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Span tweaks.

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`Span` has undergone some changes over the years (addition of an optional
parent, and possible inlining of the context in interned spans) but the
comments and identifiers used haven't kept up. As a result, I find it
harder to understand than I should.

This commit reworks the comments, renames some identifiers, and
restructures the code slightly, all to make things clearer. I now feel
like I understand this code again.
This commit is contained in:
Nicholas Nethercote 2023-09-06 09:07:58 +10:00
parent 69ec43001a
commit e525e4f10a
1 changed files with 128 additions and 110 deletions
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238
compiler/rustc_span/src/span_encoding.rs
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@ -1,9 +1,3 @@
// Spans are encoded using 1-bit tag and 2 different encoding formats (one for each tag value).
// One format is used for keeping span data inline,
// another contains index into an out-of-line span interner.
// The encoding format for inline spans were obtained by optimizing over crates in rustc/libstd.
// See https://internals.rust-lang.org/t/rfc-compiler-refactoring-spans/1357/28
use crate::def_id::{DefIndex, LocalDefId};
use crate::hygiene::SyntaxContext;
use crate::SPAN_TRACK;
@ -13,59 +7,69 @@ use rustc_data_structures::fx::FxIndexSet;
/// A compressed span.
///
/// Whereas [`SpanData`] is 16 bytes, which is a bit too big to stick everywhere, `Span`
/// is a form that only takes up 8 bytes, with less space for the length, parent and
/// context. The vast majority (99.9%+) of `SpanData` instances will fit within
/// those 8 bytes; any `SpanData` whose fields don't fit into a `Span` are
/// [`SpanData`] is 16 bytes, which is too big to stick everywhere. `Span` only
/// takes up 8 bytes, with less space for the length, parent and context. The
/// vast majority (99.9%+) of `SpanData` instances can be made to fit within
/// those 8 bytes. Any `SpanData` whose fields don't fit into a `Span` are
/// stored in a separate interner table, and the `Span` will index into that
/// table. Interning is rare enough that the cost is low, but common enough
/// that the code is exercised regularly.
///
/// An earlier version of this code used only 4 bytes for `Span`, but that was
/// slower because only 80--90% of spans could be stored inline (even less in
/// very large crates) and so the interner was used a lot more.
/// very large crates) and so the interner was used a lot more. That version of
/// the code also predated the storage of parents.
///
/// Inline (compressed) format with no parent:
/// - `span.base_or_index == span_data.lo`
/// - `span.len_or_tag == len == span_data.hi - span_data.lo` (must be `<= MAX_LEN`)
/// - `span.ctxt_or_tag == span_data.ctxt` (must be `<= MAX_CTXT`)
/// There are four different span forms.
///
/// Interned format with inline `SyntaxContext`:
/// - `span.base_or_index == index` (indexes into the interner table)
/// - `span.len_or_tag == LEN_TAG` (high bit set, all other bits are zero)
/// - `span.ctxt_or_tag == span_data.ctxt` (must be `<= MAX_CTXT`)
/// Inline-context format (requires non-huge length, non-huge context, and no parent):
/// - `span.lo_or_index == span_data.lo`
/// - `span.len_with_tag_or_marker == len == span_data.hi - span_data.lo` (must be `<= MAX_LEN`)
/// - `span.ctxt_or_parent_or_marker == span_data.ctxt` (must be `<= MAX_CTXT`)
///
/// Inline (compressed) format with root context:
/// - `span.base_or_index == span_data.lo`
/// - `span.len_or_tag == len == span_data.hi - span_data.lo` (must be `<= MAX_LEN`)
/// - `span.len_or_tag` has top bit (`PARENT_MASK`) set
/// - `span.ctxt == span_data.parent` (must be `<= MAX_CTXT`)
/// Inline-parent format (requires non-huge length, root context, and non-huge parent):
/// - `span.lo_or_index == span_data.lo`
/// - `span.len_with_tag_or_marker & !PARENT_TAG == len == span_data.hi - span_data.lo`
/// (must be `<= MAX_LEN`)
/// - `span.len_with_tag_or_marker` has top bit (`PARENT_TAG`) set
/// - `span.ctxt_or_parent_or_marker == span_data.parent` (must be `<= MAX_CTXT`)
///
/// Interned format:
/// - `span.base_or_index == index` (indexes into the interner table)
/// - `span.len_or_tag == LEN_TAG` (high bit set, all other bits are zero)
/// - `span.ctxt_or_tag == CTXT_TAG`
/// Partially-interned format (requires non-huge context):
/// - `span.lo_or_index == index` (indexes into the interner table)
/// - `span.len_with_tag_or_marker == BASE_LEN_INTERNED_MARKER`
/// - `span.ctxt_or_parent_or_marker == span_data.ctxt` (must be `<= MAX_CTXT`)
///
/// The inline form uses 0 for the tag value (rather than 1) so that we don't
/// need to mask out the tag bit when getting the length, and so that the
/// dummy span can be all zeroes.
