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libexpr: Use EvalProfiler for FunctionCallTrace

Browse source 
This wires up the {pre,post}FunctionCallHook machinery
in EvalState::callFunction and migrates FunctionCallTrace
to use the new EvalProfiler mechanisms for tracing.

Note that branches when the hook gets called are marked with [[unlikely]]
as a hint to the compiler that this is not a hot path. For non-tracing
evaluation this should be a 100% predictable branch, so the performance
cost is nonexistent.

Some measurements to prove support this point:

```
nix build .#nix-cli
nix build github:nixos/nix/d692729759e4e370361cc5105fbeb0e33137ca9e#nix-cli --out-link before
```

(Before)

```
$ taskset -c 2,3 hyperfine "GC_INITIAL_HEAP_SIZE=16g before/bin/nix eval nixpkgs#gnome --no-eval-cache" --warmup 4
Benchmark 1: GC_INITIAL_HEAP_SIZE=16g before/bin/nix eval nixpkgs#gnome --no-eval-cache
  Time (mean ± σ):      2.517 s ±  0.032 s    [User: 1.464 s, System: 0.476 s]
  Range (min … max):    2.464 s …  2.557 s    10 runs
```

(After)

```
$ taskset -c 2,3 hyperfine "GC_INITIAL_HEAP_SIZE=16g result/bin/nix eval nixpkgs#gnome --no-eval-cache" --warmup 4
Benchmark 1: GC_INITIAL_HEAP_SIZE=16g result/bin/nix eval nixpkgs#gnome --no-eval-cache
  Time (mean ± σ):      2.499 s ±  0.022 s    [User: 1.448 s, System: 0.478 s]
  Range (min … max):    2.472 s …  2.537 s    10 runs
```
This commit is contained in:
Sergei Zimmerman 2025-05-16 22:29:18 +00:00
parent 6b4a86a9e3
commit fa6f69f9c5
No known key found for this signature in database
GPG key ID: A9B0B557CA632325
4 changed files with 38 additions and 13 deletions
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15
src/libexpr/eval.cc
View file

@ -372,6 +372,10 @@ EvalState::EvalState(
);
createBaseEnv(settings);
/* Register function call tracer. */
if (settings.traceFunctionCalls)
profiler.addProfiler(make_ref<FunctionCallTrace>());
}
@ -1526,9 +1530,14 @@ void EvalState::callFunction(Value & fun, std::span<Value *> args, Value & vRes,
{
auto _level = addCallDepth(pos);
auto trace = settings.traceFunctionCalls
? std::make_unique<FunctionCallTrace>(positions[pos])
: nullptr;
auto neededHooks = profiler.getNeededHooks();
if (neededHooks.test(EvalProfiler::preFunctionCall)) [[unlikely]]
profiler.preFunctionCallHook(*this, fun, args, pos);
Finally traceExit_{[&](){
if (profiler.getNeededHooks().test(EvalProfiler::postFunctionCall)) [[unlikely]]
profiler.postFunctionCallHook(*this, fun, args, pos);
}};
forceValue(fun, pos);