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Rollup merge of rust-lang#72497 - RalfJung:tag-term, r=oli-obk
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tag/niche terminology cleanup

The term "discriminant" was used in two ways throughout the compiler:
* every enum variant has a corresponding discriminant, that can be given explicitly with `Variant = N`.
* that discriminant is then encoded in memory to store which variant is active -- but this encoded form of the discriminant was also often called "discriminant", even though it is conceptually quite different (e.g., it can be smaller in size, or even use niche-filling).

After discussion with @eddyb, this renames the second term to "tag". The way the tag is encoded can be either `TagEncoding::Direct` (formerly `DiscriminantKind::Tag`) or `TagEncoding::Niche` (formerly `DiscrimianntKind::Niche`).

This finally resolves some long-standing confusion I had about the handling of variant indices and discriminants, which surfaced in rust-lang#72419.

(There is also a `DiscriminantKind` type in libcore, it remains unaffected. I think this corresponds to the discriminant, not the tag, so that seems all right.)

r? @eddyb
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@Manishearth
Manishearth committed Jun 16, 2020
2 parents 259b3f6 + 10c8d2a commit dd83a9b
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Showing 13 changed files with 181 additions and 180 deletions.
108 changes: 53 additions & 55 deletions src/librustc_codegen_llvm/debuginfo/metadata.rs
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
use self::EnumDiscriminantInfo::*;
use self::EnumTagInfo::*;
use self::MemberDescriptionFactory::*;
use self::RecursiveTypeDescription::*;

Expand Down Expand Up @@ -40,7 +40,7 @@ use rustc_middle::{bug, span_bug};
use rustc_session::config::{self, DebugInfo};
use rustc_span::symbol::{Interner, Symbol};
use rustc_span::{self, SourceFile, SourceFileHash, Span};
use rustc_target::abi::{Abi, Align, DiscriminantKind, HasDataLayout, Integer, LayoutOf};
use rustc_target::abi::{Abi, Align, HasDataLayout, Integer, LayoutOf, TagEncoding};
use rustc_target::abi::{Int, Pointer, F32, F64};
use rustc_target::abi::{Primitive, Size, VariantIdx, Variants};

Expand Down Expand Up @@ -1335,7 +1335,7 @@ fn generator_layout_and_saved_local_names(
struct EnumMemberDescriptionFactory<'ll, 'tcx> {
enum_type: Ty<'tcx>,
layout: TyAndLayout<'tcx>,
discriminant_type_metadata: Option<&'ll DIType>,
tag_type_metadata: Option<&'ll DIType>,
containing_scope: &'ll DIScope,
span: Span,
}
Expand Down Expand Up @@ -1385,7 +1385,7 @@ impl EnumMemberDescriptionFactory<'ll, 'tcx> {
cx,
self.layout,
variant_info,
NoDiscriminant,
NoTag,
self_metadata,
self.span,
);
Expand All @@ -1409,19 +1409,19 @@ impl EnumMemberDescriptionFactory<'ll, 'tcx> {
}]
}
Variants::Multiple {
discr_kind: DiscriminantKind::Tag,
discr_index,
tag_encoding: TagEncoding::Direct,
tag_field,
ref variants,
..
} => {
let discriminant_info = if fallback {
RegularDiscriminant {
discr_field: Field::from(discr_index),
discr_type_metadata: self.discriminant_type_metadata.unwrap(),
let tag_info = if fallback {
RegularTag {
tag_field: Field::from(tag_field),
tag_type_metadata: self.tag_type_metadata.unwrap(),
}
} else {
// This doesn't matter in this case.
NoDiscriminant
NoTag
};
variants
.iter_enumerated()
Expand All @@ -1432,7 +1432,7 @@ impl EnumMemberDescriptionFactory<'ll, 'tcx> {
cx,
variant,
variant_info,
discriminant_info,
tag_info,
self_metadata,
self.span,
);
Expand Down Expand Up @@ -1467,11 +1467,11 @@ impl EnumMemberDescriptionFactory<'ll, 'tcx> {
.collect()
}
Variants::Multiple {
discr_kind:
DiscriminantKind::Niche { ref niche_variants, niche_start, dataful_variant },
ref discr,
tag_encoding:
TagEncoding::Niche { ref niche_variants, niche_start, dataful_variant },
ref tag,
ref variants,
discr_index,
tag_field,
} => {
if fallback {
let variant = self.layout.for_variant(cx, dataful_variant);
Expand All @@ -1480,7 +1480,7 @@ impl EnumMemberDescriptionFactory<'ll, 'tcx> {
cx,
variant,
variant_info_for(dataful_variant),
OptimizedDiscriminant,
OptimizedTag,
self.containing_scope,
self.span,
);
Expand Down Expand Up @@ -1524,8 +1524,8 @@ impl EnumMemberDescriptionFactory<'ll, 'tcx> {
cx,
&mut name,
self.layout,
self.layout.fields.offset(discr_index),
self.layout.field(cx, discr_index).size,
self.layout.fields.offset(tag_field),
self.layout.field(cx, tag_field).size,
);
variant_info_for(*niche_variants.start()).map_struct_name(|variant_name| {
name.push_str(variant_name);
Expand All @@ -1552,7 +1552,7 @@ impl EnumMemberDescriptionFactory<'ll, 'tcx> {
cx,
variant,
variant_info,
OptimizedDiscriminant,
OptimizedTag,
self_metadata,
self.span,
);
Expand All @@ -1573,7 +1573,7 @@ impl EnumMemberDescriptionFactory<'ll, 'tcx> {
let value = (i.as_u32() as u128)
.wrapping_sub(niche_variants.start().as_u32() as u128)
.wrapping_add(niche_start);
let value = truncate(value, discr.value.size(cx));
let value = truncate(value, tag.value.size(cx));
// NOTE(eddyb) do *NOT* remove this assert, until
// we pass the full 128-bit value to LLVM, otherwise
// truncation will be silent and remain undetected.
Expand Down Expand Up @@ -1603,7 +1603,7 @@ struct VariantMemberDescriptionFactory<'ll, 'tcx> {
/// Cloned from the `layout::Struct` describing the variant.
offsets: Vec<Size>,
args: Vec<(String, Ty<'tcx>)>,
discriminant_type_metadata: Option<&'ll DIType>,
tag_type_metadata: Option<&'ll DIType>,
span: Span,
}

