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evaluate inherit (from) exprs only once per directive

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desugaring inherit-from to syntactic duplication of the source expr also
duplicates side effects of the source expr (such as trace calls) and
expensive computations (such as derivationStrict).
This commit is contained in:
pennae 2024-01-27 16:33:34 +01:00
parent ecf8b12d60
commit cefd0302b5
9 changed files with 109 additions and 15 deletions
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7
doc/manual/rl-next/inherit-from-by-need.md Normal file
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@ -0,0 +1,7 @@
---
synopsis: "`inherit (x) ...` evaluates `x` only once"
prs: 9847
---
`inherit (x) a b ...` now evaluates the expression `x` only once for all inherited attributes rather than once for each inherited attribute.
This does not usually have a measurable impact, but side-effects (such as `builtins.trace`) would be duplicated and expensive expressions (such as derivations) could cause a measurable slowdown.

24
src/libexpr/eval.cc
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@ -1186,6 +1186,18 @@ void ExprPath::eval(EvalState & state, Env & env, Value & v)
}
Env * ExprAttrs::buildInheritFromEnv(EvalState & state, Env & up)
{
Env & inheritEnv = state.allocEnv(inheritFromExprs->size());
inheritEnv.up = &up;
Displacement displ = 0;
for (auto from : *inheritFromExprs)
inheritEnv.values[displ++] = from->maybeThunk(state, up);
return &inheritEnv;
}
void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
{
v.mkAttrs(state.buildBindings(attrs.size() + dynamicAttrs.size()).finish());
@ -1197,6 +1209,7 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
Env & env2(state.allocEnv(attrs.size()));
env2.up = &env;
dynamicEnv = &env2;
Env * inheritEnv = inheritFromExprs ? buildInheritFromEnv(state, env2) : nullptr;
AttrDefs::iterator overrides = attrs.find(state.sOverrides);
bool hasOverrides = overrides != attrs.end();
@ -1209,9 +1222,9 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
Value * vAttr;
if (hasOverrides && !i.second.inherited()) {
vAttr = state.allocValue();
mkThunk(*vAttr, *i.second.chooseByKind(&env2, &env, &env2), i.second.e);
mkThunk(*vAttr, *i.second.chooseByKind(&env2, &env, inheritEnv), i.second.e);
} else
vAttr = i.second.e->maybeThunk(state, *i.second.chooseByKind(&env2, &env, &env2));
vAttr = i.second.e->maybeThunk(state, *i.second.chooseByKind(&env2, &env, inheritEnv));
env2.values[displ++] = vAttr;
v.attrs->push_back(Attr(i.first, vAttr, i.second.pos));
}
@ -1244,10 +1257,11 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
}
else {
Env * inheritEnv = inheritFromExprs ? buildInheritFromEnv(state, env) : nullptr;
for (auto & i : attrs) {
v.attrs->push_back(Attr(
i.first,
i.second.e->maybeThunk(state, *i.second.chooseByKind(&env, &env, &env)),
i.second.e->maybeThunk(state, *i.second.chooseByKind(&env, &env, inheritEnv)),
i.second.pos));
}
}
@ -1282,6 +1296,8 @@ void ExprLet::eval(EvalState & state, Env & env, Value & v)
Env & env2(state.allocEnv(attrs->attrs.size()));
env2.up = &env;
Env * inheritEnv = attrs->inheritFromExprs ? attrs->buildInheritFromEnv(state, env2) : nullptr;
/* The recursive attributes are evaluated in the new environment,
while the inherited attributes are evaluated in the original
environment. */
@ -1289,7 +1305,7 @@ void ExprLet::eval(EvalState & state, Env & env, Value & v)
for (auto & i : attrs->attrs) {
env2.values[displ++] = i.second.e->maybeThunk(
state,
*i.second.chooseByKind(&env2, &env, &env2));
*i.second.chooseByKind(&env2, &env, inheritEnv));
}
auto dts = state.debugRepl

