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Lazify SourceFile::lines.

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`SourceFile::lines` is a big part of metadata. It's stored in a compressed form
(a difference list) to save disk space. Decoding it is a big fraction of
compile time for very small crates/programs.

This commit introduces a new type `SourceFileLines` which has a `Lines`
form and a `Diffs` form. The latter is used when the metadata is first
read, and it is only decoded into the `Lines` form when line data is
actually needed. This avoids the decoding cost for many files,
especially in `std`. It's a performance win of up to 15% for tiny
crates/programs where metadata decoding is a high part of compilation
costs.

A `Lock` is needed because the methods that access lines data (which can
trigger decoding) take `&self` rather than `&mut self`. To allow for this,
`SourceFile::lines` now takes a `FnMut` that operates on the lines slice rather
than returning the lines slice.
This commit is contained in:
Nicholas Nethercote 2022-05-30 15:59:45 +10:00
parent bef2b7cd1c
commit 0b81d7cdc6
8 changed files with 213 additions and 116 deletions
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13
compiler/rustc_span/src/source_map.rs
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@ -331,7 +331,7 @@ impl SourceMap {
name_hash: u128,
source_len: usize,
cnum: CrateNum,
mut file_local_lines: Vec<BytePos>,
file_local_lines: Lock<SourceFileLines>,
mut file_local_multibyte_chars: Vec<MultiByteChar>,
mut file_local_non_narrow_chars: Vec<NonNarrowChar>,
mut file_local_normalized_pos: Vec<NormalizedPos>,
@ -355,8 +355,15 @@ impl SourceMap {
// form rather than pre-computing the offset into a local variable. The
// compiler backend can optimize away the repeated computations in a
// way that won't trigger overflow checks.
for pos in &mut file_local_lines {
*pos = (*pos - original_start_pos) + start_pos;
match &mut *file_local_lines.borrow_mut() {
SourceFileLines::Lines { lines } => {
for pos in lines {
*pos = (*pos - original_start_pos) + start_pos;
}
}
SourceFileLines::Diffs { line_start, .. } => {
*line_start = (*line_start - original_start_pos) + start_pos;
}
}
for mbc in &mut file_local_multibyte_chars {
mbc.pos = (mbc.pos - original_start_pos) + start_pos;