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Optimize empty list constants

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This avoids a Value allocation for empty list constants. During a `nix
search nixpkgs`, about 82% of all thunked lists are empty, so this
removes about 3 million Value allocations.

Performance comparison on `nix search github:NixOS/nixpkgs/e1fa12d4f6c6fe19ccb59cac54b5b3f25e160870 --no-eval-cache`:

maximum RSS:        median = 3845432.0000  mean = 3845432.0000  stddev =      0.0000  min = 3845432.0000  max = 3845432.0000  [rejected?, p=0.00000, Δ=-70084.00000±0.00000]
soft page faults:   median = 965395.0000  mean = 965394.6667  stddev =      1.1181  min = 965392.0000  max = 965396.0000  [rejected?, p=0.00000, Δ=-17929.77778±38.59610]
system CPU time:    median =      1.8029  mean =      1.7702  stddev =      0.0621  min =      1.6749  max =      1.8417  [rejected, p=0.00064, Δ=-0.12873±0.09905]
user CPU time:      median =     14.1022  mean =     14.0633  stddev =      0.1869  min =     13.8118  max =     14.3190  [not rejected, p=0.03006, Δ=-0.18248±0.24928]
elapsed time:       median =     15.8205  mean =     15.8618  stddev =      0.2312  min =     15.5033  max =     16.1670  [not rejected, p=0.00558, Δ=-0.28963±0.29434]
This commit is contained in:
Eelco Dolstra 2024-01-02 12:39:16 +01:00
parent a21c762dab
commit 3f796514b3
3 changed files with 15 additions and 0 deletions
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11
src/libexpr/eval.cc
View file

@ -554,6 +554,8 @@ EvalState::EvalState(
static_assert(sizeof(Env) <= 16, "environment must be <= 16 bytes");
vEmptyList.mkList(0);
/* Initialise the Nix expression search path. */
if (!evalSettings.pureEval) {
for (auto & i : _searchPath.elements)
@ -1384,6 +1386,15 @@ void ExprList::eval(EvalState & state, Env & env, Value & v)
}
Value * ExprList::maybeThunk(EvalState & state, Env & env)
{
if (elems.empty()) {
return &state.vEmptyList;
}
return Expr::maybeThunk(state, env);
}
void ExprVar::eval(EvalState & state, Env & env, Value & v)
{
Value * v2 = state.lookupVar(&env, *this, false);

3
src/libexpr/eval.hh
View file

@ -305,6 +305,9 @@ public:
return *errorBuilder;
}
/* Empty list constant. */
Value vEmptyList;
private:
/* Cache for calls to addToStore(); maps source paths to the store

1
src/libexpr/nixexpr.hh
View file

@ -299,6 +299,7 @@ struct ExprList : Expr
std::vector<Expr *> elems;
ExprList() { };
COMMON_METHODS
Value * maybeThunk(EvalState & state, Env & env) override;
PosIdx getPos() const override
{