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Merge pull request #13387 from NaN-git/opt-listToAttrs

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libexpr: don't allocate additional set in `builtins.listToAttrs`
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Robert Hensing 2025-07-02 11:55:39 +02:00 • committed by GitHub
commit 04a731b4b9
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1 changed files with 32 additions and 10 deletions
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42
src/libexpr/primops.cc
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@ -2962,25 +2962,47 @@ static void prim_listToAttrs(EvalState & state, const PosIdx pos, Value * * args
{
state.forceList(*args[0], pos, "while evaluating the argument passed to builtins.listToAttrs");
auto attrs = state.buildBindings(args[0]->listSize());
// Step 1. Sort the name-value attrsets in place using the memory we allocate for the result
size_t listSize = args[0]->listSize();
auto & bindings = *state.allocBindings(listSize);
using ElemPtr = decltype(&bindings[0].value);
std::set<Symbol> seen;
for (auto v2 : args[0]->listItems()) {
for (const auto & [n, v2] : enumerate(args[0]->listItems())) {
state.forceAttrs(*v2, pos, "while evaluating an element of the list passed to builtins.listToAttrs");
auto j = state.getAttr(state.sName, v2->attrs(), "in a {name=...; value=...;} pair");
auto name = state.forceStringNoCtx(*j->value, j->pos, "while evaluating the `name` attribute of an element of the list passed to builtins.listToAttrs");
auto sym = state.symbols.create(name);
if (seen.insert(sym).second) {
auto j2 = state.getAttr(state.sValue, v2->attrs(), "in a {name=...; value=...;} pair");
attrs.insert(sym, j2->value, j2->pos);
}
// (ab)use Attr to store a Value * * instead of a Value *, so that we can stabilize the sort using the Value * *
bindings[n] = Attr(sym, std::bit_cast<Value *>(&v2));
}
v.mkAttrs(attrs);
std::sort(&bindings[0], &bindings[listSize], [](const Attr & a, const Attr & b) {
// Note that .value is actually a Value * * that corresponds to the position in the list
return a < b || (!(a > b) && std::bit_cast<ElemPtr>(a.value) < std::bit_cast<ElemPtr>(b.value));
});
// Step 2. Unpack the bindings in place and skip name-value pairs with duplicate names
Symbol prev;
for (size_t n = 0; n < listSize; n++) {
auto attr = bindings[n];
if (prev == attr.name) {
continue;
}
// Note that .value is actually a Value * *; see earlier comments
Value * v2 = *std::bit_cast<ElemPtr>(attr.value);
auto j = state.getAttr(state.sValue, v2->attrs(), "in a {name=...; value=...;} pair");
prev = attr.name;
bindings.push_back({prev, j->value, j->pos});
}
// help GC and clear end of allocated array
for (size_t n = bindings.size(); n < listSize; n++) {
bindings[n] = Attr{};
}
v.mkAttrs(&bindings);
}
static RegisterPrimOp primop_listToAttrs({