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reduce the size of Env by one pointer

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since `up` and `values` are both pointer-aligned the type field will
also be pointer-aligned, wasting 48 bits of space on most machines. we
can get away with removing the type field altogether by encoding some
information into the `with` expr that created the env to begin with,
reducing the GC load for the absolutely massive amount of single-entry
envs we create for lambdas. this reduces memory usage of system eval by
quite a bit (reducing heap size of our system eval from 8.4GB to 8.23GB)
and gives similar savings in eval time.

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

before:

  Time (mean ± σ):      5.576 s ±  0.003 s    [User: 5.197 s, System: 0.378 s]
  Range (min … max):    5.572 s …  5.581 s    10 runs

after:

  Time (mean ± σ):      5.408 s ±  0.002 s    [User: 5.019 s, System: 0.388 s]
  Range (min … max):    5.405 s …  5.411 s    10 runs
This commit is contained in:
pennae 2023-12-22 18:19:53 +01:00
parent ee439734e9
commit 1fe66852ff
9 changed files with 75 additions and 34 deletions
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29
src/libexpr/eval.cc
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@ -543,7 +543,7 @@ EvalState::EvalState(
, env1AllocCache(std::allocate_shared<void *>(traceable_allocator<void *>(), nullptr))
#endif
, baseEnv(allocEnv(128))
, staticBaseEnv{std::make_shared<StaticEnv>(false, nullptr)}
, staticBaseEnv{std::make_shared<StaticEnv>(nullptr, nullptr)}
{
corepkgsFS->setPathDisplay("<nix", ">");
internalFS->setPathDisplay("«nix-internal»", "");
@ -781,7 +781,7 @@ void printStaticEnvBindings(const SymbolTable & st, const StaticEnv & se)
// just for the current level of Env, not the whole chain.
void printWithBindings(const SymbolTable & st, const Env & env)
{
if (env.type == Env::HasWithAttrs) {
if (!env.values[0]->isThunk()) {
std::cout << "with: ";
std::cout << ANSI_MAGENTA;
Bindings::iterator j = env.values[0]->attrs->begin();
@ -835,7 +835,7 @@ void mapStaticEnvBindings(const SymbolTable & st, const StaticEnv & se, const En
if (env.up && se.up) {
mapStaticEnvBindings(st, *se.up, *env.up, vm);
if (env.type == Env::HasWithAttrs) {
if (!env.values[0]->isThunk()) {
// add 'with' bindings.
Bindings::iterator j = env.values[0]->attrs->begin();
while (j != env.values[0]->attrs->end()) {
@ -973,22 +973,23 @@ inline Value * EvalState::lookupVar(Env * env, const ExprVar & var, bool noEval)
if (!var.fromWith) return env->values[var.displ];
// This early exit defeats the `maybeThunk` optimization for variables from `with`,
// The added complexity of handling this appears to be similarly in cost, or
// the cases where applicable were insignificant in the first place.
if (noEval) return nullptr;
auto * fromWith = var.fromWith;
while (1) {
if (env->type == Env::HasWithExpr) {
if (noEval) return 0;
Value * v = allocValue();
evalAttrs(*env->up, (Expr *) env->values[0], *v, noPos, "<borked>");
env->values[0] = v;
env->type = Env::HasWithAttrs;
}
forceAttrs(*env->values[0], fromWith->pos, "while evaluating the first subexpression of a with expression");
Bindings::iterator j = env->values[0]->attrs->find(var.name);
if (j != env->values[0]->attrs->end()) {
if (countCalls) attrSelects[j->pos]++;
return j->value;
}
if (!env->prevWith)
if (!fromWith->parentWith)
error("undefined variable '%1%'", symbols[var.name]).atPos(var.pos).withFrame(*env, var).debugThrow<UndefinedVarError>();
for (size_t l = env->prevWith; l; --l, env = env->up) ;
for (size_t l = fromWith->prevWith; l; --l, env = env->up) ;
fromWith = fromWith->parentWith;
}
}
@ -1816,9 +1817,7 @@ void ExprWith::eval(EvalState & state, Env & env, Value & v)
{
Env & env2(state.allocEnv(1));
env2.up = &env;
env2.prevWith = prevWith;
env2.type = Env::HasWithExpr;
env2.values[0] = (Value *) attrs;
env2.values[0] = attrs->maybeThunk(state, env);
body->eval(state, env2, v);
}