-
Notifications
You must be signed in to change notification settings - Fork 12.7k
Commit
This commit does not belong to any branch on this repository, and may belong to a fork outside of the repository.
Compress "small" spans to 32 bits and intern "large" spans
- Loading branch information
1 parent
3a7b960
commit f069c88
Showing
6 changed files
with
227 additions
and
32 deletions.
There are no files selected for viewing
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Original file line number | Diff line number | Diff line change |
---|---|---|
@@ -0,0 +1,181 @@ | ||
// 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. | ||
|
||
// Spans are encoded using 2-bit tag and 4 different encoding formats for each tag. | ||
// Three formats are used for keeping span data inline, | ||
// the fourth one contains index into out-of-line span interner. | ||
// The encoding formats 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 super::*; | ||
|
||
/// 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); | ||
|
||
/// 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); | ||
|
||
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 }, | ||
}) | ||
} | ||
|
||
#[inline] | ||
pub fn data(self) -> SpanData { | ||
decode(self) | ||
} | ||
} | ||
|
||
// Tags | ||
const TAG_INLINE0: u32 = 0b00; | ||
const TAG_INLINE1: u32 = 0b01; | ||
const TAG_INLINE2: u32 = 0b10; | ||
const TAG_INTERNED: u32 = 0b11; | ||
const TAG_MASK: u32 = 0b11; | ||
|
||
// Fields indexes | ||
const BASE_INDEX: usize = 0; | ||
const LEN_INDEX: usize = 1; | ||
const CTXT_INDEX: usize = 2; | ||
|
||
// Tag = 0b00, inline format 0. | ||
// ----------------------------------- | ||
// | base 31:8 | len 7:2 | tag 1:0 | | ||
// ----------------------------------- | ||
const INLINE0_SIZES: [u32; 3] = [24, 6, 0]; | ||
const INLINE0_OFFSETS: [u32; 3] = [8, 2, 2]; | ||
|
||
// Tag = 0b01, inline format 1. | ||
// ----------------------------------- | ||
// | base 31:10 | len 9:2 | tag 1:0 | | ||
// ----------------------------------- | ||
const INLINE1_SIZES: [u32; 3] = [22, 8, 0]; | ||
const INLINE1_OFFSETS: [u32; 3] = [10, 2, 2]; | ||
|
||
// Tag = 0b10, inline format 2. | ||
// ------------------------------------------------ | ||
// | base 31:14 | len 13:13 | ctxt 12:2 | tag 1:0 | | ||
// ------------------------------------------------ | ||
const INLINE2_SIZES: [u32; 3] = [18, 1, 11]; | ||
const INLINE2_OFFSETS: [u32; 3] = [14, 13, 2]; | ||
|
||
// Tag = 0b11, interned format. | ||
// ------------------------ | ||
// | index 31:3 | tag 1:0 | | ||
// ------------------------ | ||
const INTERNED_INDEX_SIZE: u32 = 30; | ||
const INTERNED_INDEX_OFFSET: u32 = 2; | ||
|
||
fn encode(sd: &SpanData) -> Span { | ||
let (base, len, ctxt) = (sd.lo.0, sd.hi.0 - sd.lo.0, sd.ctxt.0); | ||
|
||
// Can we fit the span data into this encoding? | ||
let fits = |sizes: [u32; 3]| { | ||
(base >> sizes[BASE_INDEX]) == 0 && (len >> sizes[LEN_INDEX]) == 0 && | ||
(ctxt >> sizes[CTXT_INDEX]) == 0 | ||
}; | ||
// Turn fields into a single `u32` value. | ||
let compose = |offsets: [u32; 3], tag| { | ||
(base << offsets[BASE_INDEX]) | (len << offsets[LEN_INDEX]) | | ||
(ctxt << offsets[CTXT_INDEX]) | tag | ||
}; | ||
|
||
let val = if fits(INLINE0_SIZES) { | ||
compose(INLINE0_OFFSETS, TAG_INLINE0) | ||
} else if fits(INLINE1_SIZES) { | ||
compose(INLINE1_OFFSETS, TAG_INLINE1) | ||
} else if fits(INLINE2_SIZES) { | ||
compose(INLINE2_OFFSETS, TAG_INLINE2) | ||
} else { | ||
let index = with_span_interner(|interner| interner.intern(sd)); | ||
if (index >> INTERNED_INDEX_SIZE) == 0 { | ||
(index << INTERNED_INDEX_OFFSET) | TAG_INTERNED | ||
} else { | ||
panic!("too many spans in a crate"); | ||
} | ||
}; | ||
Span(val) | ||
} | ||
|
||
fn decode(span: Span) -> SpanData { | ||
let val = span.0; | ||
|
||
// Extract a field at position `pos` having size `size`. | ||
let extract = |pos, size| { | ||
let mask = ((!0u32) as u64 >> (32 - size)) as u32; // Can't shift u32 by 32 | ||
(val >> pos) & mask | ||
}; | ||
|
||
let (base, len, ctxt) = match val & TAG_MASK { | ||
TAG_INLINE0 => ( | ||
extract(INLINE0_OFFSETS[BASE_INDEX], INLINE0_SIZES[BASE_INDEX]), | ||
extract(INLINE0_OFFSETS[LEN_INDEX], INLINE0_SIZES[LEN_INDEX]), | ||
extract(INLINE0_OFFSETS[CTXT_INDEX], INLINE0_SIZES[CTXT_INDEX]), | ||
), | ||
TAG_INLINE1 => ( | ||
extract(INLINE1_OFFSETS[BASE_INDEX], INLINE1_SIZES[BASE_INDEX]), | ||
extract(INLINE1_OFFSETS[LEN_INDEX], INLINE1_SIZES[LEN_INDEX]), | ||
extract(INLINE1_OFFSETS[CTXT_INDEX], INLINE1_SIZES[CTXT_INDEX]), | ||
), | ||
TAG_INLINE2 => ( | ||
extract(INLINE2_OFFSETS[BASE_INDEX], INLINE2_SIZES[BASE_INDEX]), | ||
extract(INLINE2_OFFSETS[LEN_INDEX], INLINE2_SIZES[LEN_INDEX]), | ||
extract(INLINE2_OFFSETS[CTXT_INDEX], INLINE2_SIZES[CTXT_INDEX]), | ||
), | ||
TAG_INTERNED => { | ||
let index = extract(INTERNED_INDEX_OFFSET, INTERNED_INDEX_SIZE); | ||
return with_span_interner(|interner| *interner.get(index)); | ||
} | ||
_ => unreachable!() | ||
}; | ||
SpanData { lo: BytePos(base), hi: BytePos(base + len), ctxt: SyntaxContext(ctxt) } | ||
} | ||
|
||
#[derive(Default)] | ||
struct SpanInterner { | ||
spans: HashMap<SpanData, u32>, | ||
span_data: Vec<SpanData>, | ||
} | ||
|
||
impl SpanInterner { | ||
fn intern(&mut self, span_data: &SpanData) -> u32 { | ||
if let Some(index) = self.spans.get(span_data) { | ||
return *index; | ||
} | ||
|
||
let index = self.spans.len() as u32; | ||
self.span_data.push(*span_data); | ||
self.spans.insert(*span_data, index); | ||
index | ||
} | ||
|
||
fn get(&self, index: u32) -> &SpanData { | ||
&self.span_data[index as usize] | ||
} | ||
} | ||
|
||
// If an interner exists in TLS, return it. Otherwise, prepare a fresh one. | ||
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())) | ||
} |