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* Memoise checkVarDefs since internally produced terms (i.e., not the

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result of parsing) can have very heavy sharing, causing exponential
  complexity if we naively recurse into them.  ATerms are graphs, not
  trees!
This commit is contained in:
Eelco Dolstra 2005-11-04 15:17:05 +00:00
parent 1f285cf556
commit 5bf939885a
1 changed files with 24 additions and 9 deletions
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33
src/libexpr/nixexpr.cc
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@ -301,8 +301,16 @@ Expr substitute(const ATermMap & subs, Expr e)
}
void checkVarDefs(const ATermMap & defs, Expr e)
/* We use memoisation to prevent exponential complexity on heavily
shared ATerms (remember, an ATerm is a graph, not a tree!). Note
that using an STL set is fine here wrt to ATerm garbage collection
since all the ATerms in the set are already reachable from
somewhere else. */
static void checkVarDefs2(set<Expr> & done, const ATermMap & defs, Expr e)
{
if (done.find(e) != done.end()) return;
done.insert(e);
ATerm name, pos, value;
ATermList formals;
ATerm with, body;
@ -320,22 +328,22 @@ void checkVarDefs(const ATermMap & defs, Expr e)
Expr deflt;
if (!matchNoDefFormal(*i, name))
if (matchDefFormal(*i, name, deflt))
checkVarDefs(defs, deflt);
checkVarDefs2(done, defs, deflt);
else
abort();
defs2.set(name, (ATerm) ATempty);
}
checkVarDefs(defs2, body);
checkVarDefs2(done, defs2, body);
}
else if (matchFunction1(e, name, body, pos)) {
ATermMap defs2(defs);
defs2.set(name, (ATerm) ATempty);
checkVarDefs(defs2, body);
checkVarDefs2(done, defs2, body);
}
else if (matchRec(e, rbnds, nrbnds)) {
checkVarDefs(defs, (ATerm) nrbnds);
checkVarDefs2(done, defs, (ATerm) nrbnds);
ATermMap defs2(defs);
for (ATermIterator i(rbnds); i; ++i) {
if (!matchBind(*i, name, value, pos)) abort(); /* can't happen */
@ -345,25 +353,32 @@ void checkVarDefs(const ATermMap & defs, Expr e)
if (!matchBind(*i, name, value, pos)) abort(); /* can't happen */
defs2.set(name, (ATerm) ATempty);
}
checkVarDefs(defs2, (ATerm) rbnds);
checkVarDefs2(done, defs2, (ATerm) rbnds);
}
else if (matchWith(e, with, body, pos)) {
/* We can't check the body without evaluating the definitions
(which is an arbitrary expression), so we don't do that
here but only when actually evaluating the `with'. */
checkVarDefs(defs, with);
checkVarDefs2(done, defs, with);
}
else if (ATgetType(e) == AT_APPL) {
int arity = ATgetArity(ATgetAFun(e));
for (int i = 0; i < arity; ++i)
checkVarDefs(defs, ATgetArgument(e, i));
checkVarDefs2(done, defs, ATgetArgument(e, i));
}
else if (ATgetType(e) == AT_LIST)
for (ATermIterator i((ATermList) e); i; ++i)
checkVarDefs(defs, *i);
checkVarDefs2(done, defs, *i);
}
void checkVarDefs(const ATermMap & defs, Expr e)
{
set<Expr> done;
checkVarDefs2(done, defs, e);
}