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merged 2 commits into from
Sep 9, 2023
Merged

Span tweaks #115594

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e525e4f
`Span` tweaks.
nnethercote Sep 5, 2023
5790372
Optimize `Span::is_dummy`.
nnethercote Sep 6, 2023
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12 changes: 1 addition & 11 deletions compiler/rustc_span/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -510,10 +510,6 @@ impl SpanData {
pub fn is_dummy(self) -> bool {
self.lo.0 == 0 && self.hi.0 == 0
}
#[inline]
pub fn is_visible(self, sm: &SourceMap) -> bool {
!self.is_dummy() && sm.is_span_accessible(self.span())
}
/// Returns `true` if `self` fully encloses `other`.
pub fn contains(self, other: Self) -> bool {
self.lo <= other.lo && other.hi <= self.hi
Expand Down Expand Up @@ -573,15 +569,9 @@ impl Span {
self.data().with_parent(ctxt)
}

/// Returns `true` if this is a dummy span with any hygienic context.
#[inline]
pub fn is_dummy(self) -> bool {
self.data_untracked().is_dummy()
}

#[inline]
pub fn is_visible(self, sm: &SourceMap) -> bool {
self.data_untracked().is_visible(sm)
!self.is_dummy() && sm.is_span_accessible(self)
}

/// Returns `true` if this span comes from any kind of macro, desugaring or inlining.
Expand Down
255 changes: 145 additions & 110 deletions compiler/rustc_span/src/span_encoding.rs
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Original file line number Diff line number Diff line change
@@ -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;
Expand All @@ -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.
///
/// There are four different span forms.
///
/// 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`)
/// 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`)
///
/// 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-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`)
///
/// 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`)
/// 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`)
///
/// 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`
/// 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 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.
/// 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
Expand All @@ -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]
Expand All @@ -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());

if len <= MAX_LEN && ctxt2 <= MAX_CTXT {
let len_or_tag = len as u16;
debug_assert_eq!(len_or_tag & PARENT_MASK, 0);
let (lo2, len, ctxt2) = (lo.0, hi.0 - lo.0, ctxt.as_u32());

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]
Expand All @@ -148,56 +159,80 @@ 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])
}
}

/// Returns `true` if this is a dummy span with any hygienic context.
#[inline]
pub fn is_dummy(self) -> bool {
if self.len_with_tag_or_marker != BASE_LEN_INTERNED_MARKER {
// Inline-context or inline-parent format.
let lo = self.lo_or_index;
let len = (self.len_with_tag_or_marker & !PARENT_TAG) as u32;
debug_assert!(len <= MAX_LEN);
lo == 0 && len == 0
} else {
// Fully-interned or partially-interned format.
let index = self.lo_or_index;
let data = with_span_interner(|interner| interner.spans[index as usize]);
data.lo == BytePos(0) && data.hi == BytePos(0)
}
}

/// 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)
}
}
}
}
Expand Down