #27023: must add any necessary padding to `size`
220 // (to make it a multiple of `align`) before returning it.
222 // Namely, the returned size should be, in C notation:
224 // `size + ((size & (align-1)) ? align : 0)`
226 // emulated via the semi-standard fast bit trick:
228 // `(size + (align-1)) & -align`
229 let addend = fx.bcx.ins().iadd_imm(align, -1);
230 let add = fx.bcx.ins().iadd(size, addend);
231 let zero = fx.bcx.ins().iconst(fx.pointer_type, 0);
232 let neg = fx.bcx.ins().isub(zero, align);
233 let size = fx.bcx.ins().band(add, neg);