rust/src/unsize.rs

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//! Codegen of the [`PointerCast::Unsize`] operation.
//!
//! [`PointerCast::Unsize`]: `rustc_middle::ty::adjustment::PointerCast::Unsize`
use crate::prelude::*;
// Adapted from https://github.com/rust-lang/rust/blob/2a663555ddf36f6b041445894a8c175cd1bc718c/src/librustc_codegen_ssa/base.rs#L159-L307
/// Retrieve the information we are losing (making dynamic) in an unsizing
/// adjustment.
///
/// The `old_info` argument is a bit funny. It is intended for use
/// in an upcast, where the new vtable for an object will be derived
/// from the old one.
pub(crate) fn unsized_info<'tcx>(
fx: &mut FunctionCx<'_, '_, 'tcx>,
source: Ty<'tcx>,
target: Ty<'tcx>,
old_info: Option<Value>,
) -> Value {
let (source, target) =
fx.tcx.struct_lockstep_tails_erasing_lifetimes(source, target, ParamEnv::reveal_all());
match (&source.kind(), &target.kind()) {
(&ty::Array(_, len), &ty::Slice(_)) => fx
.bcx
.ins()
.iconst(fx.pointer_type, len.eval_target_usize(fx.tcx, ParamEnv::reveal_all()) as i64),
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(
&ty::Dynamic(ref data_a, _, src_dyn_kind),
&ty::Dynamic(ref data_b, _, target_dyn_kind),
) => {
assert_eq!(src_dyn_kind, target_dyn_kind);
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let old_info =
old_info.expect("unsized_info: missing old info for trait upcasting coercion");
if data_a.principal_def_id() == data_b.principal_def_id() {
// A NOP cast that doesn't actually change anything, should be allowed even with invalid vtables.
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return old_info;
}
// trait upcasting coercion
let vptr_entry_idx =
fx.tcx.vtable_trait_upcasting_coercion_new_vptr_slot((source, target));
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if let Some(entry_idx) = vptr_entry_idx {
let entry_idx = u32::try_from(entry_idx).unwrap();
let entry_offset = entry_idx * fx.pointer_type.bytes();
let vptr_ptr = Pointer::new(old_info).offset_i64(fx, entry_offset.into()).load(
fx,
fx.pointer_type,
crate::vtable::vtable_memflags(),
);
vptr_ptr
} else {
old_info
}
}
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(_, ty::Dynamic(data, ..)) => crate::vtable::get_vtable(fx, source, data.principal()),
_ => bug!("unsized_info: invalid unsizing {:?} -> {:?}", source, target),
}
}
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/// Coerce `src` to `dst_ty`.
fn unsize_ptr<'tcx>(
fx: &mut FunctionCx<'_, '_, 'tcx>,
src: Value,
src_layout: TyAndLayout<'tcx>,
dst_layout: TyAndLayout<'tcx>,
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old_info: Option<Value>,
) -> (Value, Value) {
match (&src_layout.ty.kind(), &dst_layout.ty.kind()) {
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(&ty::Ref(_, a, _), &ty::Ref(_, b, _))
| (&ty::Ref(_, a, _), &ty::RawPtr(ty::TypeAndMut { ty: b, .. }))
| (&ty::RawPtr(ty::TypeAndMut { ty: a, .. }), &ty::RawPtr(ty::TypeAndMut { ty: b, .. })) => {
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(src, unsized_info(fx, *a, *b, old_info))
}
(&ty::Adt(def_a, _), &ty::Adt(def_b, _)) if def_a.is_box() && def_b.is_box() => {
let (a, b) = (src_layout.ty.boxed_ty(), dst_layout.ty.boxed_ty());
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(src, unsized_info(fx, a, b, old_info))
}
(&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => {
assert_eq!(def_a, def_b);
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if src_layout == dst_layout {
return (src, old_info.unwrap());
}
let mut result = None;
for i in 0..src_layout.fields.count() {
let src_f = src_layout.field(fx, i);
assert_eq!(src_layout.fields.offset(i).bytes(), 0);
assert_eq!(dst_layout.fields.offset(i).bytes(), 0);
if src_f.is_zst() {
continue;
}
assert_eq!(src_layout.size, src_f.size);
let dst_f = dst_layout.field(fx, i);
assert_ne!(src_f.ty, dst_f.ty);
assert_eq!(result, None);
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result = Some(unsize_ptr(fx, src, src_f, dst_f, old_info));
}
result.unwrap()
}
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_ => bug!("unsize_ptr: called on bad types"),
}
}
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/// Coerces `src` to `dst_ty` which is guaranteed to be a `dyn*` type.
pub(crate) fn cast_to_dyn_star<'tcx>(
fx: &mut FunctionCx<'_, '_, 'tcx>,
src: Value,
src_ty_and_layout: TyAndLayout<'tcx>,
dst_ty: Ty<'tcx>,
old_info: Option<Value>,
) -> (Value, Value) {
assert!(
matches!(dst_ty.kind(), ty::Dynamic(_, _, ty::DynStar)),
"destination type must be a dyn*"
);
(src, unsized_info(fx, src_ty_and_layout.ty, dst_ty, old_info))
}
/// Coerce `src`, which is a reference to a value of type `src_ty`,
/// to a value of type `dst_ty` and store the result in `dst`
pub(crate) fn coerce_unsized_into<'tcx>(
fx: &mut FunctionCx<'_, '_, 'tcx>,
src: CValue<'tcx>,
dst: CPlace<'tcx>,
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) {
let src_ty = src.layout().ty;
let dst_ty = dst.layout().ty;
let mut coerce_ptr = || {
let (base, info) =
if fx.layout_of(src.layout().ty.builtin_deref(true).unwrap().ty).is_unsized() {
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let (old_base, old_info) = src.load_scalar_pair(fx);
unsize_ptr(fx, old_base, src.layout(), dst.layout(), Some(old_info))
} else {
let base = src.load_scalar(fx);
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unsize_ptr(fx, base, src.layout(), dst.layout(), None)
};
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dst.write_cvalue(fx, CValue::by_val_pair(base, info, dst.layout()));
};
match (&src_ty.kind(), &dst_ty.kind()) {
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(&ty::Ref(..), &ty::Ref(..))
