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//! Functions for reading and writing discriminants of multi-variant layouts (enums and generators).
use rustc_middle::ty::layout::{LayoutOf, PrimitiveExt};
use rustc_middle::{mir, ty};
use rustc_target::abi::{self, TagEncoding};
use rustc_target::abi::{VariantIdx, Variants};
use super::{ImmTy, InterpCx, InterpResult, Machine, OpTy, PlaceTy, Scalar};
impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
/// Writes the discriminant of the given variant.
#[instrument(skip(self), level = "trace")]
pub fn write_discriminant(
&mut self,
variant_index: VariantIdx,
dest: &PlaceTy<'tcx, M::Provenance>,
) -> InterpResult<'tcx> {
// Layout computation excludes uninhabited variants from consideration
// therefore there's no way to represent those variants in the given layout.
// Essentially, uninhabited variants do not have a tag that corresponds to their
// discriminant, so we cannot do anything here.
// When evaluating we will always error before even getting here, but ConstProp 'executes'
// dead code, so we cannot ICE here.
if dest.layout.for_variant(self, variant_index).abi.is_uninhabited() {
throw_ub!(UninhabitedEnumVariantWritten)
}
match dest.layout.variants {
abi::Variants::Single { index } => {
assert_eq!(index, variant_index);
}
abi::Variants::Multiple {
tag_encoding: TagEncoding::Direct,
tag: tag_layout,
tag_field,
..
} => {
// No need to validate that the discriminant here because the
// `TyAndLayout::for_variant()` call earlier already checks the variant is valid.
let discr_val =
dest.layout.ty.discriminant_for_variant(*self.tcx, variant_index).unwrap().val;
// raw discriminants for enums are isize or bigger during
// their computation, but the in-memory tag is the smallest possible
// representation
let size = tag_layout.size(self);
let tag_val = size.truncate(discr_val);
let tag_dest = self.place_field(dest, tag_field)?;
self.write_scalar(Scalar::from_uint(tag_val, size), &tag_dest)?;
}
abi::Variants::Multiple {
tag_encoding:
TagEncoding::Niche { untagged_variant, ref niche_variants, niche_start },
tag: tag_layout,
tag_field,
..
} => {
// No need to validate that the discriminant here because the
// `TyAndLayout::for_variant()` call earlier already checks the variant is valid.
if variant_index != untagged_variant {
let variants_start = niche_variants.start().as_u32();
let variant_index_relative = variant_index
.as_u32()
.checked_sub(variants_start)
.expect("overflow computing relative variant idx");
// We need to use machine arithmetic when taking into account `niche_start`:
// tag_val = variant_index_relative + niche_start_val
let tag_layout = self.layout_of(tag_layout.primitive().to_int_ty(*self.tcx))?;
let niche_start_val = ImmTy::from_uint(niche_start, tag_layout);
let variant_index_relative_val =
ImmTy::from_uint(variant_index_relative, tag_layout);
let tag_val = self.binary_op(
mir::BinOp::Add,
&variant_index_relative_val,
&niche_start_val,
)?;
// Write result.
let niche_dest = self.place_field(dest, tag_field)?;
self.write_immediate(*tag_val, &niche_dest)?;
}
}
}
Ok(())
}
/// Read discriminant, return the runtime value as well as the variant index.
/// Can also legally be called on non-enums (e.g. through the discriminant_value intrinsic)!
#[instrument(skip(self), level = "trace")]
pub fn read_discriminant(
&self,
op: &OpTy<'tcx, M::Provenance>,
) -> InterpResult<'tcx, (Scalar<M::Provenance>, VariantIdx)> {
trace!("read_discriminant_value {:#?}", op.layout);
// Get type and layout of the discriminant.
let discr_layout = self.layout_of(op.layout.ty.discriminant_ty(*self.tcx))?;
trace!("discriminant type: {:?}", discr_layout.ty);
// We use "discriminant" to refer to the value associated with a particular enum variant.
// This is not to be confused with its "variant index", which is just determining its position in the
// declared list of variants -- they can differ with explicitly assigned discriminants.
// We use "tag" to refer to how the discriminant is encoded in memory, which can be either
// straight-forward (`TagEncoding::Direct`) or with a niche (`TagEncoding::Niche`).
let (tag_scalar_layout, tag_encoding, tag_field) = match op.layout.variants {
Variants::Single { index } => {
let discr = match op.layout.ty.discriminant_for_variant(*self.tcx, index) {
Some(discr) => {
// This type actually has discriminants.
assert_eq!(discr.ty, discr_layout.ty);
Scalar::from_uint(discr.val, discr_layout.size)
}
None => {
// On a type without actual discriminants, variant is 0.
assert_eq!(index.as_u32(), 0);
Scalar::from_uint(index.as_u32(), discr_layout.size)
}
};
return Ok((discr, index));
}
Variants::Multiple { tag, ref tag_encoding, tag_field, .. } => {
(tag, tag_encoding, tag_field)
}
};
// There are *three* layouts that come into play here:
// - The discriminant has a type for typechecking. This is `discr_layout`, and is used for
// the `Scalar` we return.