44
src/libexpr/nixexpr.cc
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@ -80,7 +80,7 @@ void ExprAttrs::showBindings(const SymbolTable & symbols, std::ostream & str) co
return sa < sb;
});
std::vector<Symbol> inherits;
std::map<Expr *, std::vector<Symbol>> inheritsFrom;
std::map<ExprInheritFrom *, std::vector<Symbol>> inheritsFrom;
for (auto & i : sorted) {
switch (i->second.kind) {
case AttrDef::Kind::Plain:
@ -90,7 +90,8 @@ void ExprAttrs::showBindings(const SymbolTable & symbols, std::ostream & str) co
break;
case AttrDef::Kind::InheritedFrom: {
auto & select = dynamic_cast<ExprSelect &>(*i->second.e);
inheritsFrom[select.e].push_back(i->first);
auto & from = dynamic_cast<ExprInheritFrom &>(*select.e);
inheritsFrom[&from].push_back(i->first);
break;
}
}
@ -102,7 +103,7 @@ void ExprAttrs::showBindings(const SymbolTable & symbols, std::ostream & str) co
}
for (const auto & [from, syms] : inheritsFrom) {
str << "inherit (";
from->show(symbols, str);
(*inheritFromExprs)[from->displ]->show(symbols, str);
str << ")";
for (auto sym : syms) str << " " << symbols[sym];
str << "; ";
@ -328,6 +329,12 @@ void ExprVar::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
this->level = withLevel;
}
void ExprInheritFrom::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
{
if (es.debugRepl)
es.exprEnvs.insert(std::make_pair(this, env));
}
void ExprSelect::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
{
if (es.debugRepl)
@ -351,6 +358,27 @@ void ExprOpHasAttr::bindVars(EvalState & es, const std::shared_ptr<const StaticE
i.expr->bindVars(es, env);
}
std::shared_ptr<const StaticEnv> ExprAttrs::bindInheritSources(
EvalState & es, const std::shared_ptr<const StaticEnv> & env)
{
if (!inheritFromExprs)
return nullptr;
// the inherit (from) source values are inserted into an env of its own, which
// does not introduce any variable names.
// analysis must see an empty env, or an env that contains only entries with
// otherwise unused names to not interfere with regular names. the parser
// has already filled all exprs that access this env with appropriate level
// and displacement, and nothing else is allowed to access it. ideally we'd
// not even *have* an expr that grabs anything from this env since it's fully
// invisible, but the evaluator does not allow for this yet.
auto inner = std::make_shared<StaticEnv>(nullptr, env.get(), 0);
for (auto from : *inheritFromExprs)
from->bindVars(es, env);
return inner;
}
void ExprAttrs::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
{
if (es.debugRepl)
@ -368,8 +396,9 @@ void ExprAttrs::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv>
// No need to sort newEnv since attrs is in sorted order.
auto inheritFromEnv = bindInheritSources(es, newEnv);
for (auto & i : attrs)
i.second.e->bindVars(es, i.second.chooseByKind(newEnv, env, newEnv));
i.second.e->bindVars(es, i.second.chooseByKind(newEnv, env, inheritFromEnv));
for (auto & i : dynamicAttrs) {
i.nameExpr->bindVars(es, newEnv);
@ -377,8 +406,10 @@ void ExprAttrs::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv>
}
}
else {
auto inheritFromEnv = bindInheritSources(es, env);
for (auto & i : attrs)
i.second.e->bindVars(es, i.second.chooseByKind(env, env, env));
i.second.e->bindVars(es, i.second.chooseByKind(env, env, inheritFromEnv));
for (auto & i : dynamicAttrs) {
i.nameExpr->bindVars(es, env);
@ -446,8 +477,9 @@ void ExprLet::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
// No need to sort newEnv since attrs->attrs is in sorted order.
auto inheritFromEnv = attrs->bindInheritSources(es, newEnv);
for (auto & i : attrs->attrs)
i.second.e->bindVars(es, i.second.chooseByKind(newEnv, env, newEnv));
i.second.e->bindVars(es, i.second.chooseByKind(newEnv, env, inheritFromEnv));
if (es.debugRepl)
es.exprEnvs.insert(std::make_pair(this, newEnv));

16
src/libexpr/nixexpr.hh
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@ -135,6 +135,18 @@ struct ExprVar : Expr
COMMON_METHODS
};
struct ExprInheritFrom : ExprVar
{
ExprInheritFrom(PosIdx pos, Displacement displ): ExprVar(pos, {})
{
this->level = 0;
this->displ = displ;
this->fromWith = nullptr;
}
void bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env);
};
struct ExprSelect : Expr
{
PosIdx pos;
@ -195,6 +207,7 @@ struct ExprAttrs : Expr
};
typedef std::map<Symbol, AttrDef> AttrDefs;
AttrDefs attrs;
std::unique_ptr<std::vector<Expr *>> inheritFromExprs;
struct DynamicAttrDef {
Expr * nameExpr, * valueExpr;
PosIdx pos;
@ -208,6 +221,9 @@ struct ExprAttrs : Expr
PosIdx getPos() const override { return pos; }
COMMON_METHODS
std::shared_ptr<const StaticEnv> bindInheritSources(
EvalState & es, const std::shared_ptr<const StaticEnv> & env);
Env * buildInheritFromEnv(EvalState & state, Env & up);
void showBindings(const SymbolTable & symbols, std::ostream & str) const;
};

