(proof-new) Separating optimizations for strings skolem caching (#6064)

This PR ensures that several optimizations are not performed in the reference implementation of skolem sharing (d_useOpts=false). This is to ensure that the many of the rules in the strings proof checker do not depend on the rewriter.

These errors were caught by the LFSC proof checker.

2 files changed, 15 insertions, 8 deletions

diff --git a/src/theory/strings/skolem_cache.cpp b/src/theory/strings/skolem_cache.cpp
index b68eb4a04..3c47b8f28 100644
--- a/src/theory/strings/skolem_cache.cpp
+++ b/src/theory/strings/skolem_cache.cpp
@@ -62,9 +62,13 @@ Node SkolemCache::mkTypedSkolemCached(
   Trace("skolem-cache") << "mkTypedSkolemCached start: (" << id << ", " << a
                         << ", " << b << ")" << std::endl;
   SkolemId idOrig = id;
-  a = a.isNull() ? a : Rewriter::rewrite(a);
-  b = b.isNull() ? b : Rewriter::rewrite(b);
-
+  // do not rewrite beforehand if we are not using optimizations, this is so
+  // that the proof checker does not depend on the rewriter.
+  if (d_useOpts)
+  {
+    a = a.isNull() ? a : Rewriter::rewrite(a);
+    b = b.isNull() ? b : Rewriter::rewrite(b);
+  }
   std::tie(id, a, b) = normalizeStringSkolem(id, a, b);
 
   // optimization: if we aren't asking for the purification skolem for constant
@@ -79,6 +83,7 @@ Node SkolemCache::mkTypedSkolemCached(
   std::map<SkolemId, Node>::iterator it = d_skolemCache[a][b].find(id);
   if (it != d_skolemCache[a][b].end())
   {
+    Trace("skolem-cache") << "...return existing " << it->second << std::endl;
     // already cached
     return it->second;
   }
@@ -120,6 +125,7 @@ Node SkolemCache::mkTypedSkolemCached(
     }
     break;
   }
+  Trace("skolem-cache") << "...returned " << sk << std::endl;
   d_allSkolems.insert(sk);
   d_skolemCache[a][b][id] = sk;
   return sk;
@@ -265,9 +271,11 @@ SkolemCache::normalizeStringSkolem(SkolemId id, Node a, Node b)
     b = Node::null();
   }
 
-  a = a.isNull() ? a : Rewriter::rewrite(a);
-  b = b.isNull() ? b : Rewriter::rewrite(b);
-
+  if (d_useOpts)
+  {
+    a = a.isNull() ? a : Rewriter::rewrite(a);
+    b = b.isNull() ? b : Rewriter::rewrite(b);
+  }
   Trace("skolem-cache") << "normalizeStringSkolem end: (" << id << ", " << a
                         << ", " << b << ")" << std::endl;
   return std::make_tuple(id, a, b);
diff --git a/src/theory/strings/theory_strings_preprocess.cpp b/src/theory/strings/theory_strings_preprocess.cpp
index f0afe5f26..eb897b2e5 100644
--- a/src/theory/strings/theory_strings_preprocess.cpp
+++ b/src/theory/strings/theory_strings_preprocess.cpp
@@ -69,7 +69,6 @@ Node StringsPreprocess::reduce(Node t,
     Node m = t[2];
     Node skt = sc->mkSkolemCached(t, SkolemCache::SK_PURIFY, "sst");
     Node t12 = nm->mkNode(PLUS, n, m);
-    t12 = Rewriter::rewrite(t12);
     Node lt0 = nm->mkNode(STRING_LENGTH, s);
     //start point is greater than or equal zero
     Node c1 = nm->mkNode(GEQ, n, zero);
@@ -101,7 +100,7 @@ Node StringsPreprocess::reduce(Node t,
     Node lemma = nm->mkNode(ITE, cond, b1, b2);
 
     // assert:
-    // IF    n >=0 AND n < len( s ) AND m > 0
+    // IF    n >=0 AND len( s ) > n AND m > 0
     // THEN: s = sk1 ++ skt ++ sk2 AND
     //       len( sk1 ) = n AND
     //       ( len( sk2 ) = len( s )-(n+m) OR len( sk2 ) = 0 ) AND