Merge pull request #9658 from pennae/env-diet

reduce the size of Env by one pointer

(cherry picked from commit 83f5622545a2fc31eb7e7d5105f64ed6dd3058b3)
Change-Id: I5636290526d0165cfc61aee1e7a5b94db4a26cef
This commit is contained in:
eldritch horrors 2024-03-04 07:37:45 +01:00
parent cd326a2aa4
commit 6b279cd10e
9 changed files with 75 additions and 34 deletions

View file

@ -0,0 +1,7 @@
---
synopsis: Reduce eval memory usage and wall time
prs: 9658
---
Reduce the size of the `Env` struct used in the evaluator by a pointer, or 8 bytes on most modern machines.
This reduces memory usage during eval by around 2% and wall time by around 3%.

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@ -0,0 +1,31 @@
---
synopsis: Better error reporting for `with` expressions
prs: 9658
---
`with` expressions using non-attrset values to resolve variables are now reported with proper positions.
Previously an incorrect `with` expression would report no position at all, making it hard to determine where the error originated:
```
nix-repl> with 1; a
error:
<borked>
at «none»:0: (source not available)
error: value is an integer while a set was expected
```
Now position information is preserved and reported as with most other errors:
```
nix-repl> with 1; a
error:
… while evaluating the first subexpression of a with expression
at «string»:1:1:
1| with 1; a
| ^
error: value is an integer while a set was expected
```

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@ -109,7 +109,7 @@ NixRepl::NixRepl(const SearchPath & searchPath, nix::ref<Store> store, ref<EvalS
: AbstractNixRepl(state)
, debugTraceIndex(0)
, getValues(getValues)
, staticEnv(new StaticEnv(false, state->staticBaseEnv.get()))
, staticEnv(new StaticEnv(nullptr, state->staticBaseEnv.get()))
, historyFile(getDataDir() + "/nix/repl-history")
{
}

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@ -73,8 +73,6 @@ Env & EvalState::allocEnv(size_t size)
#endif
env = (Env *) allocBytes(sizeof(Env) + size * sizeof(Value *));
env->type = Env::Plain;
/* We assume that env->values has been cleared by the allocator; maybeThunk() and lookupVar fromWith expect this. */
return *env;

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@ -519,7 +519,7 @@ EvalState::EvalState(
, env1AllocCache(std::allocate_shared<void *>(traceable_allocator<void *>(), nullptr))
#endif
, baseEnv(allocEnv(128))
, staticBaseEnv{std::make_shared<StaticEnv>(false, nullptr)}
, staticBaseEnv{std::make_shared<StaticEnv>(nullptr, nullptr)}
{
countCalls = getEnv("NIX_COUNT_CALLS").value_or("0") != "0";
@ -788,7 +788,7 @@ void printStaticEnvBindings(const SymbolTable & st, const StaticEnv & se)
// just for the current level of Env, not the whole chain.
void printWithBindings(const SymbolTable & st, const Env & env)
{
if (env.type == Env::HasWithAttrs) {
if (!env.values[0]->isThunk()) {
std::cout << "with: ";
std::cout << ANSI_MAGENTA;
Bindings::iterator j = env.values[0]->attrs->begin();
@ -842,7 +842,7 @@ void mapStaticEnvBindings(const SymbolTable & st, const StaticEnv & se, const En
if (env.up && se.up) {
mapStaticEnvBindings(st, *se.up, *env.up, vm);
if (env.type == Env::HasWithAttrs) {
if (!env.values[0]->isThunk()) {
// add 'with' bindings.
Bindings::iterator j = env.values[0]->attrs->begin();
while (j != env.values[0]->attrs->end()) {
@ -980,22 +980,23 @@ inline Value * EvalState::lookupVar(Env * env, const ExprVar & var, bool noEval)
if (!var.fromWith) return env->values[var.displ];
// This early exit defeats the `maybeThunk` optimization for variables from `with`,
// The added complexity of handling this appears to be similarly in cost, or
// the cases where applicable were insignificant in the first place.
if (noEval) return nullptr;
auto * fromWith = var.fromWith;
while (1) {
if (env->type == Env::HasWithExpr) {
if (noEval) return 0;
Value * v = allocValue();
evalAttrs(*env->up, (Expr *) env->values[0], *v, noPos, "<borked>");
env->values[0] = v;
env->type = Env::HasWithAttrs;
}
forceAttrs(*env->values[0], fromWith->pos, "while evaluating the first subexpression of a with expression");
Bindings::iterator j = env->values[0]->attrs->find(var.name);
if (j != env->values[0]->attrs->end()) {
if (countCalls) attrSelects[j->pos]++;
return j->value;
}
if (!env->prevWith)
if (!fromWith->parentWith)
error("undefined variable '%1%'", symbols[var.name]).atPos(var.pos).withFrame(*env, var).debugThrow<UndefinedVarError>();
for (size_t l = env->prevWith; l; --l, env = env->up) ;
for (size_t l = fromWith->prevWith; l; --l, env = env->up) ;
fromWith = fromWith->parentWith;
}
}
@ -1854,9 +1855,7 @@ void ExprWith::eval(EvalState & state, Env & env, Value & v)
{
Env & env2(state.allocEnv(1));
env2.up = &env;
env2.prevWith = prevWith;
env2.type = Env::HasWithExpr;
env2.values[0] = (Value *) attrs;
env2.values[0] = attrs->maybeThunk(state, env);
body->eval(state, env2, v);
}

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@ -115,11 +115,6 @@ struct Constant
struct Env
{
Env * up;
/**
* Number of of levels up to next `with` environment
*/
unsigned short prevWith:14;
enum { Plain = 0, HasWithExpr, HasWithAttrs } type:2;
Value * values[0];
};

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);
}

View file

@ -139,6 +139,7 @@ std::ostream & operator << (std::ostream & str, const Pos & pos);
struct Env;
struct Value;
class EvalState;
struct ExprWith;
struct StaticEnv;
@ -221,8 +222,11 @@ struct ExprVar : Expr
Symbol name;
/* Whether the variable comes from an environment (e.g. a rec, let
or function argument) or from a "with". */
bool fromWith;
or function argument) or from a "with".
`nullptr`: Not from a `with`.
Valid pointer: the nearest, innermost `with` expression to query first. */
ExprWith * fromWith;
/* In the former case, the value is obtained by going `level`
levels up from the current environment and getting the
@ -380,6 +384,7 @@ struct ExprWith : Expr
PosIdx pos;
Expr * attrs, * body;
size_t prevWith;
ExprWith * parentWith;
ExprWith(const PosIdx & pos, Expr * attrs, Expr * body) : pos(pos), attrs(attrs), body(body) { };
PosIdx getPos() const override { return pos; }
COMMON_METHODS
@ -473,14 +478,14 @@ extern ExprBlackHole eBlackHole;
runtime. */
struct StaticEnv
{
bool isWith;
ExprWith * isWith;
const StaticEnv * up;
// Note: these must be in sorted order.
typedef std::vector<std::pair<Symbol, Displacement>> Vars;
Vars vars;
StaticEnv(bool isWith, const StaticEnv * up, size_t expectedSize = 0) : isWith(isWith), up(up) {
StaticEnv(ExprWith * isWith, const StaticEnv * up, size_t expectedSize = 0) : isWith(isWith), up(up) {
vars.reserve(expectedSize);
};

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@ -233,7 +233,7 @@ 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) {