SILCombine: remove unneeded reference counting instructions from global_value

Delete all reference count instructions on a global_value if the only other uses are projections (ref_element_addr and ref_tail_addr).

Author: eeckstein

lib/SILOptimizer/SILCombiner/SILCombiner.h

@@ -301,6 +301,7 @@ class SILCombiner :
 
   SILInstruction *visitMarkDependenceInst(MarkDependenceInst *MDI);
   SILInstruction *visitClassifyBridgeObjectInst(ClassifyBridgeObjectInst *CBOI);
+  SILInstruction *visitGlobalValueInst(GlobalValueInst *globalValue);
   SILInstruction *visitConvertFunctionInst(ConvertFunctionInst *CFI);
   SILInstruction *
   visitConvertEscapeToNoEscapeInst(ConvertEscapeToNoEscapeInst *Cvt);

lib/SILOptimizer/SILCombiner/SILCombinerMiscVisitors.cpp

@@ -2283,3 +2283,40 @@ SILCombiner::visitClassifyBridgeObjectInst(ClassifyBridgeObjectInst *cboi) {
 
   return nullptr;
 }
+
+/// Returns true if reference counting and debug_value users of a global_value
+/// can be deleted.
+static bool checkGlobalValueUsers(SILValue val,
+                                  SmallVectorImpl<SILInstruction *> &toDelete) {
+  for (Operand *use : val->getUses()) {
+    SILInstruction *user = use->getUser();
+    if (isa<RefCountingInst>(user) || isa<DebugValueInst>(user)) {
+      toDelete.push_back(user);
+      continue;
+    }
+    if (auto *upCast = dyn_cast<UpcastInst>(user)) {
+      if (!checkGlobalValueUsers(upCast, toDelete))
+        return false;
+      continue;
+    }
+    // Projection instructions don't access the object header, so they don't
+    // prevent deleting reference counting instructions.
+    if (isa<RefElementAddrInst>(user) || isa<RefTailAddrInst>(user))
+      continue;
+    return false;
+  }
+  return true;
+}
+
+SILInstruction *
+SILCombiner::visitGlobalValueInst(GlobalValueInst *globalValue) {
+  // Delete all reference count instructions on a global_value if the only other
+  // users are projections (ref_element_addr and ref_tail_addr).
+  SmallVector<SILInstruction *, 8> toDelete;
+  if (!checkGlobalValueUsers(globalValue, toDelete))
+    return nullptr;
+  for (SILInstruction *inst : toDelete) {
+    eraseInstFromFunction(*inst);
+  }
+  return nullptr;
+}

test/SILOptimizer/sil_combine.sil

@@ -4203,4 +4203,26 @@ bb1(%result : $Builtin.NativeObject):
 
 bb2(%error : $Error):
   unreachable
-}
+}
+
+sil_global private @outlined_global : $_ContiguousArrayStorage<Int>
+
+// CHECK-LABEL: sil @test_global_value
+// CHECK-NOT:   retain
+// CHECK-NOT:   release
+// CHECK-NOT:   debug_value
+// CHECK:     } // end sil function 'test_global_value'
+sil @test_global_value : $@convention(thin) () -> Int {
+bb0:
+  %0 = global_value @outlined_global : $_ContiguousArrayStorage<Int>
+  strong_retain %0 : $_ContiguousArrayStorage<Int>
+  debug_value %0 : $_ContiguousArrayStorage<Int>, let, name "x"
+  %2 = upcast %0 : $_ContiguousArrayStorage<Int> to $__ContiguousArrayStorageBase
+  strong_retain %2 : $__ContiguousArrayStorageBase
+  %13 = ref_tail_addr [immutable] %2 : $__ContiguousArrayStorageBase, $Int
+  %16 = load %13 : $*Int
+  strong_release %2 : $__ContiguousArrayStorageBase
+  strong_release %0 : $_ContiguousArrayStorage<Int>
+  return %16 : $Int
+} // end sil function 'test_global_value'
+