lix/src/libexpr/eval.cc
Eelco Dolstra c4ac2a164a * The recent change in nixpkgs of calling `stdenv.mkDerivation'
instead of `derivation' triggered a huge slowdown in the Nix
  expression evaluator.  Total execution time of `nix-env -qa' went up
  by a factor of 60 or so.

  This scalability problem was caused by expressions such as

    (x: y: ... x ...) a b

  where `a' is a large term (say, the one in
  `all-packages-generic.nix').  Then the first beta-reduction would
  produce

    (y: ... a ...) b

  by substituting `a' for `x'.  The second beta-reduction would then
  substitute `b' for `y' into the body `... a ...', which is a large
  term due to `a', and thus causes a large traversal to be performed
  by substitute() in the second reduction.  This is however entirely
  redundant, since `a' cannot contain free variables (since we never
  substitute below a weak head normal form).

  The solution is to wrap substituted terms into a `Closed'
  constructor, i.e.,

    subst(subs, Var(x)) = Closed(e) iff subs[x] = e

  have substitution not descent into closed terms,

    subst(subs, Closed(x)) = Closed(x)

  and otherwise ignore them for evaluation,

    eval(Closed(x)) = eval(x).

* Fix a typo that caused incorrect substitutions to be performed in
  simple lambdas, e.g., `(x: x: x) a' would reduce to `(x: a)'.
2004-03-30 15:05:35 +00:00