11
src/libexpr/parser-state.hh
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@ -118,13 +118,24 @@ inline void ParserState::addAttr(ExprAttrs * attrs, AttrPath && attrPath, Expr *
auto ae = dynamic_cast<ExprAttrs *>(e);
auto jAttrs = dynamic_cast<ExprAttrs *>(j->second.e);
if (jAttrs && ae) {
if (ae->inheritFromExprs && !jAttrs->inheritFromExprs)
jAttrs->inheritFromExprs = std::make_unique<std::vector<Expr *>>();
for (auto & ad : ae->attrs) {
auto j2 = jAttrs->attrs.find(ad.first);
if (j2 != jAttrs->attrs.end()) // Attr already defined in iAttrs, error.
dupAttr(ad.first, j2->second.pos, ad.second.pos);
jAttrs->attrs.emplace(ad.first, ad.second);
if (ad.second.kind == ExprAttrs::AttrDef::Kind::InheritedFrom) {
auto & sel = dynamic_cast<ExprSelect &>(*ad.second.e);
auto & from = dynamic_cast<ExprInheritFrom &>(*sel.e);
from.displ += jAttrs->inheritFromExprs->size();
}
}
jAttrs->dynamicAttrs.insert(jAttrs->dynamicAttrs.end(), ae->dynamicAttrs.begin(), ae->dynamicAttrs.end());
if (ae->inheritFromExprs) {
jAttrs->inheritFromExprs->insert(jAttrs->inheritFromExprs->end(),
ae->inheritFromExprs->begin(), ae->inheritFromExprs->end());
}
} else {
dupAttr(attrPath, pos, j->second.pos);
}

7
src/libexpr/parser.y
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@ -321,14 +321,17 @@ binds
}
| binds INHERIT '(' expr ')' attrs ';'
{ $$ = $1;
/* !!! Should ensure sharing of the expression in $4. */
if (!$$->inheritFromExprs)
$$->inheritFromExprs = std::make_unique<std::vector<Expr *>>();
$$->inheritFromExprs->push_back($4);
auto from = new nix::ExprInheritFrom(state->at(@4), $$->inheritFromExprs->size() - 1);
for (auto & i : *$6) {
if ($$->attrs.find(i.symbol) != $$->attrs.end())
state->dupAttr(i.symbol, state->at(@6), $$->attrs[i.symbol].pos);
$$->attrs.emplace(
i.symbol,
ExprAttrs::AttrDef(
new ExprSelect(CUR_POS, $4, i.symbol),
new ExprSelect(CUR_POS, from, i.symbol),
state->at(@6),
ExprAttrs::AttrDef::Kind::InheritedFrom));
}

1
tests/functional/lang/eval-okay-inherit-from.err.exp
View File

@ -1,2 +1 @@
trace: used
trace: used

2
tests/functional/lang/eval-okay-inherit-from.exp
View File

@ -1 +1 @@
[ 1 2 { __overrides = { y = { d = [ ]; }; }; c = [ ]; d = 4; x = { c = [ ]; }; y = «repeated»; } ]
[ 1 2 { __overrides = { y = { d = [ ]; }; }; c = [ ]; d = 4; x = { c = [ ]; }; y = «repeated»; } { inner = { c = 3; d = 4; }; } ]

12
tests/functional/lang/eval-okay-inherit-from.nix
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@ -2,5 +2,15 @@ let
inherit (builtins.trace "used" { a = 1; b = 2; }) a b;
x.c = 3;
y.d = 4;
merged = {
inner = {
inherit (y) d;
};
inner = {
inherit (x) c;
};
};
in
[ a b rec { x.c = []; inherit (x) c; inherit (y) d; __overrides.y.d = []; } ]
[ a b rec { x.c = []; inherit (x) c; inherit (y) d; __overrides.y.d = []; } merged ]