SIL: add SILArgument::isErased()

Similar to SILInstruction, it's now possible to check if an argument was erased from a SILBasicBlock.

Author: eeckstein

include/swift/SIL/SILArgument.h

@@ -101,6 +101,14 @@ class SILArgument : public ValueBase {
 
   SILBasicBlock *getParent() const { return parentBlock; }
 
+  /// Returns true if this argument is erased from a basic block.
+  ///
+  /// Note that SILArguments which are erased from a SILBasicBlock are not
+  /// destroyed and freed, but are kept in memory. So it's safe to keep a
+  /// pointer to an erased argument and then at a later time check if its
+  /// erased.
+  bool isErased() const { return !parentBlock; }
+
   SILFunction *getFunction();
   const SILFunction *getFunction() const;
 

include/swift/SIL/SILBasicBlock.h

@@ -308,7 +308,7 @@ public SwiftObjectHeader {
                                     const ValueDecl *D = nullptr);
 
   /// Remove all block arguments.
-  void dropAllArguments() { ArgumentList.clear(); }
+  void dropAllArguments();
 
   //===--------------------------------------------------------------------===//
   // Successors

lib/SIL/IR/SILBasicBlock.cpp

@@ -180,6 +180,7 @@ SILFunctionArgument *SILBasicBlock::replaceFunctionArgument(
 
   SILFunctionArgument *NewArg = new (M) SILFunctionArgument(Ty, Kind, D);
   NewArg->setParent(this);
+  ArgumentList[i]->parentBlock = nullptr;
 
   // TODO: When we switch to malloc/free allocation we'll be leaking memory
   // here.
@@ -203,6 +204,7 @@ SILPhiArgument *SILBasicBlock::replacePhiArgument(unsigned i, SILType Ty,
 
   SILPhiArgument *NewArg = new (M) SILPhiArgument(Ty, Kind, D);
   NewArg->setParent(this);
+  ArgumentList[i]->parentBlock = nullptr;
 
   // TODO: When we switch to malloc/free allocation we'll be leaking memory
   // here.
@@ -257,9 +259,17 @@ SILPhiArgument *SILBasicBlock::insertPhiArgument(unsigned AtArgPos, SILType Ty,
   return arg;
 }
 
+void SILBasicBlock::dropAllArguments() {
+  for (SILArgument *arg : ArgumentList) {
+    arg->parentBlock = nullptr;
+  }
+  ArgumentList.clear();
+}
+
 void SILBasicBlock::eraseArgument(int Index) {
   assert(getArgument(Index)->use_empty() &&
          "Erasing block argument that has uses!");
+  ArgumentList[Index]->parentBlock = nullptr;
   ArgumentList.erase(ArgumentList.begin() + Index);
 }