From 5bf939885a8bfca9b66a2440bf52330c5fde6477 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <e.dolstra@tudelft.nl>
Date: Fri, 4 Nov 2005 15:17:05 +0000
Subject: [PATCH] * Memoise checkVarDefs since internally produced terms (i.e.,
 not the   result of parsing) can have very heavy sharing, causing exponential
   complexity if we naively recurse into them.  ATerms are graphs, not  
 trees!

---
 src/libexpr/nixexpr.cc | 33 ++++++++++++++++++++++++---------
 1 file changed, 24 insertions(+), 9 deletions(-)

diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index 289e923c3..6fb72687d 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -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);
 }