libexpr: Actually cache line information in PosTable (backport #13211)#13580
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mergify[bot] merged 2 commits into2.29-maintenancefrom Jul 30, 2025
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For heavier objects it doesn't make sense to return a std::optional with the copy of the data, when it can be used by const reference. (cherry picked from commit 4711720)
Previous code had a sneaky bug due to which no caching
actually happened:
```cpp
auto linesForInput = (*lines)[origin->offset];
```
That should have been:
```cpp
auto & linesForInput = (*lines)[origin->offset];
```
See [1].
Now that it also makes sense to make the cache bound in side
in order not to memoize all the sources without freeing any memory.
The default cache size has been chosen somewhat arbitrarily to be ~64k
origins. For reference, 25.05 nixpkgs has ~50k .nix files.
Simple benchmark:
```nix
let
pkgs = import <nixpkgs> { };
in
builtins.foldl' (acc: el: acc + el.line) 0 (
builtins.genList (x: builtins.unsafeGetAttrPos "gcc" pkgs) 10000
)
```
(After)
```
$ hyperfine "result/bin/nix eval -f ./test.nix"
Benchmark 1: result/bin/nix eval -f ./test.nix
Time (mean ± σ): 292.7 ms ± 3.9 ms [User: 131.0 ms, System: 120.5 ms]
Range (min … max): 288.1 ms … 300.5 ms 10 runs
```
(Before)
```
hyperfine "nix eval -f ./test.nix"
Benchmark 1: nix eval -f ./test.nix
Time (mean ± σ): 666.7 ms ± 6.4 ms [User: 428.3 ms, System: 191.2 ms]
Range (min … max): 659.7 ms … 681.3 ms 10 runs
```
If the origin happens to be a `all-packages.nix` or similar in size then the
difference is much more dramatic.
[1]: https://www.github.com/lix-project/lix/commit/22e3f0e9875082be7f4eec8e3caeb134a7f1c05f
(cherry picked from commit 5ea81f5)
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Motivation
Previous code had a sneaky bug due to which no caching
actually happened:
auto linesForInput = (*lines)[origin->offset];That should have been:
auto & linesForInput = (*lines)[origin->offset];See 1.
Now that it also makes sense to make the cache bound in size
in order not to memoize all the sources without freeing any memory.
The default cache size has been chosen somewhat arbitrarily to be ~64k
origins. For reference, 25.05 nixpkgs has ~50k .nix files.
Simple benchmark:
(After)
(Before)
If the origin happens to be a
all-packages.nixor similar in size then thedifference is much more dramatic.
Context
#12645 (comment)
@roberth As it turns out the
PosTablewas indeed supposed to do the caching, but didn't due to the bug I described above.Add 👍 to pull requests you find important.
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This is an automatic backport of pull request #13211 done by [Mergify](https://mergify.com).