/// Fully-interned format (all cases not covered above):
/// - `span.lo_or_index == index` (indexes into the interner table)
/// - `span.len_with_tag_or_marker == BASE_LEN_INTERNED_MARKER`
/// - `span.ctxt_or_parent_or_marker == CTXT_INTERNED_MARKER`
///
/// The partially-interned form requires looking in the interning table for
/// lo and length, but the context is stored inline as well as interned.
/// This is useful because context lookups are often done in isolation, and
/// inline lookups are quicker.
///
/// Notes about the choice of field sizes:
/// - `base` is 32 bits in both `Span` and `SpanData`, which means that `base`
/// values never cause interning. The number of bits needed for `base`
/// - `lo` is 32 bits in both `Span` and `SpanData`, which means that `lo`
/// values never cause interning. The number of bits needed for `lo`
/// depends on the crate size. 32 bits allows up to 4 GiB of code in a crate.
/// - `len` is 15 bits in `Span` (a u16, minus 1 bit for the tag) and 32 bits
/// in `SpanData`, which means that large `len` values will cause interning.
/// The number of bits needed for `len` does not depend on the crate size.
/// The most common numbers of bits for `len` are from 0 to 7, with a peak usually
/// at 3 or 4, and then it drops off quickly from 8 onwards. 15 bits is enough
/// for 99.99%+ of cases, but larger values (sometimes 20+ bits) might occur
/// dozens of times in a typical crate.
/// - `ctxt_or_tag` is 16 bits in `Span` and 32 bits in `SpanData`, which means that
/// large `ctxt` values will cause interning. The number of bits needed for
/// `ctxt` values depend partly on the crate size and partly on the form of
/// the code. No crates in `rustc-perf` need more than 15 bits for `ctxt_or_tag`,
/// but larger crates might need more than 16 bits.
/// Having no compression on this field means there is no performance cliff
/// if a crate exceeds a particular size.
/// - `len` is ~15 bits in `Span` (a u16, minus 1 bit for PARENT_TAG) and 32
/// bits in `SpanData`, which means that large `len` values will cause
/// interning. The number of bits needed for `len` does not depend on the
/// crate size. The most common numbers of bits for `len` are from 0 to 7,
/// with a peak usually at 3 or 4, and then it drops off quickly from 8
/// onwards. 15 bits is enough for 99.99%+ of cases, but larger values
/// (sometimes 20+ bits) might occur dozens of times in a typical crate.
/// - `ctxt_or_parent_or_marker` is 16 bits in `Span` and two 32 bit fields in
/// `SpanData`, which means intering will happen if `ctxt` is large, if
/// `parent` is large, or if both values are non-zero. The number of bits
/// needed for `ctxt` values depend partly on the crate size and partly on
/// the form of the code. No crates in `rustc-perf` need more than 15 bits
/// for `ctxt_or_parent_or_marker`, but larger crates might need more than 16
/// bits. The number of bits needed for `parent` hasn't been measured,
/// because `parent` isn't currently used by default.
