Compress "small" spans to 32 bits and intern "large" spans

src/libsyntax_pos/span_encoding.rs

@@ -0,0 +1,143 @@
+// 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 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 {BytePos, SpanData};
+use hygiene::SyntaxContext;
+
+use rustc_data_structures::fx::FxHashMap;
+use std::cell::RefCell;
+
+/// 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_INLINE: u32 = 0;
+const TAG_INTERNED: u32 = 1;
+const TAG_MASK: u32 = 1;
+
+// Fields indexes
+const BASE_INDEX: usize = 0;
+const LEN_INDEX: usize = 1;
+const CTXT_INDEX: usize = 2;
+
+// 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];
+
+// Tag = 1, interned format.
+// ------------------------
+// | index 31:1 | tag 0:0 |
+// ------------------------
+const INTERNED_INDEX_SIZE: u32 = 31;
+const INTERNED_INDEX_OFFSET: u32 = 1;
+
+#[inline]
+fn encode(sd: &SpanData) -> Span {
+    let (base, len, ctxt) = (sd.lo.0, sd.hi.0 - sd.lo.0, sd.ctxt.0);
+
+    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)
+}
+
+#[inline]
+fn decode(span: Span) -> SpanData {
+    let val = span.0;
+
+    // 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
+    };
+
+    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) }
+}
+
+#[derive(Default)]
+struct SpanInterner {
+    spans: FxHashMap<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
+    }
+
+    #[inline]
+    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.
+#[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()))
+}