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comments and refactor variable names

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This commit is contained in:
Ralf Jung 2020-05-23 12:10:13 +02:00
parent 0f18203e85
commit 5a3971cdb8
1 changed files with 46 additions and 38 deletions
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84
src/librustc_mir/interpret/operand.rs
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@ -580,7 +580,14 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
) -> InterpResult<'tcx, (Scalar<M::PointerTag>, VariantIdx)> {
trace!("read_discriminant_value {:#?}", rval.layout);
let (discr_scalar_layout, discr_kind, discr_index) = match rval.layout.variants {
// We use "discriminant" to refer to the value associated with a particualr 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 (`DiscriminantKind::Tag`) or with a niche (`DiscriminantKind::Niche`).
// Unfortunately, the rest of the compiler calls the latter "discriminant", too, which makes things
// rather confusing.
let (tag_scalar_layout, tag_kind, tag_index) = match rval.layout.variants {
Variants::Single { index } => {
let discr = match rval.layout.ty.discriminant_for_variant(*self.tcx, index) {
Some(discr) => {
@ -602,31 +609,31 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
};
// There are *three* types/layouts that come into play here:
// - The field storing the discriminant has a layout, which my be a pointer.
// This is `discr_val.layout`; we just use it for sanity checks.
// - The discriminant has a layout for tag storing purposes, which is always an integer.
// This is `discr_layout` and is used to interpret the value we read from the
// discriminant field.
// - The discriminant also has a type for typechecking, and that type's
// layout can be *different*. This is `discr_ty`, and is used for the `Scalar`
// we return. If necessary, a cast from `discr_layout` is performed.
// - The discriminant has a type for typechecking. This is `discr_ty`, and is used for
// the `Scalar` we return.
// - The discriminant gets encoded as a tag/niche, with layout `tag_layout`.
// This is always an integer, and used to interpret the value we read from the
// tag field. For the return value, a cast to `discr_ty` 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 discr_layout = self.layout_of(discr_scalar_layout.value.to_int_ty(*self.tcx))?;
let tag_layout = self.layout_of(tag_scalar_layout.value.to_int_ty(*self.tcx))?;
// Read discriminant value and sanity-check `discr_layout`.
let discr_val = self.read_immediate(self.operand_field(rval, discr_index)?)?;
assert_eq!(discr_layout.size, discr_val.layout.size);
assert_eq!(discr_layout.abi.is_signed(), discr_val.layout.abi.is_signed());
let discr_val = discr_val.to_scalar()?;
trace!("discriminant value: {:?}", discr_val);
// Read tag and sanity-check `tag_layout`.
let tag_val = self.read_immediate(self.operand_field(rval, tag_index)?)?;
assert_eq!(tag_layout.size, tag_val.layout.size);
assert_eq!(tag_layout.abi.is_signed(), tag_val.layout.abi.is_signed());
let tag_val = tag_val.to_scalar()?;
trace!("tag value: {:?}", tag_val);
// Get type used by typechecking.
let discr_ty = match rval.layout.ty.kind {
ty::Adt(adt, _) => {
let discr_int_ty = Integer::from_attr(self, adt.repr.discr_type());
// The signedness of tag and discriminant is the same.
discr_int_ty.to_ty(*self.tcx, discr_layout.abi.is_signed())
discr_int_ty.to_ty(*self.tcx, tag_layout.abi.is_signed())
}
ty::Generator(_, substs, _) => {
let substs = substs.as_generator();
@ -636,17 +643,17 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
};
// Figure out which discriminant and variant this corresponds to.
Ok(match *discr_kind {
Ok(match *tag_kind {
DiscriminantKind::Tag => {
let discr_bits = self
.force_bits(discr_val, discr_layout.size)
.map_err(|_| err_ub!(InvalidDiscriminant(discr_val.erase_tag())))?;
// Cast discriminant bits to the right type.
let discr_ty_layout = self.layout_of(discr_ty)?;
let tag_bits = self
.force_bits(tag_val, tag_layout.size)
.map_err(|_| err_ub!(InvalidDiscriminant(tag_val.erase_tag())))?;
// Cast bits from tag layout to discriminant layout.
let discr_layout = self.layout_of(discr_ty)?;
let discr_val_cast =
self.cast_from_scalar(discr_bits, discr_layout, discr_ty);
let discr_bits = discr_val_cast.assert_bits(discr_ty_layout.size);
// Find variant index for this tag, and catch invalid discriminants.
self.cast_from_scalar(tag_bits, tag_layout, discr_ty);
let discr_bits = discr_val_cast.assert_bits(discr_layout.size);
// Convert discriminant to variant index, and catch invalid discriminants.
let index = match rval.layout.ty.kind {
ty::Adt(adt, _) => {
adt.discriminants(self.tcx.tcx).find(|(_, var)| var.val == discr_bits)
@ -659,36 +666,36 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
}
_ => bug!("tagged layout for non-adt non-generator"),
}
.ok_or_else(|| err_ub!(InvalidDiscriminant(discr_val.erase_tag())))?;
.ok_or_else(|| err_ub!(InvalidDiscriminant(tag_val.erase_tag())))?;
// Return the cast value, and the index.
(discr_val_cast, index.0)
}
DiscriminantKind::Niche { dataful_variant, ref niche_variants, niche_start } => {
// Compute the variant this discriminant corresponds to. With niche layout,
// tag and variant index are the same.
// 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 discr_val.to_bits_or_ptr(discr_layout.size, self) {
let variant = match tag_val.to_bits_or_ptr(tag_layout.size, self) {
Err(ptr) => {
// The niche must be just 0 (which an inbounds pointer value never is)
let ptr_valid = niche_start == 0
&& variants_start == variants_end
&& !self.memory.ptr_may_be_null(ptr);
if !ptr_valid {
throw_ub!(InvalidDiscriminant(discr_val.erase_tag()))
throw_ub!(InvalidDiscriminant(tag_val.erase_tag()))
}
dataful_variant
}
Ok(bits_discr) => {
Ok(tag_bits) => {
// We need to use machine arithmetic to get the relative variant idx:
// variant_index_relative = discr_val - niche_start_val
let discr_val = ImmTy::from_uint(bits_discr, discr_layout);
let niche_start_val = ImmTy::from_uint(niche_start, discr_layout);
// 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, discr_val, niche_start_val)?;
self.binary_op(mir::BinOp::Sub, tag_val, niche_start_val)?;
let variant_index_relative = variant_index_relative_val
.to_scalar()?
.assert_bits(discr_val.layout.size);
.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)
@ -712,7 +719,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
}
};
// Compute the size of the scalar we need to return.
// FIXME: Why do we not need to do a cast here like we do above?
// No need to cast, because the variant index directly serves as discriminant and is
// encoded in the tag.
let size = self.layout_of(discr_ty)?.size;
(Scalar::from_uint(variant.as_u32(), size), variant)
}