Allow DCE of load [copy]

Author: meg-gupta

lib/SILOptimizer/Transforms/DeadCodeElimination.cpp

@@ -43,21 +43,18 @@ namespace {
 // FIXME: Reconcile the similarities between this and
 //        isInstructionTriviallyDead.
 static bool seemsUseful(SILInstruction *I) {
-  // begin_access is defined to have side effects, but this is not relevant for
-  // DCE.
-  if (isa<BeginAccessInst>(I))
+  // Even though begin_access/begin_borrow/destroy_value/copy_value have
+  // side-effects, they can be DCE'ed if they do not have useful
+  // dependencies/reverse dependencies
+  if (isa<BeginAccessInst>(I) || isa<BeginBorrowInst>(I) ||
+      isa<CopyValueInst>(I) || isa<DestroyValueInst>(I))
     return false;
 
-  // Even though begin_borrow/destroy_value/copy_value have side-effects, they
-  // can be DCE'ed if they do not have useful dependencies/reverse dependencies
-  if (isa<BeginBorrowInst>(I))
-    return false;
-
-  if (isa<DestroyValueInst>(I))
-    return false;
-
-  if (isa<CopyValueInst>(I))
-    return false;
+  // A load [copy] is okay to be DCE'ed if there are no useful dependencies
+  if (auto *load = dyn_cast<LoadInst>(I)) {
+    if (load->getOwnershipQualifier() == LoadOwnershipQualifier::Copy)
+      return false;
+  }
 
   if (I->mayHaveSideEffects())
     return true;

test/SILOptimizer/dead_code_elimination_nontrivial_ossa.sil

@@ -218,6 +218,21 @@ bb3(%2 : @owned $Klass):
   return %res : $()
 }
 
+// CHECK-LABEL: sil [ossa] @dce_deadcopy7 :
+// CHECK-NOT: load [copy]
+// CHECK-LABEL: } // end sil function 'dce_deadcopy7'
+sil [ossa] @dce_deadcopy7 : $@convention(thin) (@in Klass) -> () {
+bb0(%0 : $*Klass):
+  %1 = load [copy] %0 : $*Klass
+  br bb1(%1 : $Klass)
+
+bb1(%2 : @owned $Klass):
+  destroy_value %2 : $Klass
+  destroy_addr %0 : $*Klass
+  %res = tuple ()
+  return %res : $()
+}
+
 // CHECK-LABEL: sil [ossa] @dce_destructure1 :
 // CHECK: bb0(%0 : {{.*}})
 // CHECK-NEXT: destroy_value %0