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encode black holes as tApp values

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checking for isBlackhole in the forceValue hot path is rather more
expensive than necessary, and with a little bit of trickery we can move
such handling into the isApp case. small performance benefit, but under
some circumstances we've seen 2% improvement as well.

〉 nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'

before:

  Time (mean ± σ):      4.429 s ±  0.002 s    [User: 3.929 s, System: 0.500 s]
  Range (min … max):    4.427 s …  4.433 s    10 runs

after:

  Time (mean ± σ):      4.396 s ±  0.002 s    [User: 3.894 s, System: 0.501 s]
  Range (min … max):    4.393 s …  4.399 s    10 runs
This commit is contained in:
pennae 2023-12-10 08:24:45 +01:00
parent 0218e4e6c3
commit 78353deb02
7 changed files with 93 additions and 32 deletions
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13
src/libexpr/eval-inline.hh
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@ -104,11 +104,16 @@ void EvalState::forceValue(Value & v, Callable getPos)
}
}
else if (v.isApp()) {
PosIdx pos = getPos();
callFunction(*v.app.left, *v.app.right, v, pos);
try {
callFunction(*v.app.left, *v.app.right, v, noPos);
} catch (InfiniteRecursionError & e) {
// only one black hole can *throw* in any given eval stack so we need not
// check whether the position is set already.
if (v.isBlackhole())
e.err.errPos = positions[getPos()];
throw;
}
}
else if (v.isBlackhole())
error("infinite recursion encountered").atPos(getPos()).template debugThrow<EvalError>();
}