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Auto merge of #119455 - Mark-Simulacrum:relative-spans, r=cjgillot
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Embed length of offset/position into Span tag byte

This cuts the average bytes/relative span from 3.5 to 3.2 on libcore, ultimately saving ~400kb of data.
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bors committed Mar 6, 2024
2 parents b77e018 + 51dfe49 commit 8039906
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Showing 3 changed files with 24 additions and 6 deletions.
6 changes: 5 additions & 1 deletion compiler/rustc_metadata/src/rmeta/decoder.rs
Original file line number Diff line number Diff line change
Expand Up @@ -504,7 +504,11 @@ impl<'a, 'tcx> SpanDecoder for DecodeContext<'a, 'tcx> {
let data = if tag.kind() == SpanKind::Indirect {
// Skip past the tag we just peek'd.
self.read_u8();
let offset_or_position = self.read_usize();
// indirect tag lengths are safe to access, since they're (0, 8)
let bytes_needed = tag.length().unwrap().0 as usize;
let mut total = [0u8; usize::BITS as usize / 8];
total[..bytes_needed].copy_from_slice(self.read_raw_bytes(bytes_needed));
let offset_or_position = usize::from_le_bytes(total);
let position = if tag.is_relative_offset() {
start - offset_or_position
} else {
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20 changes: 16 additions & 4 deletions compiler/rustc_metadata/src/rmeta/encoder.rs
Original file line number Diff line number Diff line change
Expand Up @@ -172,11 +172,19 @@ impl<'a, 'tcx> SpanEncoder for EncodeContext<'a, 'tcx> {
// previously saved offset must be smaller than the current position.
let offset = self.opaque.position() - last_location;
if offset < last_location {
SpanTag::indirect(true).encode(self);
offset.encode(self);
let needed = bytes_needed(offset);
SpanTag::indirect(true, needed as u8).encode(self);
self.opaque.write_with(|dest| {
*dest = offset.to_le_bytes();
needed
});
} else {
SpanTag::indirect(false).encode(self);
last_location.encode(self);
let needed = bytes_needed(last_location);
SpanTag::indirect(false, needed as u8).encode(self);
self.opaque.write_with(|dest| {
*dest = last_location.to_le_bytes();
needed
});
}
}
Entry::Vacant(v) => {
Expand Down Expand Up @@ -212,6 +220,10 @@ impl<'a, 'tcx> SpanEncoder for EncodeContext<'a, 'tcx> {
}
}

fn bytes_needed(n: usize) -> usize {
(usize::BITS - n.leading_zeros()).div_ceil(u8::BITS) as usize
}

impl<'a, 'tcx> Encodable<EncodeContext<'a, 'tcx>> for SpanData {
fn encode(&self, s: &mut EncodeContext<'a, 'tcx>) {
// Don't serialize any `SyntaxContext`s from a proc-macro crate,
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4 changes: 3 additions & 1 deletion compiler/rustc_metadata/src/rmeta/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -530,11 +530,13 @@ impl SpanTag {
SpanTag(data)
}

fn indirect(relative: bool) -> SpanTag {
fn indirect(relative: bool, length_bytes: u8) -> SpanTag {
let mut tag = SpanTag(SpanKind::Indirect as u8);
if relative {
tag.0 |= 0b100;
}
assert!(length_bytes <= 8);
tag.0 |= length_bytes << 3;
tag
}

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