[semantic-arc] Attempt to handle indirect_mutating parameters that are never written to.

It only handles local writes, so we can not handle chains of inout parameters.
That being said, I wonder if we could hook this up to an analysis to determine
if the inout has this same property in callees.

Author: gottesmm

Diff for: lib/SILOptimizer/Mandatory/SemanticARCOpts.cpp

@@ -17,6 +17,7 @@
 #include "swift/SIL/DebugUtils.h"
 #include "swift/SIL/MemAccessUtils.h"
 #include "swift/SIL/OwnershipUtils.h"
+#include "swift/SIL/Projection.h"
 #include "swift/SIL/SILArgument.h"
 #include "swift/SIL/SILBuilder.h"
 #include "swift/SIL/SILInstruction.h"
@@ -150,6 +151,129 @@ LiveRange::LiveRange(SILValue value)
   }
 }
 
+//===----------------------------------------------------------------------===//
+//                        Address Written To Analysis
+//===----------------------------------------------------------------------===//
+
+namespace {
+
+/// A simple analysis that checks if a specific def (in our case an inout
+/// argument) is ever written to. This is conservative, local and processes
+/// recursively downwards from def->use.
+struct IsAddressWrittenToDefUseAnalysis {
+  llvm::SmallDenseMap<SILValue, bool, 8> isWrittenToCache;
+
+  bool operator()(SILValue value) {
+    auto iter = isWrittenToCache.try_emplace(value, true);
+
+    // If we are already in the map, just return that.
+    if (!iter.second)
+      return iter.first->second;
+
+    // Otherwise, compute our value, cache it and return.
+    bool result = isWrittenToHelper(value);
+    iter.first->second = result;
+    return result;
+  }
+
+private:
+  bool isWrittenToHelper(SILValue value);
+};
+
+} // end anonymous namespace
+
+bool IsAddressWrittenToDefUseAnalysis::isWrittenToHelper(SILValue initialValue) {
+  SmallVector<Operand *, 8> worklist(initialValue->getUses());
+  while (!worklist.empty()) {
+    auto *op = worklist.pop_back_val();
+    SILInstruction *user = op->getUser();
+
+    if (Projection::isAddressProjection(user) ||
+        isa<ProjectBlockStorageInst>(user)) {
+      for (SILValue r : user->getResults()) {
+        llvm::copy(r->getUses(), std::back_inserter(worklist));
+      }
+      continue;
+    }
+
+    if (auto *oeai = dyn_cast<OpenExistentialAddrInst>(user)) {
+      // Mutable access!
+      if (oeai->getAccessKind() != OpenedExistentialAccess::Immutable) {
+        return true;
+      }
+
+      //  Otherwise, look through it and continue.
+      llvm::copy(oeai->getUses(), std::back_inserter(worklist));
+      continue;
+    }
+
+    // load_borrow and incidental uses are fine as well.
+    if (isa<LoadBorrowInst>(user) || isIncidentalUse(user)) {
+      continue;
+    }
+
+    // Look through immutable begin_access.
+    if (auto *bai = dyn_cast<BeginAccessInst>(user)) {
+      // If we do not have a read, return true.
+      if (bai->getAccessKind() != SILAccessKind::Read) {
+        return true;
+      }
+
+      // Otherwise, add the users to the worklist and continue.
+      llvm::copy(bai->getUses(), std::back_inserter(worklist));
+      continue;
+    }
+
+    // As long as we do not have a load [take], we are fine.
+    if (auto *li = dyn_cast<LoadInst>(user)) {
+      if (li->getOwnershipQualifier() == LoadOwnershipQualifier::Take) {
+        return true;
+      }
+      continue;
+    }
+
+    // If we have a FullApplySite, see if we use the value as an
+    // indirect_guaranteed parameter. If we use it as inout, we need
+    // interprocedural analysis that we do not perform here.
+    if (auto fas = FullApplySite::isa(user)) {
+      if (fas.getArgumentConvention(*op) ==
+          SILArgumentConvention::Indirect_In_Guaranteed)
+        continue;
+
+      // Otherwise, be conservative and return true.
+      return true;
+    }
+
+    // Copy addr that read are just loads.
+    if (auto *cai = dyn_cast<CopyAddrInst>(user)) {
+      // If our value is the destination, this is a write.
+      if (cai->getDest() == op->get()) {
+        return true;
+      }
+
+      // Ok, so we are Src by process of elimination. Make sure we are not being
+      // taken.
+      if (cai->isTakeOfSrc()) {
+        return true;
+      }
+
+      // Otherwise, we are safe and can continue.
+      continue;
+    }
+
+    // If we did not recognize the user, just return conservatively that it was
+    // written to.
+    LLVM_DEBUG(llvm::dbgs()
+               << "Function: " << user->getFunction()->getName() << "\n");
+    LLVM_DEBUG(llvm::dbgs() << "Value: " << op->get());
+    LLVM_DEBUG(llvm::dbgs() << "Unknown instruction!: " << *user);
+    return true;
+  }
+
+  // Ok, we finished our worklist and this address is not being written to.
+  return false;
+}
+
 //===----------------------------------------------------------------------===//
 //                               Implementation
 //===----------------------------------------------------------------------===//
@@ -185,6 +309,7 @@ struct SemanticARCOptVisitor
   SILFunction &F;
   Optional<DeadEndBlocks> TheDeadEndBlocks;
   ValueLifetimeAnalysis::Frontier lifetimeFrontier;
+  IsAddressWrittenToDefUseAnalysis isAddressWrittenToDefUseAnalysis;
 
   explicit SemanticARCOptVisitor(SILFunction &F) : F(F) {}
 
@@ -712,12 +837,15 @@ class StorageGuaranteesLoadVisitor
       return answer(false);
     }
 
