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{libexpr,libcmd}: Make debugger significantly faster

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The underlying issue is that debugger code path was
calling PosTable::operator[] in each eval method.
This has become incredibly expensive since 5d9fdab3de.

While we are it it, I've reworked the code to
not use std::shared_ptr where it really isn't necessary.

As I've documented in previous commits, this is actually
more a workaround for recursive header dependencies now
and is only necessary in `error.hh` code.

Some ad-hoc benchmarking:

After this commit:

```
Benchmark 1: nix eval nixpkgs#hello --impure --ignore-try --no-eval-cache --debugger
  Time (mean ± σ):     784.2 ms ±   7.1 ms    [User: 561.4 ms, System: 147.7 ms]
  Range (min … max):   773.5 ms … 792.6 ms    10 runs
```

On master 3604c7c51:

```
Benchmark 1: nix eval nixpkgs#hello --impure --ignore-try --no-eval-cache --debugger
  Time (mean ± σ):     22.914 s ±  0.178 s    [User: 18.524 s, System: 4.151 s]
  Range (min … max):   22.738 s … 23.290 s    10 runs
```

(cherry picked from commit adbd08399c)
This commit is contained in:
Sergei Zimmerman 2025-03-13 16:24:30 +00:00 • committed by Mergify
parent 593e0eebeb
commit 11919bc471
4 changed files with 47 additions and 29 deletions
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11
src/libcmd/repl.cc
View file

@ -140,16 +140,13 @@ static std::ostream & showDebugTrace(std::ostream & out, const PosTable & positi
out << ANSI_RED "error: " << ANSI_NORMAL;
out << dt.hint.str() << "\n";
// prefer direct pos, but if noPos then try the expr.
auto pos = dt.pos
? dt.pos
: positions[dt.expr.getPos() ? dt.expr.getPos() : noPos];
auto pos = dt.getPos(positions);
if (pos) {
out << *pos;
if (auto loc = pos->getCodeLines()) {
out << pos;
if (auto loc = pos.getCodeLines()) {
out << "\n";
printCodeLines(out, "", *pos, *loc);
printCodeLines(out, "", pos, *loc);
out << "\n";
}
}