Merge pull request #84760 from eeckstein/fix-sil-combine

SILCombine: be  careful about deleting trivially dead `destructure_struct` instructions

Author: eeckstein

lib/SILOptimizer/Utils/InstOptUtils.cpp

@@ -179,6 +179,14 @@ bool swift::isInstructionTriviallyDead(SILInstruction *inst) {
   if (isa<BorrowedFromInst>(inst))
     return false;
 
+  // A dead `destructure_struct` with an owned argument can appear for a non-copyable or
+  // non-escapable struct which has only trivial elements. The instruction is not trivially
+  // dead because it ends the lifetime of its operand.
+  if (isa<DestructureStructInst>(inst) &&
+      inst->getOperand(0)->getOwnershipKind() == OwnershipKind::Owned) {
+    return false;
+  }
+
   // These invalidate enums so "write" memory, but that is not an essential
   // operation so we can remove these if they are trivially dead.
   if (isa<UncheckedTakeEnumDataAddrInst>(inst))

test/SILOptimizer/sil_combine_moveonly.sil

@@ -11,6 +11,11 @@ struct S : ~Copyable {
   deinit {}
 }
 
+struct S2 : ~Copyable {
+  @_hasStorage var a: Int { get set }
+  @_hasStorage var b: Int { get set }
+}
+
 struct FileDescriptor: ~Copyable {
   var fd: Builtin.Int64
 
@@ -169,3 +174,14 @@ bb0:
   %13 = tuple ()
   return %13
 }
+
+// CHECK-LABEL: sil [ossa] @dont_remove_destructure_struct_of_non_copyable :
+// CHECK:         destructure_struct
+// CHECK-LABEL: } // end sil function 'dont_remove_destructure_struct_of_non_copyable'
+sil [ossa] @dont_remove_destructure_struct_of_non_copyable : $@convention(method) (@owned S2) -> () {
+bb0(%0 : @owned $S2):
+  (%1, %2) = destructure_struct %0
+  %r = tuple ()
+  return %r
+}
+