refactor(tvix/eval): improve representation of chunk/span mapping
This switches out the previous compressed representation (count of instructions per span) with a representation where the chunk's span list stores the index of the first operation that belongs to a span, and finds the right span by using a binary search when looking them up. This improves the lookup complexity from O(n) to O(log n). This improvement was suggested and (mostly) implemented by GPT-4. I only fixed up some names and updated the logic for deleting spans (which it only did not do because I didn't tell it about that). The code was verified by producing a complex error before/after the change and ensuring that all spans in the error match exactly. Co-Authored-By: GPT-4 Change-Id: Ibfa12cc6973af1c9b0ae55bb464d1975209771f5 Reviewed-on: https://cl.tvl.fyi/c/depot/+/8385 Reviewed-by: ezemtsov <eugene.zemtsov@gmail.com> Tested-by: BuildkiteCI Autosubmit: tazjin <tazjin@tvl.su>
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1 changed files with 22 additions and 24 deletions
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@ -21,8 +21,8 @@ struct SourceSpan {
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/// Span into the [codemap::Codemap].
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span: codemap::Span,
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/// Number of instructions derived from this span.
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count: usize,
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/// Index of the first operation covered by this span.
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start: usize,
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}
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/// A chunk is a representation of a sequence of bytecode
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@ -61,7 +61,7 @@ impl Chunk {
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pub fn push_op(&mut self, data: OpCode, span: codemap::Span) -> CodeIdx {
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let idx = self.code.len();
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self.code.push(data);
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self.push_span(span);
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self.push_span(span, idx);
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CodeIdx(idx)
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}
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@ -76,19 +76,11 @@ impl Chunk {
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// Simply drop the last op.
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self.code.pop();
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// If the last span only had this op, drop it, otherwise
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// decrease its operation counter.
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match self.spans.last_mut() {
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// If the last span had more than one op, decrease the
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// counter.
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Some(span) if span.count > 1 => span.count -= 1,
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// Otherwise, drop it.
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Some(_) => {
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if let Some(span) = self.spans.last() {
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// If the last span started at this op, drop it.
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if span.start == self.code.len() {
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self.spans.pop();
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}
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None => unreachable!(),
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}
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}
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@ -100,31 +92,37 @@ impl Chunk {
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// Span tracking implementation
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fn push_span(&mut self, span: codemap::Span) {
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fn push_span(&mut self, span: codemap::Span, start: usize) {
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match self.spans.last_mut() {
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// We do not need to insert the same span again, as this
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// instruction was compiled from the same span as the last
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// one.
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Some(last) if last.span == span => last.count += 1,
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Some(last) if last.span == span => {}
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// In all other cases, this is a new source span.
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_ => self.spans.push(SourceSpan { span, count: 1 }),
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_ => self.spans.push(SourceSpan { span, start }),
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}
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}
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/// Retrieve the [codemap::Span] from which the instruction at
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/// `offset` was compiled.
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pub fn get_span(&self, offset: CodeIdx) -> codemap::Span {
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let mut pos = 0;
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let position = self
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.spans
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.binary_search_by(|span| span.start.cmp(&offset.0));
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for span in &self.spans {
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pos += span.count;
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if pos > offset.0 {
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return span.span;
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let span = match position {
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Ok(index) => &self.spans[index],
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Err(index) => {
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if index == 0 {
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&self.spans[0]
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} else {
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&self.spans[index - 1]
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}
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}
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};
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panic!("compiler error: chunk missing span for offset {}", offset.0);
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span.span
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}
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/// Write the disassembler representation of the operation at
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