///
/// In order to reliably use parented spans in incremental compilation,
/// the dependency to the parent definition's span. This is performed
@ -74,19 +78,22 @@ use rustc_data_structures::fx::FxIndexSet;
#[derive(Clone, Copy, Eq, PartialEq, Hash)]
#[rustc_pass_by_value]
pub struct Span {
base_or_index: u32,
len_or_tag: u16,
ctxt_or_tag: u16,
lo_or_index: u32,
len_with_tag_or_marker: u16,
ctxt_or_parent_or_marker: u16,
}
const LEN_TAG: u16 = 0b1111_1111_1111_1111;
const PARENT_MASK: u16 = 0b1000_0000_0000_0000;
const MAX_LEN: u32 = 0b0111_1111_1111_1111;
const CTXT_TAG: u32 = 0b1111_1111_1111_1111;
const MAX_CTXT: u32 = CTXT_TAG - 1;
// `MAX_LEN` is chosen so that `PARENT_TAG | MAX_LEN` is distinct from
// `BASE_LEN_INTERNED_MARKER`. (If `MAX_LEN` was 1 higher, this wouldn't be true.)
const MAX_LEN: u32 = 0b0111_1111_1111_1110;
const MAX_CTXT: u32 = 0b0111_1111_1111_1110;
const PARENT_TAG: u16 = 0b1000_0000_0000_0000;
const BASE_LEN_INTERNED_MARKER: u16 = 0b1111_1111_1111_1111;
const CTXT_INTERNED_MARKER: u16 = 0b1111_1111_1111_1111;
/// Dummy span, both position and length are zero, syntax context is zero as well.
pub const DUMMY_SP: Span = Span { base_or_index: 0, len_or_tag: 0, ctxt_or_tag: 0 };
/// The dummy span has zero position, length, and context, and no parent.
pub const DUMMY_SP: Span =
Span { lo_or_index: 0, len_with_tag_or_marker: 0, ctxt_or_parent_or_marker: 0 };
impl Span {
#[inline]
@ -100,39 +107,43 @@ impl Span {
std::mem::swap(&mut lo, &mut hi);
}
let (base, len, ctxt2) = (lo.0, hi.0 - lo.0, ctxt.as_u32());
let (lo2, len, ctxt2) = (lo.0, hi.0 - lo.0, ctxt.as_u32());
if len <= MAX_LEN && ctxt2 <= MAX_CTXT {
let len_or_tag = len as u16;
debug_assert_eq!(len_or_tag & PARENT_MASK, 0);
if let Some(parent) = parent {
// Inline format with parent.
let len_or_tag = len_or_tag | PARENT_MASK;
let parent2 = parent.local_def_index.as_u32();
if ctxt2 == SyntaxContext::root().as_u32()
&& parent2 <= MAX_CTXT
&& len_or_tag < LEN_TAG
{
debug_assert_ne!(len_or_tag, LEN_TAG);
return Span { base_or_index: base, len_or_tag, ctxt_or_tag: parent2 as u16 };
}
} else {
// Inline format with ctxt.
debug_assert_ne!(len_or_tag, LEN_TAG);
if len <= MAX_LEN {
if ctxt2 <= MAX_CTXT && parent.is_none() {
// Inline-context format.
return Span {
base_or_index: base,
len_or_tag: len as u16,
ctxt_or_tag: ctxt2 as u16,
lo_or_index: lo2,
len_with_tag_or_marker: len as u16,
ctxt_or_parent_or_marker: ctxt2 as u16,
};
} else if ctxt2 == SyntaxContext::root().as_u32()
&& let Some(parent) = parent
&& let parent2 = parent.local_def_index.as_u32()
&& parent2 <= MAX_CTXT
{
// Inline-parent format.
return Span {
lo_or_index: lo2,
len_with_tag_or_marker: PARENT_TAG | len as u16,
ctxt_or_parent_or_marker: parent2 as u16
};
}
}
// Interned format.
// Partially-interned or fully-interned format.
let index =
with_span_interner(|interner| interner.intern(&SpanData { lo, hi, ctxt, parent }));
let ctxt_or_tag = if ctxt2 <= MAX_CTXT { ctxt2 } else { CTXT_TAG } as u16;
Span { base_or_index: index, len_or_tag: LEN_TAG, ctxt_or_tag }
let ctxt_or_parent_or_marker = if ctxt2 <= MAX_CTXT {
ctxt2 as u16 // partially-interned
} else {
CTXT_INTERNED_MARKER // fully-interned
};
Span {
lo_or_index: index,
len_with_tag_or_marker: BASE_LEN_INTERNED_MARKER,
ctxt_or_parent_or_marker,
}
}
#[inline]
@ -148,56 +159,63 @@ impl Span {
/// This function must not be used outside the incremental engine.
