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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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2
src/libexpr/primops.cc
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@ -214,7 +214,7 @@ static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * v
Env * env = &state.allocEnv(vScope->attrs->size());
env->up = &state.baseEnv;
auto staticEnv = std::make_shared<StaticEnv>(false, state.staticBaseEnv.get(), vScope->attrs->size());
auto staticEnv = std::make_shared<StaticEnv>(nullptr, state.staticBaseEnv.get(), vScope->attrs->size());
unsigned int displ = 0;
for (auto & attr : *vScope->attrs) {