[opt] dead thin to thick in mand combine

lib/SILOptimizer/Mandatory/MandatoryCombine.cpp

@@ -27,6 +27,7 @@
 #define DEBUG_TYPE "sil-mandatory-combiner"
 #include "swift/Basic/LLVM.h"
 #include "swift/Basic/STLExtras.h"
+#include "swift/SIL/InstructionUtils.h"
 #include "swift/SIL/SILInstructionWorklist.h"
 #include "swift/SIL/SILVisitor.h"
 #include "swift/SILOptimizer/PassManager/Passes.h"
@@ -129,6 +130,7 @@ class MandatoryCombiner final
   /// Base visitor that does not do anything.
   SILInstruction *visitSILInstruction(SILInstruction *) { return nullptr; }
   SILInstruction *visitApplyInst(ApplyInst *instruction);
+  SILInstruction *visitThinToThickFunctionInst(ThinToThickFunctionInst *i);
 };
 
 } // end anonymous namespace
@@ -225,6 +227,17 @@ bool MandatoryCombiner::doOneIteration(SILFunction &function,
   return madeChange;
 }
 
+template <class InstT> static FunctionRefInst *getRemovableRef(InstT *i) {
+  // If the only use of the function_ref is us, then remove it.
+  auto funcRef = dyn_cast<FunctionRefInst>(i->getCallee());
+  if (funcRef &&
+      (funcRef->use_empty() ||
+       (funcRef->getSingleUse() && funcRef->getSingleUse()->getUser() == i))) {
+    return funcRef;
+  }
+  return nullptr;
+}
+
 //===----------------------------------------------------------------------===//
 //                     MandatoryCombiner Visitor Methods
 //===----------------------------------------------------------------------===//
@@ -287,6 +300,19 @@ SILInstruction *MandatoryCombiner::visitApplyInst(ApplyInst *instruction) {
   return nullptr;
 }
 
+/// Try to remove thing to thick instructions that are no longer used
+SILInstruction *
+MandatoryCombiner::visitThinToThickFunctionInst(ThinToThickFunctionInst *i) {
+  auto *ref = getRemovableRef(i);
+  if (tryDeleteDeadClosure(i, instModCallbacks, /*needKeepArgsAlive=*/false)) {
+    if (ref) {
+      instModCallbacks.deleteInst(ref);
+    }
+  }
+
+  return nullptr;
+}
+
 //===----------------------------------------------------------------------===//
 //                            Top Level Entrypoint
 //===----------------------------------------------------------------------===//

test/SILOptimizer/mandatory_combine_closure_cleanup.sil

@@ -0,0 +1,32 @@
+// RUN: %target-sil-opt -enable-objc-interop -enable-sil-verify-all %s -mandatory-combine | %FileCheck %s
+
+import Builtin
+
+func test()
+
+sil [ossa] @closure : $@convention(thin) (Builtin.Int32) -> Builtin.Int32
+
+// CHECK-LABEL: remove_dead_thin_to_thick
+// CHECK: bb0
+// TODO: one of these will be removed by #30429.
+// CHECK-NEXT: tuple
+// CHECK-NEXT: tuple
+// CHECK-NEXT: return
+// CHECK-LABEL: end sil function 'remove_dead_thin_to_thick'
+sil hidden [ossa] @remove_dead_thin_to_thick : $@convention(thin) () -> () {
+bb0:
+  %0 = function_ref @closure : $@convention(thin) (Builtin.Int32) -> Builtin.Int32
+  %1 = thin_to_thick_function %0 : $@convention(thin) (Builtin.Int32) -> Builtin.Int32 to $@callee_guaranteed (Builtin.Int32) -> Builtin.Int32
+  debug_value %1 : $@callee_guaranteed (Builtin.Int32) -> Builtin.Int32, let, name "c"
+
+  %3 = begin_borrow %1 : $@callee_guaranteed (Builtin.Int32) -> Builtin.Int32
+  %4 = copy_value %3 : $@callee_guaranteed (Builtin.Int32) -> Builtin.Int32
+  // This is all we got from the closure so we should be able to remove *everything* in this function.
+  %5 = tuple ()
+  destroy_value %4 : $@callee_guaranteed (Builtin.Int32) -> Builtin.Int32
+  end_borrow %3 : $@callee_guaranteed (Builtin.Int32) -> Builtin.Int32
+  destroy_value %1 : $@callee_guaranteed (Builtin.Int32) -> Builtin.Int32
+
+  %9 = tuple ()
+  return %9 : $()
+} // end sil function 'remove_dead_thin_to_thick'