| (&ty::Ref(..), &ty::RawPtr(..))
| (&ty::RawPtr(..), &ty::RawPtr(..)) => coerce_ptr(),
(&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => {
assert_eq!(def_a, def_b);
for i in 0..def_a.variant(VariantIdx::new(0)).fields.len() {
let src_f = src.value_field(fx, mir::Field::new(i));
let dst_f = dst.place_field(fx, mir::Field::new(i));
if dst_f.layout().is_zst() {
continue;
}
if src_f.layout().ty == dst_f.layout().ty {
dst_f.write_cvalue(fx, src_f);
} else {
coerce_unsized_into(fx, src_f, dst_f);
}
}
}
_ => bug!("coerce_unsized_into: invalid coercion {:?} -> {:?}", src_ty, dst_ty),
}
}
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pub(crate) fn coerce_dyn_star<'tcx>(
fx: &mut FunctionCx<'_, '_, 'tcx>,
src: CValue<'tcx>,
dst: CPlace<'tcx>,
) {
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let (data, extra) = if let ty::Dynamic(_, _, ty::DynStar) = src.layout().ty.kind() {
let (data, vtable) = src.load_scalar_pair(fx);
(data, Some(vtable))
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} else {
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let data = src.load_scalar(fx);
(data, None)
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};
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let (data, vtable) = cast_to_dyn_star(fx, data, src.layout(), dst.layout().ty, extra);
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dst.write_cvalue(fx, CValue::by_val_pair(data, vtable, dst.layout()));
}
// Adapted from https://github.com/rust-lang/rust/blob/2a663555ddf36f6b041445894a8c175cd1bc718c/src/librustc_codegen_ssa/glue.rs
pub(crate) fn size_and_align_of_dst<'tcx>(
fx: &mut FunctionCx<'_, '_, 'tcx>,
layout: TyAndLayout<'tcx>,
info: Value,
) -> (Value, Value) {
assert!(layout.is_unsized() || layout.abi == Abi::Uninhabited);
match layout.ty.kind() {
ty::Dynamic(..) => {
// load size/align from vtable
(crate::vtable::size_of_obj(fx, info), crate::vtable::min_align_of_obj(fx, info))
}
ty::Slice(_) | ty::Str => {
let unit = layout.field(fx, 0);
// The info in this case is the length of the str, so the size is that
// times the unit size.
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(
fx.bcx.ins().imul_imm(info, unit.size.bytes() as i64),
fx.bcx.ins().iconst(fx.pointer_type, unit.align.abi.bytes() as i64),
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)
}
_ => {
// First get the size of all statically known fields.
// Don't use size_of because it also rounds up to alignment, which we
// want to avoid, as the unsized field's alignment could be smaller.
assert!(!layout.ty.is_simd());
let i = layout.fields.count() - 1;
let sized_size = layout.fields.offset(i).bytes();
let sized_align = layout.align.abi.bytes();
let sized_align = fx.bcx.ins().iconst(fx.pointer_type, sized_align as i64);
// Recurse to get the size of the dynamically sized field (must be
// the last field).
let field_layout = layout.field(fx, i);
let (unsized_size, mut unsized_align) = size_and_align_of_dst(fx, field_layout, info);
// FIXME (#26403, #27023): We should be adding padding
// to `sized_size` (to accommodate the `unsized_align`
// required of the unsized field that follows) before
// summing it with `sized_size`. (Note that since #26403
// is unfixed, we do not yet add the necessary padding
// here. But this is where the add would go.)
// Return the sum of sizes and max of aligns.
let size = fx.bcx.ins().iadd_imm(unsized_size, sized_size as i64);
// Packed types ignore the alignment of their fields.
if let ty::Adt(def, _) = layout.ty.kind() {
if def.repr().packed() {
unsized_align = sized_align;
}
}
// Choose max of two known alignments (combined value must
// be aligned according to more restrictive of the two).
let cmp = fx.bcx.ins().icmp(IntCC::UnsignedGreaterThan, sized_align, unsized_align);
let align = fx.bcx.ins().select(cmp, sized_align, unsized_align);
// Issue #27023: must add any necessary padding to `size`
// (to make it a multiple of `align`) before returning it.
//
// Namely, the returned size should be, in C notation:
//
// `size + ((size & (align-1)) ? align : 0)`
//
// emulated via the semi-standard fast bit trick:
//
// `(size + (align-1)) & -align`
let addend = fx.bcx.ins().iadd_imm(align, -1);
let add = fx.bcx.ins().iadd(size, addend);
let neg = fx.bcx.ins().ineg(align);
let size = fx.bcx.ins().band(add, neg);
(size, align)
}
}
}