Expand All @@ -1617,7 +1617,7 @@ impl VariantMemberDescriptionFactory<'ll, 'tcx> {
MemberDescription {
name: name.to_string(),
type_metadata: if use_enum_fallback(cx) {
match self.discriminant_type_metadata {
match self.tag_type_metadata {
// Discriminant is always the first field of our variant
// when using the enum fallback.
Some(metadata) if i == 0 => metadata,
Expand All @@ -1637,11 +1637,14 @@ impl VariantMemberDescriptionFactory<'ll, 'tcx> {
}
}

// FIXME: terminology here should be aligned with `abi::TagEncoding`.
// `OptimizedTag` is `TagEncoding::Niche`, `RegularTag` is `TagEncoding::Direct`.
// `NoTag` should be removed; users should use `Option<EnumTagInfo>` instead.
#[derive(Copy, Clone)]
enum EnumDiscriminantInfo<'ll> {
RegularDiscriminant { discr_field: Field, discr_type_metadata: &'ll DIType },
OptimizedDiscriminant,
NoDiscriminant,
enum EnumTagInfo<'ll> {
RegularTag { tag_field: Field, tag_type_metadata: &'ll DIType },
OptimizedTag,
NoTag,
}

#[derive(Copy, Clone)]
Expand Down Expand Up @@ -1706,7 +1709,7 @@ fn describe_enum_variant(
cx: &CodegenCx<'ll, 'tcx>,
layout: layout::TyAndLayout<'tcx>,
variant: VariantInfo<'_, 'tcx>,
discriminant_info: EnumDiscriminantInfo<'ll>,
discriminant_info: EnumTagInfo<'ll>,
containing_scope: &'ll DIScope,
span: Span,
) -> (&'ll DICompositeType, MemberDescriptionFactory<'ll, 'tcx>) {
Expand All @@ -1722,12 +1725,12 @@ fn describe_enum_variant(
let (offsets, args) = if use_enum_fallback(cx) {
// If this is not a univariant enum, there is also the discriminant field.
let (discr_offset, discr_arg) = match discriminant_info {
RegularDiscriminant { discr_field, .. } => {
RegularTag { tag_field, .. } => {
// We have the layout of an enum variant, we need the layout of the outer enum
let enum_layout = cx.layout_of(layout.ty);
let offset = enum_layout.fields.offset(discr_field.as_usize());
let offset = enum_layout.fields.offset(tag_field.as_usize());
let args =
("RUST$ENUM$DISR".to_owned(), enum_layout.field(cx, discr_field.as_usize()).ty);
("RUST$ENUM$DISR".to_owned(), enum_layout.field(cx, tag_field.as_usize()).ty);
(Some(offset), Some(args))
}
_ => (None, None),
Expand Down Expand Up @@ -1757,8 +1760,8 @@ fn describe_enum_variant(
let member_description_factory = VariantMDF(VariantMemberDescriptionFactory {
offsets,
args,
discriminant_type_metadata: match discriminant_info {
RegularDiscriminant { discr_type_metadata, .. } => Some(discr_type_metadata),
tag_type_metadata: match discriminant_info {
RegularTag { tag_type_metadata, .. } => Some(tag_type_metadata),
_ => None,
},
span,
Expand Down Expand Up @@ -1880,18 +1883,18 @@ fn prepare_enum_metadata(