#[inline]
pub fn data_untracked(self) -> SpanData {
if self.len_or_tag != LEN_TAG {
// Inline format.
if self.len_or_tag & PARENT_MASK == 0 {
debug_assert!(self.len_or_tag as u32 <= MAX_LEN);
if self.len_with_tag_or_marker != BASE_LEN_INTERNED_MARKER {
if self.len_with_tag_or_marker & PARENT_TAG == 0 {
// Inline-context format.
let len = self.len_with_tag_or_marker as u32;
debug_assert!(len <= MAX_LEN);
SpanData {
lo: BytePos(self.base_or_index),
hi: BytePos(self.base_or_index + self.len_or_tag as u32),
ctxt: SyntaxContext::from_u32(self.ctxt_or_tag as u32),
lo: BytePos(self.lo_or_index),
hi: BytePos(self.lo_or_index + len),
ctxt: SyntaxContext::from_u32(self.ctxt_or_parent_or_marker as u32),
parent: None,
}
} else {
let len = self.len_or_tag & !PARENT_MASK;
debug_assert!(len as u32 <= MAX_LEN);
let parent =
LocalDefId { local_def_index: DefIndex::from_u32(self.ctxt_or_tag as u32) };
// Inline-parent format.
let len = (self.len_with_tag_or_marker & !PARENT_TAG) as u32;
debug_assert!(len <= MAX_LEN);
let parent = LocalDefId {
local_def_index: DefIndex::from_u32(self.ctxt_or_parent_or_marker as u32),
};
SpanData {
lo: BytePos(self.base_or_index),
hi: BytePos(self.base_or_index + len as u32),
lo: BytePos(self.lo_or_index),
hi: BytePos(self.lo_or_index + len),
ctxt: SyntaxContext::root(),
parent: Some(parent),
}
}
} else {
// Interned format.
let index = self.base_or_index;
// Fully-interned or partially-interned format. In either case,
// the interned value contains all the data, so we don't need to
// distinguish them.
let index = self.lo_or_index;
with_span_interner(|interner| interner.spans[index as usize])
}
}
/// This function is used as a fast path when decoding the full `SpanData` is not necessary.
/// It's a cut-down version of `data_untracked`.
#[inline]
pub fn ctxt(self) -> SyntaxContext {
let ctxt_or_tag = self.ctxt_or_tag as u32;
// Check for interned format.
if self.len_or_tag == LEN_TAG {
if ctxt_or_tag == CTXT_TAG {
// Fully interned format.
let index = self.base_or_index;
with_span_interner(|interner| interner.spans[index as usize].ctxt)
if self.len_with_tag_or_marker != BASE_LEN_INTERNED_MARKER {
if self.len_with_tag_or_marker & PARENT_TAG == 0 {
// Inline-context format.
SyntaxContext::from_u32(self.ctxt_or_parent_or_marker as u32)
} else {
// Interned format with inline ctxt.
SyntaxContext::from_u32(ctxt_or_tag)
// Inline-parent format. We know that the SyntaxContext is root.
SyntaxContext::root()
}
} else if self.len_or_tag & PARENT_MASK == 0 {
// Inline format with inline ctxt.
SyntaxContext::from_u32(ctxt_or_tag)
} else {
// Inline format with inline parent.
// We know that the SyntaxContext is root.
SyntaxContext::root()
if self.ctxt_or_parent_or_marker != CTXT_INTERNED_MARKER {
// Partially-interned format. This path avoids looking up the
// interned value, and is the whole point of the
// partially-interned format.
SyntaxContext::from_u32(self.ctxt_or_parent_or_marker as u32)
} else {
// Fully-interned format.
let index = self.lo_or_index;
with_span_interner(|interner| interner.spans[index as usize].ctxt)
}
}
}
}