Fix `SPLIT_EQ_STRIP_R`/`SPLIT_EQ_STRIP_L` rewrites (#6550)

Fixes #6520. The `SPLIT_EQ_STRIP_R`/`SPLIT_EQ_STRIP_L` rewrites were
applied too aggressively. Those rewrites attempt to rewrite string
equalities between concatenations where the prefix on one side is
provably shorter than the prefix on the other side. The length of the
shorter prefix is then stripped from the longer prefix. However, cvc5
was not checking whether it was able to strip the length of the full
prefix. If cvc5 cannot strip the full length of the shorter prefix, then
the rewrite does not apply because parts of the shorter prefix would
have to be kept. This commit adds an additional condition that checks
whether the length of the full prefix was stripped.

diff --git a/src/theory/strings/sequences_rewriter.cpp b/src/theory/strings/sequences_rewriter.cpp
index 84127e8e378d63bbeaa8fab43a99863eea152525..d3c7501853ec2557b534c7b61d7b02ab37f68158 100644 (file)
--- a/src/theory/strings/sequences_rewriter.cpp
+++ b/src/theory/strings/sequences_rewriter.cpp
@@ -468,13 +468,20 @@ Node SequencesRewriter::rewriteStrEqualityExt(Node node)
             if (StringsEntail::stripSymbolicLength(
                     pfxv1, rpfxv1, 1, lenPfx0, true))
             {
-              std::vector<Node> sfxv0(v0.begin() + i, v0.end());
-              pfxv1.insert(pfxv1.end(), v1.begin() + j, v1.end());
-              Node ret = nm->mkNode(kind::AND,
-                                    pfx0.eqNode(utils::mkConcat(rpfxv1, stype)),
-                                    utils::mkConcat(sfxv0, stype)
-                                        .eqNode(utils::mkConcat(pfxv1, stype)));
-              return returnRewrite(node, ret, Rewrite::SPLIT_EQ_STRIP_R);
+              // The rewrite requires the full left-hand prefix length to be
+              // stripped (otherwise we would have to keep parts of the
+              // left-hand prefix).
+              if (lenPfx0.isConst() && lenPfx0.getConst<Rational>().isZero())
+              {
+                std::vector<Node> sfxv0(v0.begin() + i, v0.end());
+                pfxv1.insert(pfxv1.end(), v1.begin() + j, v1.end());
+                Node ret =
+                    nm->mkNode(kind::AND,
+                               pfx0.eqNode(utils::mkConcat(rpfxv1, stype)),
+                               utils::mkConcat(sfxv0, stype)
+                                   .eqNode(utils::mkConcat(pfxv1, stype)));
+                return returnRewrite(node, ret, Rewrite::SPLIT_EQ_STRIP_R);
+              }
             }
 
             // If the prefix of the right-hand side is (strictly) longer than
@@ -496,13 +503,20 @@ Node SequencesRewriter::rewriteStrEqualityExt(Node node)
             if (StringsEntail::stripSymbolicLength(
                     pfxv0, rpfxv0, 1, lenPfx1, true))
             {
-              pfxv0.insert(pfxv0.end(), v0.begin() + i, v0.end());
-              std::vector<Node> sfxv1(v1.begin() + j, v1.end());
-              Node ret = nm->mkNode(kind::AND,
-                                    utils::mkConcat(rpfxv0, stype).eqNode(pfx1),
-                                    utils::mkConcat(pfxv0, stype)
-                                        .eqNode(utils::mkConcat(sfxv1, stype)));
-              return returnRewrite(node, ret, Rewrite::SPLIT_EQ_STRIP_L);
+              // The rewrite requires the full right-hand prefix length to be
+              // stripped (otherwise we would have to keep parts of the
+              // right-hand prefix).
+              if (lenPfx1.isConst() && lenPfx1.getConst<Rational>().isZero())
+              {
+                pfxv0.insert(pfxv0.end(), v0.begin() + i, v0.end());
+                std::vector<Node> sfxv1(v1.begin() + j, v1.end());
+                Node ret =
+                    nm->mkNode(kind::AND,
+                               utils::mkConcat(rpfxv0, stype).eqNode(pfx1),
+                               utils::mkConcat(pfxv0, stype)
+                                   .eqNode(utils::mkConcat(sfxv1, stype)));
+                return returnRewrite(node, ret, Rewrite::SPLIT_EQ_STRIP_L);
+              }
             }
 
             // If the prefix of the left-hand side is (strictly) longer than

diff --git a/test/regress/CMakeLists.txt b/test/regress/CMakeLists.txt
index 8ed170da904ccc20532afc29a1c3b3af6570a6cf..5e9b38e8de8ececee4d8022a2736166cc95e1202 100644 (file)
--- a/test/regress/CMakeLists.txt
+++ b/test/regress/CMakeLists.txt
@@ -1128,6 +1128,7 @@ set(regress_0_tests
   regress0/strings/issue5915-repl-ctn-rewrite.smt2
   regress0/strings/issue6203-3-unfold-trivial-true.smt2
   regress0/strings/issue6510-seq-bool.smt2
+  regress0/strings/issue6520.smt2
   regress0/strings/itos-entail.smt2
   regress0/strings/large-model.smt2
   regress0/strings/leadingzero001.smt2

diff --git a/test/regress/regress0/strings/issue6520.smt2 b/test/regress/regress0/strings/issue6520.smt2
new file mode 100644 (file)
index 0000000..23a9383
--- /dev/null
+++ b/test/regress/regress0/strings/issue6520.smt2
@@ -0,0 +1,8 @@
+; COMMAND-LINE: --ext-rew-prep
+(set-logic QF_SLIA)
+(declare-fun a () String)
+(declare-fun b () String)
+(declare-fun c () String)
+(assert (= (str.++ "AB" b c) (str.++ c "B" a)))
+(set-info :status sat)
+(check-sat)