1 // Spans are encoded using 1-bit tag and 2 different encoding formats (one for each tag value).
2 // One format is used for keeping span data inline,
3 // another contains index into an out-of-line span interner.
4 // The encoding format for inline spans were obtained by optimizing over crates in rustc/libstd.
5 // See https://internals.rust-lang.org/t/rfc-compiler-refactoring-spans/1357/28
7 use crate::hygiene::SyntaxContext;
8 use crate::SESSION_GLOBALS;
9 use crate::{BytePos, SpanData};
11 use rustc_data_structures::fx::FxIndexSet;
13 /// A compressed span.
15 /// Whereas [`SpanData`] is 12 bytes, which is a bit too big to stick everywhere, `Span`
16 /// is a form that only takes up 8 bytes, with less space for the length and
17 /// context. The vast majority (99.9%+) of `SpanData` instances will fit within
18 /// those 8 bytes; any `SpanData` whose fields don't fit into a `Span` are
19 /// stored in a separate interner table, and the `Span` will index into that
20 /// table. Interning is rare enough that the cost is low, but common enough
21 /// that the code is exercised regularly.
23 /// An earlier version of this code used only 4 bytes for `Span`, but that was
24 /// slower because only 80--90% of spans could be stored inline (even less in
25 /// very large crates) and so the interner was used a lot more.
27 /// Inline (compressed) format:
28 /// - `span.base_or_index == span_data.lo`
29 /// - `span.len_or_tag == len == span_data.hi - span_data.lo` (must be `<= MAX_LEN`)
30 /// - `span.ctxt == span_data.ctxt` (must be `<= MAX_CTXT`)
33 /// - `span.base_or_index == index` (indexes into the interner table)
34 /// - `span.len_or_tag == LEN_TAG` (high bit set, all other bits are zero)
35 /// - `span.ctxt == 0`
37 /// The inline form uses 0 for the tag value (rather than 1) so that we don't
38 /// need to mask out the tag bit when getting the length, and so that the
39 /// dummy span can be all zeroes.
41 /// Notes about the choice of field sizes:
42 /// - `base` is 32 bits in both `Span` and `SpanData`, which means that `base`
43 /// values never cause interning. The number of bits needed for `base`
44 /// depends on the crate size. 32 bits allows up to 4 GiB of code in a crate.
45 /// - `len` is 15 bits in `Span` (a u16, minus 1 bit for the tag) and 32 bits
46 /// in `SpanData`, which means that large `len` values will cause interning.
47 /// The number of bits needed for `len` does not depend on the crate size.
48 /// The most common numbers of bits for `len` are from 0 to 7, with a peak usually
49 /// at 3 or 4, and then it drops off quickly from 8 onwards. 15 bits is enough
50 /// for 99.99%+ of cases, but larger values (sometimes 20+ bits) might occur
51 /// dozens of times in a typical crate.
52 /// - `ctxt` is 16 bits in `Span` and 32 bits in `SpanData`, which means that
53 /// large `ctxt` values will cause interning. The number of bits needed for
54 /// `ctxt` values depend partly on the crate size and partly on the form of
55 /// the code. No crates in `rustc-perf` need more than 15 bits for `ctxt`,
56 /// but larger crates might need more than 16 bits.
58 #[derive(Clone, Copy, Eq, PartialEq, Hash)]
65 const LEN_TAG: u16 = 0b1000_0000_0000_0000;
66 const MAX_LEN: u32 = 0b0111_1111_1111_1111;
67 const MAX_CTXT: u32 = 0b1111_1111_1111_1111;
69 /// Dummy span, both position and length are zero, syntax context is zero as well.
70 pub const DUMMY_SP: Span = Span { base_or_index: 0, len_or_tag: 0, ctxt_or_zero: 0 };
74 pub fn new(mut lo: BytePos, mut hi: BytePos, ctxt: SyntaxContext) -> Self {
76 std::mem::swap(&mut lo, &mut hi);
79 let (base, len, ctxt2) = (lo.0, hi.0 - lo.0, ctxt.as_u32());
81 if len <= MAX_LEN && ctxt2 <= MAX_CTXT {
83 Span { base_or_index: base, len_or_tag: len as u16, ctxt_or_zero: ctxt2 as u16 }
86 let index = with_span_interner(|interner| interner.intern(&SpanData { lo, hi, ctxt }));
87 Span { base_or_index: index, len_or_tag: LEN_TAG, ctxt_or_zero: 0 }
92 pub fn data(self) -> SpanData {
93 if self.len_or_tag != LEN_TAG {
95 debug_assert!(self.len_or_tag as u32 <= MAX_LEN);
97 lo: BytePos(self.base_or_index),
98 hi: BytePos(self.base_or_index + self.len_or_tag as u32),
99 ctxt: SyntaxContext::from_u32(self.ctxt_or_zero as u32),
103 debug_assert!(self.ctxt_or_zero == 0);
104 let index = self.base_or_index;
105 with_span_interner(|interner| interner.spans[index as usize])
111 pub struct SpanInterner {
112 spans: FxIndexSet<SpanData>,
116 fn intern(&mut self, span_data: &SpanData) -> u32 {
117 let (index, _) = self.spans.insert_full(*span_data);
122 // If an interner exists, return it. Otherwise, prepare a fresh one.
124 fn with_span_interner<T, F: FnOnce(&mut SpanInterner) -> T>(f: F) -> T {
125 SESSION_GLOBALS.with(|session_globals| f(&mut *session_globals.span_interner.lock()))