if let (
&Abi::Scalar(_),
&Variants::Multiple { discr_kind: DiscriminantKind::Tag, ref discr, .. },
&Variants::Multiple { tag_encoding: TagEncoding::Direct, ref tag, .. },
) = (&layout.abi, &layout.variants)
{
return FinalMetadata(discriminant_type_metadata(discr.value));
return FinalMetadata(discriminant_type_metadata(tag.value));
}

if use_enum_fallback(cx) {
let discriminant_type_metadata = match layout.variants {
Variants::Single { .. }
| Variants::Multiple { discr_kind: DiscriminantKind::Niche { .. }, .. } => None,
Variants::Multiple { discr_kind: DiscriminantKind::Tag, ref discr, .. } => {
Some(discriminant_type_metadata(discr.value))
| Variants::Multiple { tag_encoding: TagEncoding::Niche { .. }, .. } => None,
Variants::Multiple { tag_encoding: TagEncoding::Direct, ref tag, .. } => {
Some(discriminant_type_metadata(tag.value))
}
};

Expand Down Expand Up @@ -1927,7 +1930,7 @@ fn prepare_enum_metadata(
EnumMDF(EnumMemberDescriptionFactory {
enum_type,
layout,
discriminant_type_metadata,
tag_type_metadata: discriminant_type_metadata,
containing_scope,
span,
}),
Expand All @@ -1943,24 +1946,21 @@ fn prepare_enum_metadata(
Variants::Single { .. } => None,

Variants::Multiple {
discr_kind: DiscriminantKind::Niche { .. },
ref discr,
discr_index,
..
tag_encoding: TagEncoding::Niche { .. }, ref tag, tag_field, ..
} => {
// Find the integer type of the correct size.
let size = discr.value.size(cx);
let align = discr.value.align(cx);
let size = tag.value.size(cx);
let align = tag.value.align(cx);

let discr_type = match discr.value {
let tag_type = match tag.value {
Int(t, _) => t,
F32 => Integer::I32,
F64 => Integer::I64,
Pointer => cx.data_layout().ptr_sized_integer(),
}
.to_ty(cx.tcx, false);

let discr_metadata = basic_type_metadata(cx, discr_type);
let tag_metadata = basic_type_metadata(cx, tag_type);
unsafe {
Some(llvm::LLVMRustDIBuilderCreateMemberType(
DIB(cx),
Expand All @@ -1971,17 +1971,15 @@ fn prepare_enum_metadata(
UNKNOWN_LINE_NUMBER,
size.bits(),
align.abi.bits() as u32,
layout.fields.offset(discr_index).bits(),
layout.fields.offset(tag_field).bits(),
DIFlags::FlagArtificial,
discr_metadata,
tag_metadata,
))
}
}

Variants::Multiple {
discr_kind: DiscriminantKind::Tag, ref discr, discr_index, ..
} => {
let discr_type = discr.value.to_ty(cx.tcx);
Variants::Multiple { tag_encoding: TagEncoding::Direct, ref tag, tag_field, .. } => {
let discr_type = tag.value.to_ty(cx.tcx);
let (size, align) = cx.size_and_align_of(discr_type);

let discr_metadata = basic_type_metadata(cx, discr_type);
Expand All @@ -1995,7 +1993,7 @@ fn prepare_enum_metadata(
UNKNOWN_LINE_NUMBER,
size.bits(),
align.bits() as u32,
layout.fields.offset(discr_index).bits(),
layout.fields.offset(tag_field).bits(),
DIFlags::FlagArtificial,
discr_metadata,
))
Expand Down Expand Up @@ -2081,7 +2079,7 @@ fn prepare_enum_metadata(
EnumMDF(EnumMemberDescriptionFactory {
enum_type,
layout,
discriminant_type_metadata: None,
tag_type_metadata: None,
containing_scope,
span,
}),
Expand Down
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