+    // If we have an inout parameter that isn't ever actually written to, return
+    // false.
+    if (arg->getKnownParameterInfo().isIndirectMutating()) {
+      return answer(ARCOpt.isAddressWrittenToDefUseAnalysis(arg));
+    }
+
     // TODO: This should be extended:
     //
-    // 1. We should be able to analyze inout arguments and see if the inout
-    //    argument is never actually written to in a flow insensitive way.
-    //
-    // 2. We should be able to analyze in arguments and see if they are only
+    // 1. We should be able to analyze in arguments and see if they are only
     //    ever destroyed at the end of the function. In such a case, we may be
     //    able to also to promote load [copy] from such args to load_borrow.
     return answer(true);

Diff for: test/SILOptimizer/semantic-arc-opts.sil

@@ -1085,3 +1085,58 @@ bb0(%0 : @owned $Klass):
   %9999 = tuple()
   return %9999 : $()
 }
+
+// CHECK-LABEL: sil [ossa] @inout_argument_never_written_to_1 : $@convention(thin) (@inout NativeObjectPair) -> () {
+// CHECK-NOT: load [copy]
+// CHECK: load_borrow
+// CHECK-NOT: load [copy]
+// CHECK: } // end sil function 'inout_argument_never_written_to_1'
+sil [ossa] @inout_argument_never_written_to_1 : $@convention(thin) (@inout NativeObjectPair) -> () {
+bb0(%0 : $*NativeObjectPair):
+  %2 = load [copy] %0 : $*NativeObjectPair
+  (%3, %4) = destructure_struct %2 : $NativeObjectPair
+
+  %5 = function_ref @guaranteed_user : $@convention(thin) (@guaranteed Builtin.NativeObject) -> ()
+  apply %5(%3) : $@convention(thin) (@guaranteed Builtin.NativeObject) -> ()
+  apply %5(%4) : $@convention(thin) (@guaranteed Builtin.NativeObject) -> ()
+
+  destroy_value %3 : $Builtin.NativeObject
+  destroy_value %4 : $Builtin.NativeObject
+
+  %9999 = tuple()
+  return %9999 : $()
+}
+
+// CHECK-LABEL: sil [ossa] @inout_argument_never_written_to_2 : $@convention(thin) (@inout NativeObjectPair) -> () {
+// CHECK-NOT: load [copy]
+// CHECK: load_borrow
+// CHECK-NOT: load [copy]
+// CHECK: } // end sil function 'inout_argument_never_written_to_2'
+sil [ossa] @inout_argument_never_written_to_2 : $@convention(thin) (@inout NativeObjectPair) -> () {
+bb0(%0 : $*NativeObjectPair):
+  %2 = struct_element_addr %0 : $*NativeObjectPair, #NativeObjectPair.obj1
+  %3 = load [copy] %2 : $*Builtin.NativeObject
+  %5 = function_ref @guaranteed_user : $@convention(thin) (@guaranteed Builtin.NativeObject) -> ()
+  apply %5(%3) : $@convention(thin) (@guaranteed Builtin.NativeObject) -> ()
+  destroy_value %3 : $Builtin.NativeObject
+  %9999 = tuple()
+  return %9999 : $()
+}
+
+// We should be able to handle this, but we can not until we teach the ARC
+// optimizer to avoid writes not within the load [copy] region.
+//
+// CHECK-LABEL: sil [ossa] @inout_argument_never_written_to_3 : $@convention(thin) (@inout NativeObjectPair, @owned Builtin.NativeObject) -> () {
+// CHECK: load [copy]
+// CHECK: } // end sil function 'inout_argument_never_written_to_3'
+sil [ossa] @inout_argument_never_written_to_3 : $@convention(thin) (@inout NativeObjectPair, @owned Builtin.NativeObject) -> () {
+bb0(%0 : $*NativeObjectPair, %1 : @owned $Builtin.NativeObject):
+  %2 = struct_element_addr %0 : $*NativeObjectPair, #NativeObjectPair.obj1
+  %3 = load [copy] %2 : $*Builtin.NativeObject
+  %5 = function_ref @guaranteed_user : $@convention(thin) (@guaranteed Builtin.NativeObject) -> ()
+  apply %5(%3) : $@convention(thin) (@guaranteed Builtin.NativeObject) -> ()
+  destroy_value %3 : $Builtin.NativeObject
+  store %1 to [assign] %2 : $*Builtin.NativeObject
+  %9999 = tuple()
+  return %9999 : $()
+}