// - The tag (encoded discriminant) has layout `tag_layout`. This is always an integer type,
// and used to interpret the value we read from the tag field.
// For the return value, a cast to `discr_layout` is performed.
// - The field storing the tag has a layout, which is very similar to `tag_layout` but
// may be a pointer. This is `tag_val.layout`; we just use it for sanity checks.
// Get layout for tag.
let tag_layout = self.layout_of(tag_scalar_layout.primitive().to_int_ty(*self.tcx))?;
// Read tag and sanity-check `tag_layout`.
let tag_val = self.read_immediate(&self.operand_field(op, tag_field)?)?;
assert_eq!(tag_layout.size, tag_val.layout.size);
assert_eq!(tag_layout.abi.is_signed(), tag_val.layout.abi.is_signed());
trace!("tag value: {}", tag_val);
// Figure out which discriminant and variant this corresponds to.
Ok(match *tag_encoding {
TagEncoding::Direct => {
let scalar = tag_val.to_scalar();
// Generate a specific error if `tag_val` is not an integer.
// (`tag_bits` itself is only used for error messages below.)
let tag_bits = scalar
.try_to_int()
.map_err(|dbg_val| err_ub!(InvalidTag(dbg_val)))?
.assert_bits(tag_layout.size);
// Cast bits from tag layout to discriminant layout.
// After the checks we did above, this cannot fail, as
// discriminants are int-like.
let discr_val =
self.cast_from_int_like(scalar, tag_val.layout, discr_layout.ty).unwrap();
let discr_bits = discr_val.assert_bits(discr_layout.size);
// Convert discriminant to variant index, and catch invalid discriminants.
let index = match *op.layout.ty.kind() {
ty::Adt(adt, _) => {
adt.discriminants(*self.tcx).find(|(_, var)| var.val == discr_bits)
}
ty::Generator(def_id, substs, _) => {
let substs = substs.as_generator();
substs
.discriminants(def_id, *self.tcx)
.find(|(_, var)| var.val == discr_bits)
}
_ => span_bug!(self.cur_span(), "tagged layout for non-adt non-generator"),
}
.ok_or_else(|| err_ub!(InvalidTag(Scalar::from_uint(tag_bits, tag_layout.size))))?;
// Return the cast value, and the index.
(discr_val, index.0)
}
TagEncoding::Niche { untagged_variant, ref niche_variants, niche_start } => {
let tag_val = tag_val.to_scalar();
// Compute the variant this niche value/"tag" corresponds to. With niche layout,
// discriminant (encoded in niche/tag) and variant index are the same.
let variants_start = niche_variants.start().as_u32();
let variants_end = niche_variants.end().as_u32();
let variant = match tag_val.try_to_int() {
Err(dbg_val) => {
// So this is a pointer then, and casting to an int failed.
// Can only happen during CTFE.
// The niche must be just 0, and the ptr not null, then we know this is
// okay. Everything else, we conservatively reject.
let ptr_valid = niche_start == 0
&& variants_start == variants_end
&& !self.scalar_may_be_null(tag_val)?;
if !ptr_valid {
throw_ub!(InvalidTag(dbg_val))
}
untagged_variant
}
Ok(tag_bits) => {
let tag_bits = tag_bits.assert_bits(tag_layout.size);
// We need to use machine arithmetic to get the relative variant idx:
// variant_index_relative = tag_val - niche_start_val
let tag_val = ImmTy::from_uint(tag_bits, tag_layout);
let niche_start_val = ImmTy::from_uint(niche_start, tag_layout);
let variant_index_relative_val =
self.binary_op(mir::BinOp::Sub, &tag_val, &niche_start_val)?;
let variant_index_relative =
variant_index_relative_val.to_scalar().assert_bits(tag_val.layout.size);
// Check if this is in the range that indicates an actual discriminant.
if variant_index_relative <= u128::from(variants_end - variants_start) {
let variant_index_relative = u32::try_from(variant_index_relative)
.expect("we checked that this fits into a u32");
// Then computing the absolute variant idx should not overflow any more.
let variant_index = variants_start
.checked_add(variant_index_relative)
.expect("overflow computing absolute variant idx");
let variants_len = op
.layout
.ty
.ty_adt_def()
.expect("tagged layout for non adt")
.variants()
.len();
assert!(usize::try_from(variant_index).unwrap() < variants_len);
VariantIdx::from_u32(variant_index)
} else {
untagged_variant
}
}
};
// Compute the size of the scalar we need to return.
// No need to cast, because the variant index directly serves as discriminant and is
// encoded in the tag.
(Scalar::from_uint(variant.as_u32(), discr_layout.size), variant)
}
})
}
}
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