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Encode span parent in the inlined representation. #88886

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Encode span parent in the inlined representation. #88886

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68 changes: 49 additions & 19 deletions compiler/rustc_span/src/span_encoding.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -4,7 +4,7 @@
// 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::LocalDefId;
use crate::def_id::{DefIndex, LocalDefId};
use crate::hygiene::SyntaxContext;
use crate::SPAN_TRACK;
use crate::{BytePos, SpanData};
Expand All @@ -13,8 +13,8 @@ use rustc_data_structures::fx::FxIndexSet;

/// A compressed span.
///
/// Whereas [`SpanData`] is 12 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 and
/// 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
/// stored in a separate interner table, and the `Span` will index into that
Expand All @@ -25,11 +25,17 @@ use rustc_data_structures::fx::FxIndexSet;
/// 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.
///
/// Inline (compressed) format:
/// 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 == 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`)
///
/// 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)
Expand Down Expand Up @@ -76,7 +82,8 @@ pub struct Span {
ctxt_or_zero: u16,
}

const LEN_TAG: u16 = 0b1000_0000_0000_0000;
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 MAX_CTXT: u32 = 0b1111_1111_1111_1111;

Expand All @@ -97,15 +104,27 @@ impl Span {

let (base, len, ctxt2) = (lo.0, hi.0 - lo.0, ctxt.as_u32());

if len <= MAX_LEN && ctxt2 <= MAX_CTXT && parent.is_none() {
// Inline format.
Span { base_or_index: base, len_or_tag: len as u16, ctxt_or_zero: ctxt2 as u16 }
} else {
// Interned format.
let index =
with_span_interner(|interner| interner.intern(&SpanData { lo, hi, ctxt, parent }));
Span { base_or_index: index, len_or_tag: LEN_TAG, ctxt_or_zero: 0 }
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 parent = parent.local_def_index.as_u32();
if ctxt2 == SyntaxContext::root().as_u32() && parent <= MAX_CTXT {
return Span { base_or_index: base, len_or_tag, ctxt_or_zero: parent as u16 };
}
} else if ctxt2 <= MAX_CTXT {
// Inline format.
let ctxt_or_zero = ctxt2 as u16;
return Span { base_or_index: base, len_or_tag, ctxt_or_zero };
}
}

// Interned format.
let index =
with_span_interner(|interner| interner.intern(&SpanData { lo, hi, ctxt, parent }));
Span { base_or_index: index, len_or_tag: LEN_TAG, ctxt_or_zero: 0 }
}

#[inline]
Expand All @@ -123,12 +142,23 @@ impl Span {
pub fn data_untracked(self) -> SpanData {
if self.len_or_tag != LEN_TAG {
// Inline format.
debug_assert!(self.len_or_tag as u32 <= 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_zero as u32),
parent: None,
if self.len_or_tag & PARENT_MASK == 0 {
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_zero as u32),
parent: None,
}
} else {
let len = self.len_or_tag & !PARENT_MASK;
let parent =
LocalDefId { local_def_index: DefIndex::from_u32(self.ctxt_or_zero as u32) };
SpanData {
lo: BytePos(self.base_or_index),
hi: BytePos(self.base_or_index + len as u32),
ctxt: SyntaxContext::root(),
parent: Some(parent),
}
}
} else {
// Interned format.
Expand Down