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Compress "small" spans to 32 bits and intern "large" spans
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// Copyright 2017 The Rust Project Developers. See the COPYRIGHT | ||
// file at the top-level directory of this distribution and at | ||
// http://rust-lang.org/COPYRIGHT. | ||
// | ||
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or | ||
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license | ||
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your | ||
// option. This file may not be copied, modified, or distributed | ||
// except according to those terms. | ||
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// 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 | ||
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use {BytePos, SpanData}; | ||
use hygiene::SyntaxContext; | ||
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use rustc_data_structures::fx::FxHashMap; | ||
use std::cell::RefCell; | ||
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/// A compressed span. | ||
/// Contains either fields of `SpanData` inline if they are small, or index into span interner. | ||
/// The primary goal of `Span` is to be as small as possible and fit into other structures | ||
/// (that's why it uses `packed` as well). Decoding speed is the second priority. | ||
/// See `SpanData` for the info on span fields in decoded representation. | ||
#[derive(Clone, Copy, PartialEq, Eq, Hash)] | ||
#[repr(packed)] | ||
pub struct Span(u32); | ||
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/// Dummy span, both position and length are zero, syntax context is zero as well. | ||
/// This span is kept inline and encoded with format 0. | ||
pub const DUMMY_SP: Span = Span(0); | ||
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impl Span { | ||
#[inline] | ||
pub fn new(lo: BytePos, hi: BytePos, ctxt: SyntaxContext) -> Self { | ||
encode(&match lo <= hi { | ||
true => SpanData { lo, hi, ctxt }, | ||
false => SpanData { lo: hi, hi: lo, ctxt }, | ||
}) | ||
} | ||
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#[inline] | ||
pub fn data(self) -> SpanData { | ||
decode(self) | ||
} | ||
} | ||
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// Tags | ||
const TAG_INLINE: u32 = 0; | ||
const TAG_INTERNED: u32 = 1; | ||
const TAG_MASK: u32 = 1; | ||
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// Fields indexes | ||
const BASE_INDEX: usize = 0; | ||
const LEN_INDEX: usize = 1; | ||
const CTXT_INDEX: usize = 2; | ||
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// Tag = 0, inline format. | ||
// ----------------------------------- | ||
// | base 31:8 | len 7:1 | tag 0:0 | | ||
// ----------------------------------- | ||
const INLINE_SIZES: [u32; 3] = [24, 7, 0]; | ||
const INLINE_OFFSETS: [u32; 3] = [8, 1, 1]; | ||
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// Tag = 1, interned format. | ||
// ------------------------ | ||
// | index 31:1 | tag 0:0 | | ||
// ------------------------ | ||
const INTERNED_INDEX_SIZE: u32 = 31; | ||
const INTERNED_INDEX_OFFSET: u32 = 1; | ||
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#[inline] | ||
fn encode(sd: &SpanData) -> Span { | ||
let (base, len, ctxt) = (sd.lo.0, sd.hi.0 - sd.lo.0, sd.ctxt.0); | ||
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let val = if (base >> INLINE_SIZES[BASE_INDEX]) == 0 && | ||
(len >> INLINE_SIZES[LEN_INDEX]) == 0 && | ||
(ctxt >> INLINE_SIZES[CTXT_INDEX]) == 0 { | ||
(base << INLINE_OFFSETS[BASE_INDEX]) | (len << INLINE_OFFSETS[LEN_INDEX]) | | ||
(ctxt << INLINE_OFFSETS[CTXT_INDEX]) | TAG_INLINE | ||
} else { | ||
let index = with_span_interner(|interner| interner.intern(sd)); | ||
(index << INTERNED_INDEX_OFFSET) | TAG_INTERNED | ||
}; | ||
Span(val) | ||
} | ||
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#[inline] | ||
fn decode(span: Span) -> SpanData { | ||
let val = span.0; | ||
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// Extract a field at position `pos` having size `size`. | ||
let extract = |pos: u32, size: u32| { | ||
let mask = ((!0u32) as u64 >> (32 - size)) as u32; // Can't shift u32 by 32 | ||
(val >> pos) & mask | ||
}; | ||
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let (base, len, ctxt) = if val & TAG_MASK == TAG_INLINE {( | ||
extract(INLINE_OFFSETS[BASE_INDEX], INLINE_SIZES[BASE_INDEX]), | ||
extract(INLINE_OFFSETS[LEN_INDEX], INLINE_SIZES[LEN_INDEX]), | ||
extract(INLINE_OFFSETS[CTXT_INDEX], INLINE_SIZES[CTXT_INDEX]), | ||
)} else { | ||
let index = extract(INTERNED_INDEX_OFFSET, INTERNED_INDEX_SIZE); | ||
return with_span_interner(|interner| *interner.get(index)); | ||
}; | ||
SpanData { lo: BytePos(base), hi: BytePos(base + len), ctxt: SyntaxContext(ctxt) } | ||
} | ||
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#[derive(Default)] | ||
struct SpanInterner { | ||
spans: FxHashMap<SpanData, u32>, | ||
span_data: Vec<SpanData>, | ||
} | ||
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impl SpanInterner { | ||
fn intern(&mut self, span_data: &SpanData) -> u32 { | ||
if let Some(index) = self.spans.get(span_data) { | ||
return *index; | ||
} | ||
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let index = self.spans.len() as u32; | ||
self.span_data.push(*span_data); | ||
self.spans.insert(*span_data, index); | ||
index | ||
} | ||
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#[inline] | ||
fn get(&self, index: u32) -> &SpanData { | ||
&self.span_data[index as usize] | ||
} | ||
} | ||
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// If an interner exists in TLS, return it. Otherwise, prepare a fresh one. | ||
#[inline] | ||
fn with_span_interner<T, F: FnOnce(&mut SpanInterner) -> T>(f: F) -> T { | ||
thread_local!(static INTERNER: RefCell<SpanInterner> = { | ||
RefCell::new(SpanInterner::default()) | ||
}); | ||
INTERNER.with(|interner| f(&mut *interner.borrow_mut())) | ||
} |