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Code Activity

libexpr: Slim down Bindings to 8 bytes (on 64 bit systems)

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Since the only construction and push_back() calls
to Bindings happen through the `BindingsBuilder` [1] we don't
need to keep `capacity` around on the heap anymore. This saves 8 bytes
(because of the member alignment padding)
per one Bindings allocation. This isn't that much, but it does
save significant memory.

This also shows that the Bindings don't necessarily have to
be mutable, which opens up opportunities for doing small bindings
optimization and storing a 1-element Bindings directly in Value.

For the following scenario:

nix-env --query --available --out-path --file ../nixpkgs --eval-system x86_64-linux

(nixpkgs revision: ddcddd7b09a417ca9a88899f4bd43a8edb72308d)

This patch results in reduction of `sets.bytes` 13115104016 -> 12653087640,
which amounts to 462 MB less bytes allocated for Bindings.

[1]: Not actually, `getBuiltins` does mutate bindings, but this is pretty
     inconsequential and doesn't lead to problems.
This commit is contained in:
Sergei Zimmerman 2025-09-06 00:23:54 +03:00
parent 49e9c14e2f
commit 738924b705
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6 changed files with 47 additions and 48 deletions
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2
src/libexpr-c/nix_api_value.cc
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@ -594,7 +594,7 @@ nix_err nix_bindings_builder_insert(nix_c_context * context, BindingsBuilder * b
context->last_err_code = NIX_OK;
try {
auto & v = check_value_not_null(value);
nix::Symbol s = bb->builder.state.symbols.create(name);
nix::Symbol s = bb->builder.state.get().symbols.create(name);
bb->builder.insert(s, &v);
}
NIXC_CATCH_ERRS