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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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18
src/libexpr/nixexpr.cc
View file

@ -333,6 +333,8 @@ void ExprVar::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
if (es.debugRepl)
es.exprEnvs.insert(std::make_pair(this, env));
fromWith = nullptr;
/* Check whether the variable appears in the environment. If so,
set its level and displacement. */
const StaticEnv * curEnv;
@ -344,7 +346,6 @@ void ExprVar::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
} else {
auto i = curEnv->find(name);
if (i != curEnv->vars.end()) {
fromWith = false;
this->level = level;
displ = i->second;
return;
@ -360,7 +361,8 @@ void ExprVar::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
.msg = hintfmt("undefined variable '%1%'", es.symbols[name]),
.errPos = es.positions[pos]
});
fromWith = true;
for (auto * e = env.get(); e && !fromWith; e = e->up)
fromWith = e->isWith;
this->level = withLevel;
}
@ -393,7 +395,7 @@ void ExprAttrs::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv>
es.exprEnvs.insert(std::make_pair(this, env));
if (recursive) {
auto newEnv = std::make_shared<StaticEnv>(false, env.get(), recursive ? attrs.size() : 0);
auto newEnv = std::make_shared<StaticEnv>(nullptr, env.get(), recursive ? attrs.size() : 0);
Displacement displ = 0;
for (auto & i : attrs)
@ -435,7 +437,7 @@ void ExprLambda::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv>
es.exprEnvs.insert(std::make_pair(this, env));
auto newEnv = std::make_shared<StaticEnv>(
false, env.get(),
nullptr, env.get(),
(hasFormals() ? formals->formals.size() : 0) +
(!arg ? 0 : 1));
@ -471,7 +473,7 @@ void ExprLet::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
if (es.debugRepl)
es.exprEnvs.insert(std::make_pair(this, env));
auto newEnv = std::make_shared<StaticEnv>(false, env.get(), attrs->attrs.size());
auto newEnv = std::make_shared<StaticEnv>(nullptr, env.get(), attrs->attrs.size());
Displacement displ = 0;
for (auto & i : attrs->attrs)
@ -490,6 +492,10 @@ void ExprWith::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
if (es.debugRepl)
es.exprEnvs.insert(std::make_pair(this, env));
parentWith = nullptr;
for (auto * e = env.get(); e && !parentWith; e = e->up)
parentWith = e->isWith;
/* Does this `with' have an enclosing `with'? If so, record its
level so that `lookupVar' can look up variables in the previous
`with' if this one doesn't contain the desired attribute. */
@ -506,7 +512,7 @@ void ExprWith::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
es.exprEnvs.insert(std::make_pair(this, env));
attrs->bindVars(es, env);
auto newEnv = std::make_shared<StaticEnv>(true, env.get());
auto newEnv = std::make_shared<StaticEnv>(this, env.get());
body->bindVars(es, newEnv);
}