353 lines
9.5 KiB
C++

#include "eval.hh"
#include "parser.hh"
#include "primops.hh"
static void addPrimOp(ATermMap & map, const string & name, void * f)
{
map.set(name, (ATerm) ATmakeBlob(0, f));
}
static void * lookupPrimOp(ATermMap & map, ATerm name)
{
ATermBlob b = (ATermBlob) map.get(name);
if (!b) return 0;
return ATgetBlobData(b);
}
EvalState::EvalState()
: normalForms(32768, 50)
{
blackHole = ATmake("BlackHole()");
if (!blackHole) throw Error("cannot build black hole");
nrEvaluated = nrCached = 0;
addPrimOp0("true", primTrue);
addPrimOp0("false", primFalse);
addPrimOp0("null", primNull);
addPrimOp1("import", primImport);
addPrimOp1("derivation", primDerivation);
addPrimOp1("baseNameOf", primBaseNameOf);
addPrimOp1("toString", primToString);
addPrimOp1("isNull", primIsNull);
primOpsAll.add(primOps0);
primOpsAll.add(primOps1);
}
void EvalState::addPrimOp0(const string & name, PrimOp0 primOp)
{
addPrimOp(primOps0, name, (void *) primOp);
}
void EvalState::addPrimOp1(const string & name, PrimOp1 primOp)
{
addPrimOp(primOps1, name, (void *) primOp);
}
/* Substitute an argument set into the body of a function. */
static Expr substArgs(Expr body, ATermList formals, Expr arg)
{
ATMatcher m;
ATermMap subs;
Expr undefined = ATmake("Undefined");
/* Get the formal arguments. */
for (ATermIterator i(formals); i; ++i) {
Expr name, def;
if (atMatch(m, *i) >> "NoDefFormal" >> name)
subs.set(name, undefined);
else if (atMatch(m, *i) >> "DefFormal" >> name >> def)
subs.set(name, def);
else abort(); /* can't happen */
}
/* Get the actual arguments, and check that they match with the
formals. */
ATermMap args;
queryAllAttrs(arg, args);
for (ATermIterator i(args.keys()); i; ++i) {
Expr key = *i;
Expr cur = subs.get(key);
if (!cur)
throw badTerm(format("function has no formal argument `%1%'")
% aterm2String(key), arg);
subs.set(key, args.get(key));
}
/* Check that all arguments are defined. */
for (ATermIterator i(subs.keys()); i; ++i)
if (subs.get(*i) == undefined)
throw badTerm(format("formal argument `%1%' missing")
% aterm2String(*i), arg);
return substitute(subs, body);
}
/* Transform a mutually recursive set into a non-recursive set. Each
attribute is transformed into an expression that has all references
to attributes substituted with selection expressions on the
original set. E.g., e = `rec {x = f x y; y = x;}' becomes `{x = f
(e.x) (e.y); y = e.x;}'. */
ATerm expandRec(ATerm e, ATermList rbnds, ATermList nrbnds)
{
ATMatcher m;
ATerm name;
Expr e2;
/* Create the substitution list. */
ATermMap subs;
for (ATermIterator i(rbnds); i; ++i) {
if (!(atMatch(m, *i) >> "Bind" >> name >> e2))
abort(); /* can't happen */
subs.set(name, ATmake("Select(<term>, <term>)", e, name));
}
for (ATermIterator i(nrbnds); i; ++i) {
if (!(atMatch(m, *i) >> "Bind" >> name >> e2))
abort(); /* can't happen */
subs.set(name, e2);
}
/* Create the non-recursive set. */
ATermMap as;
for (ATermIterator i(rbnds); i; ++i) {
if (!(atMatch(m, *i) >> "Bind" >> name >> e2))
abort(); /* can't happen */
as.set(name, substitute(subs, e2));
}
/* Copy the non-recursive bindings. !!! inefficient */
for (ATermIterator i(nrbnds); i; ++i) {
if (!(atMatch(m, *i) >> "Bind" >> name >> e2))
abort(); /* can't happen */
as.set(name, e2);
}
return makeAttrs(as);
}
static Expr updateAttrs(Expr e1, Expr e2)
{
/* Note: e1 and e2 should be in normal form. */
ATermMap attrs;
queryAllAttrs(e1, attrs);
queryAllAttrs(e2, attrs);
return makeAttrs(attrs);
}
string evalString(EvalState & state, Expr e)
{
e = evalExpr(state, e);
ATMatcher m;
string s;
if (!(atMatch(m, e) >> "Str" >> s))
throw badTerm("string expected", e);
return s;
}
Path evalPath(EvalState & state, Expr e)
{
e = evalExpr(state, e);
ATMatcher m;
string s;
if (!(atMatch(m, e) >> "Path" >> s))
throw badTerm("path expected", e);
return s;
}
bool evalBool(EvalState & state, Expr e)
{
e = evalExpr(state, e);
ATMatcher m;
if (atMatch(m, e) >> "Bool" >> "True") return true;
else if (atMatch(m, e) >> "Bool" >> "False") return false;
else throw badTerm("expecting a boolean", e);
}
Expr evalExpr2(EvalState & state, Expr e)
{
ATMatcher m;
Expr e1, e2, e3, e4;
