From b6f49f60d951bf0bb6efcc07435a25456912a6bf Mon Sep 17 00:00:00 2001
From: Ramadhan Gerry Akbar <ramadhan.gerry@gmail.com>
Date: Tue, 2 Jun 2026 14:41:26 +0700
Subject: [PATCH] scheduling: reject reorder() of compressed dimension before
 its storage parent (fixes #403)

---
 src/index_notation/transformations.cpp | 51 ++++++++++++++++++++++++++
 test/tests-scheduling.cpp              | 24 ++++++++++++
 2 files changed, 75 insertions(+)

diff --git a/src/index_notation/transformations.cpp b/src/index_notation/transformations.cpp
index d53ec58c3..f7c51ef42 100644
--- a/src/index_notation/transformations.cpp
+++ b/src/index_notation/transformations.cpp
@@ -123,6 +123,57 @@ IndexStmt Reorder::apply(IndexStmt stmt, string* reason) const {
     *reason = "The foralls of reorder pattern: " + util::join(getreplacepattern()) + " were not directly nested.";
     return IndexStmt();
   }
+
+  // Reject reorderings that move a compressed dimension before its storage
+  // parent. For a tensor stored in CSF/CSR format the position array lookup
+  // for level L uses the level-(L-1) position variable, so the level-L loop
+  // must be nested inside the level-(L-1) loop.  Placing level-L's index
+  // variable before level-(L-1)'s index variable in the requested pattern
+  // silently generates code that reads an undefined variable (issue #403).
+  struct CheckModeOrderingDeps : public IndexNotationVisitor {
+    using IndexNotationVisitor::visit;
+    const std::vector<IndexVar>& pattern;
+    std::string* reason;
+
+    CheckModeOrderingDeps(const std::vector<IndexVar>& p, std::string* r)
+        : pattern(p), reason(r) {}
+
+    void visit(const AccessNode* node) {
+      const auto& modeOrdering = node->tensorVar.getFormat().getModeOrdering();
+      const auto& modeFormats  = node->tensorVar.getFormat().getModeFormats();
+      for (size_t m = 1; m < modeOrdering.size(); ++m) {
+        int prevMode = modeOrdering[m - 1];
+        int curMode  = modeOrdering[m];
+        // Dense levels are accessed by coordinate and do not carry a
+        // positional dependency, so no constraint applies to this pair.
+        if (modeFormats[prevMode] == ModeFormat::Dense) continue;
+
+        IndexVar prevVar = node->indexVars[prevMode];
+        IndexVar curVar  = node->indexVars[curMode];
+
+        auto prevIt = std::find(pattern.begin(), pattern.end(), prevVar);
+        auto curIt  = std::find(pattern.begin(), pattern.end(), curVar);
+        if (prevIt == pattern.end() || curIt == pattern.end()) continue;
+
+        if (curIt < prevIt) {
+          *reason = "Cannot reorder " + util::toString(curVar) +
+                    " before " + util::toString(prevVar) +
+                    ": in tensor " + node->tensorVar.getName() +
+                    ", dimension " + util::toString(curVar) +
+                    " is at a compressed level whose position array is" +
+                    " indexed by " + util::toString(prevVar) + "'s iterator.";
+          return;
+        }
+      }
+    }
+  };
+
+  CheckModeOrderingDeps checker(getreplacepattern(), reason);
+  stmt.accept(&checker);
+  if (!reason->empty()) {
+    return IndexStmt();
+  }
+
   return ForAllReplace(currentOrdering, getreplacepattern()).apply(stmt, reason);
 }
 
diff --git a/test/tests-scheduling.cpp b/test/tests-scheduling.cpp
index ee564577b..a76ccc199 100644
--- a/test/tests-scheduling.cpp
+++ b/test/tests-scheduling.cpp
@@ -998,6 +998,30 @@ TEST(scheduling_eval_test, indexVarReorder) {
   ASSERT_TENSOR_EQ(expected, a);
 }
 
+// Regression test for issue #403: reordering a compressed dimension before its
+// storage parent must be rejected with a clear error rather than silently
+// emitting code that reads an undefined position variable.
+TEST(scheduling, reorder_invalid_sparse_dependency) {
+  // SpTV: y(j) = A(i,j,k) * x(k), A stored as sss (order i,j,k).
+  // j's compressed level is indexed by i's position variable, so reorder(j,i,k)
+  // is invalid — j cannot be iterated before i.
+  Tensor<double> y("y", {4}, Sparse);
+  Tensor<double> A("A", {4, 4, 4}, {Compressed, Compressed, Compressed});
+  Tensor<double> x("x", {4}, Sparse);
+
+  IndexVar ii("i"), jj("j"), kk("k");
+  y(jj) = A(ii, jj, kk) * x(kk);
+  IndexStmt stmt = y.getAssignment().concretize();
+
+  std::string reason;
+  IndexStmt result = Reorder({jj, ii, kk}).apply(stmt, &reason);
+
+  ASSERT_FALSE(result.defined())
+      << "Expected reorder(j,i,k) to fail for sss tensor, but it succeeded";
+  ASSERT_FALSE(reason.empty())
+      << "Expected a non-empty error reason for invalid sparse reordering";
+}
+
 TEST(scheduling, divide) {
   auto dim = 256;
   float sparsity = 0.1;