Add a Block::dropAllReferences to drop all references from held instructions and call it when clearing the block. This fixes a bug where ForInst/IfInst instructions may still have references to values while being destroyed.

PiperOrigin-RevId: 229207798

mlir/include/mlir/IR/Block.h

@@ -40,6 +40,9 @@ public:
   ~Block();
 
   void clear() {
+    // Drop all references from within this block.
+    dropAllReferences();
+
     // Clear instructions in the reverse order so that uses are destroyed
     // before their defs.
     while (!empty())
@@ -162,6 +165,11 @@ public:
         const_cast<Instruction *>(&inst));
   }
 
+  /// This drops all operand uses from instructions within this block, which is
+  /// an essential step in breaking cyclic dependences between references when
+  /// they are to be deleted.
+  void dropAllReferences();
+
   //===--------------------------------------------------------------------===//
   // Terminator management
   //===--------------------------------------------------------------------===//

mlir/lib/IR/Block.cpp

@@ -87,6 +87,14 @@ Instruction *Block::findAncestorInstInBlock(Instruction *inst) {
   return currInst;
 }
 
+/// This drops all operand uses from instructions within this block, which is
+/// an essential step in breaking cyclic dependences between references when
+/// they are to be deleted.
+void Block::dropAllReferences() {
+  for (Instruction &i : *this)
+    i.dropAllReferences();
+}
+
 //===----------------------------------------------------------------------===//
 // Argument list management.
 //===----------------------------------------------------------------------===//

mlir/lib/IR/Function.cpp

@@ -37,8 +37,7 @@ Function::~Function() {
   // Instructions may have cyclic references, which need to be dropped before we
   // can start deleting them.
   for (auto &bb : *this)
-    for (auto &inst : bb)
-      inst.dropAllReferences();
+    bb.dropAllReferences();
 
   // Clean up function attributes referring to this function.
   FunctionAttr::dropFunctionReference(this);