ATerm name;
/* Normal forms. */
string cons;
if (atMatch(m, e) >> cons &&
(cons == "Str" ||
cons == "Path" ||
cons == "SubPath" ||
cons == "Uri" ||
cons == "Null" ||
cons == "Int" ||
cons == "Bool" ||
cons == "Function" ||
cons == "Function1" ||
cons == "Attrs" ||
cons == "List"))
return e;
/* The `Closed' constructor is just a way to prevent substitutions
into expressions not containing free variables. */
if (atMatch(m, e) >> "Closed" >> e1)
return evalExpr(state, e1);
/* Any encountered variables must be undeclared or primops. */
if (atMatch(m, e) >> "Var" >> name) {
PrimOp0 primOp = (PrimOp0) lookupPrimOp(state.primOps0, name);
if (primOp)
return primOp(state);
else
return e;
}
/* Function application. */
if (atMatch(m, e) >> "Call" >> e1 >> e2) {
ATermList formals;
/* Evaluate the left-hand side. */
e1 = evalExpr(state, e1);
/* Is it a primop or a function? */
if (atMatch(m, e1) >> "Var" >> name) {
PrimOp1 primOp = (PrimOp1) lookupPrimOp(state.primOps1, name);
if (primOp) return primOp(state, e2); else abort();
}
else if (atMatch(m, e1) >> "Function" >> formals >> e4)
return evalExpr(state,
substArgs(e4, formals, evalExpr(state, e2)));
else if (atMatch(m, e1) >> "Function1" >> name >> e4) {
ATermMap subs;
subs.set(name, e2);
return evalExpr(state, substitute(subs, e4));
}
else throw badTerm("expecting a function or primop", e1);
}
/* Attribute selection. */
string s1;
if (atMatch(m, e) >> "Select" >> e1 >> s1) {
Expr a = queryAttr(evalExpr(state, e1), s1);
if (!a) throw badTerm(format("missing attribute `%1%'") % s1, e);
return evalExpr(state, a);
}
/* Mutually recursive sets. */
ATermList rbnds, nrbnds;
if (atMatch(m, e) >> "Rec" >> rbnds >> nrbnds)
return expandRec(e, rbnds, nrbnds);
/* Conditionals. */
if (atMatch(m, e) >> "If" >> e1 >> e2 >> e3) {
if (evalBool(state, e1))
return evalExpr(state, e2);
else
return evalExpr(state, e3);
}
/* Assertions. */
if (atMatch(m, e) >> "Assert" >> e1 >> e2) {
if (!evalBool(state, e1)) throw badTerm("guard failed", e);
return evalExpr(state, e2);
}
/* Generic equality. */
if (atMatch(m, e) >> "OpEq" >> e1 >> e2)
return makeBool(evalExpr(state, e1) == evalExpr(state, e2));
/* Generic inequality. */
if (atMatch(m, e) >> "OpNEq" >> e1 >> e2)
return makeBool(evalExpr(state, e1) != evalExpr(state, e2));
/* Negation. */
if (atMatch(m, e) >> "OpNot" >> e1)
return makeBool(!evalBool(state, e1));
/* Implication. */
if (atMatch(m, e) >> "OpImpl" >> e1 >> e2)
return makeBool(!evalBool(state, e1) || evalBool(state, e2));
/* Conjunction (logical AND). */
if (atMatch(m, e) >> "OpAnd" >> e1 >> e2)
return makeBool(evalBool(state, e1) && evalBool(state, e2));
/* Disjunction (logical OR). */
if (atMatch(m, e) >> "OpOr" >> e1 >> e2)
return makeBool(evalBool(state, e1) || evalBool(state, e2));
/* Attribute set update (//). */
if (atMatch(m, e) >> "OpUpdate" >> e1 >> e2)
return updateAttrs(evalExpr(state, e1), evalExpr(state, e2));
/* Attribute existence test (?). */
if (atMatch(m, e) >> "OpHasAttr" >> e1 >> name) {
ATermMap attrs;
queryAllAttrs(evalExpr(state, e1), attrs);
return makeBool(attrs.get(name) != 0);
}
/* Barf. */
throw badTerm("invalid expression", e);
}
Expr evalExpr(EvalState & state, Expr e)
{
checkInterrupt();
startNest(nest, lvlVomit,
format("evaluating expression: %1%") % e);
state.nrEvaluated++;
/* Consult the memo table to quickly get the normal form of
previously evaluated expressions. */
Expr nf = state.normalForms.get(e);
if (nf) {
if (nf == state.blackHole)
throw badTerm("infinite recursion", e);
state.nrCached++;
return nf;
}
/* Otherwise, evaluate and memoize. */
state.normalForms.set(e, state.blackHole);
nf = evalExpr2(state, e);
state.normalForms.set(e, nf);
return nf;
}
Expr evalFile(EvalState & state, const Path & path)
{
startNest(nest, lvlTalkative, format("evaluating file `%1%'") % path);
Expr e = parseExprFromFile(state, path);
return evalExpr(state, e);
}
void printEvalStats(EvalState & state)
{
debug(format("evaluated %1% expressions, %2% cache hits, %3%%% efficiency")
% state.nrEvaluated % state.nrCached
% ((float) state.nrCached / (float) state.nrEvaluated * 100));
}