[TIR] Add tir::builtin::undef (#12266)

* [UnitTest] RemoveStoreUndef, simplest behavior

* [RemoveStoreUndef] First implementation

* [UnitTest] RemoveStoreUndef, stores that depend through LetStmt

* [UnitTest] RemoveStoreUndef, LetStmt handling, error on illegal usage

* [RemoveStoreUndef] Added error checking for illegal T.undef() usage

* Fix lint error

* Use const ref for list of stores to remove

* Verify that removed expression has no other side effects

* Fix lint error

include/tvm/tir/builtin.h

@@ -729,6 +729,14 @@ TVM_DLL const Op& mem_copy();
  */
 TVM_DLL const Op& assume();
 
+/*!
+ * \brief Returns an initialized but arbitrary value
+ *
+ * Compile-time representation of memory locations whose values may be
+ * altered as a result of optimizations.
+ */
+TVM_DLL const Op& undef();
+
 /*! \brief The kind of structure field info used in intrinsic */
 enum TVMStructFieldKind : int {
   // array head address

python/tvm/tir/transform/transform.py

@@ -264,6 +264,17 @@ def RemoveAssume():
     return _ffi_api.RemoveAssume()  # type: ignore
 
 
+def RemoveStoreUndef():
+    """Remove stores of undefined values from the Stmt.
+
+    Returns
+    -------
+    fpass : tvm.transform.Pass
+        The result pass
+    """
+    return _ffi_api.RemoveStoreUndef()  # type: ignore
+
+
 def BF16Legalize():
     """Legalize bf16 typed Ops.
     Runs BF16Promote, BF16CastElimination and BF16TypeLowering

src/tir/op/builtin.cc

@@ -292,6 +292,10 @@ TIR_DEFINE_BUILTIN_FUNC(assume)
     .set_attr<TCallEffectKind>("TCallEffectKind", Integer(CallEffectKind::kEmbedInfo))
     .set_num_inputs(1);
 
+TIR_DEFINE_BUILTIN_FUNC(undef)
+    .set_attr<TCallEffectKind>("TCallEffectKind", Integer(CallEffectKind::kReadState))
+    .set_num_inputs(0);
+
 }  // namespace builtin
 }  // namespace tir
 }  // namespace tvm

src/tir/transforms/remove_store_undef.cc

@@ -0,0 +1,179 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+/*!
+ * \file remove_store_undef.cc
+ * \brief Remove stores of tir::builtin::undef
+ */
+#include <tvm/runtime/registry.h>
+#include <tvm/tir/analysis.h>
+#include <tvm/tir/builtin.h>
+#include <tvm/tir/op.h>
+#include <tvm/tir/stmt.h>
+#include <tvm/tir/stmt_functor.h>
+#include <tvm/tir/transform.h>
+
+namespace tvm {
+namespace tir {
+
+class StoreUndefLocator : public StmtExprVisitor {
+ public:
+  static std::unordered_set<const BufferStoreNode*> Locate(Stmt stmt) {
+    StoreUndefLocator locator;
+    locator(std::move(stmt));
+    return locator.undef_stores_;
+  }
+
+ private:
+  StoreUndefLocator() = default;
+
+  void VisitStmt_(const BufferStoreNode* op) final {
+    bool stash_undef = false;
+    std::swap(has_undef_, stash_undef);
+    StmtExprVisitor::VisitExpr(op->value);
+    std::swap(has_undef_, stash_undef);
+    if (stash_undef) {
+      ICHECK(SideEffect(op->value) <= CallEffectKind::kReadState)
+          << "Error: T.undef() used in BufferStore expressions "
+          << "must not have other side effects";
+      undef_stores_.insert(op);
+    }
+  }
+
+  void VisitExpr_(const BufferLoadNode* op) final {
+    // This function left deliberately empty.  builtin::undef()
+    // shouldn't occur in the indices of BufferLoad.  Avoiding
+    // visiting the indices catches the builtin::undef in
+    // ValidateAllUndefRemoved.
+  }
+
+  void VisitStmt_(const LetStmtNode* op) final {
+    bool stash_undef = false;
+    std::swap(has_undef_, stash_undef);
+    StmtExprVisitor::VisitExpr(op->value);
+    std::swap(has_undef_, stash_undef);
+    if (stash_undef) {
+      ICHECK(SideEffect(op->value) <= CallEffectKind::kReadState)
+          << "Error: T.undef() used in Let expressions "
+          << "must not have other side effects";
+      var_bindings_with_undef_.insert(op->var.get());
+    }
+
+    StmtExprVisitor::VisitStmt(op->body);
+  }
+
+  void VisitExpr_(const VarNode* op) final {
+    if (var_bindings_with_undef_.count(op)) {
+      has_undef_ = true;
+    }
+  }
+
+  void VisitExpr_(const CallNode* op) final {
+    if (op->op.same_as(builtin::undef())) {
+      has_undef_ = true;
+    }
+    StmtExprVisitor::VisitExpr_(op);
+  }
+
+  bool has_undef_{false};
+
+  std::unordered_set<const VarNode*> var_bindings_with_undef_;
+  std::unordered_set<const BufferStoreNode*> undef_stores_;
+};
+
+// Remove any BufferStores whose value depends on T.undef
+class StoreUndefRemover : public StmtExprMutator {
+ public:
+  static Stmt Apply(Stmt stmt) {
+    auto to_remove = StoreUndefLocator::Locate(stmt);
+    StoreUndefRemover mutator(to_remove);
+    return mutator(std::move(stmt));
+  }
+
+ private:
+  using Parent = StmtExprMutator;
+
+  explicit StoreUndefRemover(const std::unordered_set<const BufferStoreNode*>& to_remove)
+      : to_remove_(to_remove) {}
+
+  Stmt VisitStmt_(const BufferStoreNode* op) final {
+    if (to_remove_.count(op)) {
+      return Evaluate(0);
+    } else {
+      return Parent::VisitStmt_(op);
+    }
+  }
+
+  const std::unordered_set<const BufferStoreNode*>& to_remove_;
+};
+
+// Remove any BufferStores whose value depends on T.undef
+class ContainsUndefChecker : public StmtExprVisitor {
+ public:
+  static bool Check(const Stmt& stmt) {
+    ContainsUndefChecker checker;
+    checker(stmt);
+    return checker.contains_undef;
+  }
+
+ private:
+  void VisitExpr_(const CallNode* op) final {
+    if (op->op.same_as(builtin::undef())) {
+      contains_undef = true;
+    }
+    StmtExprVisitor::VisitExpr_(op);
+  }
+
+  bool contains_undef{false};
+};
+
+namespace transform {
+Pass RemoveStoreUndefInternal() {
+  auto pass_func = [](PrimFunc f, IRModule m, PassContext ctx) {
+    auto* n = f.CopyOnWrite();
+    n->body = StoreUndefRemover::Apply(std::move(n->body));
+    return f;
+  };
+  return CreatePrimFuncPass(pass_func, 0, "tir.RemoveStoreUndefInternal", {});
+}
+
+Pass ValidateAllUndefRemoved() {
+  auto pass_func = [](PrimFunc f, IRModule m, PassContext ctx) {
+    bool contains_undef = ContainsUndefChecker::Check(f->body);
+    ICHECK(!contains_undef) << "Expected removal of BufferStore containing builtin::undef() "
+                            << "to remove all instances of builtin::undef().  "
+                            << "Instead, result was"
+                            << "\n"
+                            << f;
+    return f;
+  };
+  return CreatePrimFuncPass(pass_func, 0, "tir.ValidateAllUndefRemoved", {});
+}
+
+Pass RemoveStoreUndef() {
+  return Sequential({RemoveStoreUndefInternal(), RemoveNoOp(), ValidateAllUndefRemoved()},
+                    "tir.RemoveStoreUndef");
+}
+
+TVM_REGISTER_GLOBAL("tir.transform.RemoveStoreUndef").set_body_typed(RemoveStoreUndef);
+
+}  // namespace transform
+
+}  // namespace tir
+}  // namespace tvm

tests/python/unittest/test_tir_transform_remove_undef.py

@@ -0,0 +1,94 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+import tvm
+import tvm.testing
+from tvm.script import tir as T
+from tvm import TVMError
+
+
+class BaseBeforeAfter(tvm.testing.CompareBeforeAfter):
+    @tvm.testing.fixture
+    def transform(self):
+        return tvm.tir.transform.RemoveStoreUndef()
+
+
+class TestRemoveStoreUndef(BaseBeforeAfter):
+    """Remove a store whose value is T.undef()"""
+
+    def before(A: T.Buffer[1, "int32"]):
+        A[0] = T.undef(dtype="int32")
+
+    def expected(A: T.Buffer[1, "int32"]):
+        T.evaluate(0)
+
+
+class TestRemoveStoreUndefExpression(BaseBeforeAfter):
+    """Expressions containing T.undef() are removed"""
+
+    def before(A: T.Buffer[1, "int32"]):
+        A[0] = 1 + T.undef(dtype="int32")
+
+    def expected(A: T.Buffer[1, "int32"]):
+        T.evaluate(0)
+
+
+class TestKeepOtherCallNodes(BaseBeforeAfter):
+    """Expressions containing other CallNodes are not removed"""
+
+    def before(A: T.Buffer[1, "int32"], n: T.int32):
+        A[0] = T.shift_left(n, 1, dtype="int32")
+
+    expected = before
+
+
+class TestRemoveLetUndef(BaseBeforeAfter):
+    """Remove a store whose value is bound to T.undef()"""
+
+    def before(A: T.Buffer[1, "int32"]):
+        val = T.undef(dtype="int32")
+        A[0] = val
+
+    def expected(A: T.Buffer[1, "int32"]):
+        T.evaluate(0)
+
+
+class TestRaiseErrorForUndefAsStoreIndices(BaseBeforeAfter):
+    """Use of T.undef() as buffer indices is an error"""
+
+    def before(A: T.Buffer[1, "int32"]):
+        val = T.undef(dtype="int32")
+        A[val] = 5
+
+    expected = TVMError
+
+
+class TestRaiseErrorForUndefAsLoadIndices(BaseBeforeAfter):
+    """Use of T.undef() as buffer indices is an error
+
+    Even though this occurs as part of the BufferStore's value, the
+    T.undef() may not appear in a buffer's indices.
+    """
+
+    def before(A: T.Buffer[1, "int32"], B: T.Buffer[1, "int32"]):
+        B[0] = A[T.undef(dtype="int32")]
+
+    expected = TVMError
+
+
+if __name__ == "__main__":
+    tvm.testing.main()