Merge pull request #8686 from rjmccall/access-enforcement-selection-pass

Author: swift-ci

include/swift/SIL/SILInstruction.h

@@ -1907,6 +1907,17 @@ class BeginAccessInst
   SILValue getSource() const {
     return getOperand();
   }
+
+private:
+    /// Predicate used to filter EndAccessRange.
+  struct UseToEndAccess;
+
+public:
+  using EndAccessRange =
+    OptionalTransformRange<use_range, UseToEndAccess, use_iterator>;
+
+  /// Find all the associated end_access instructions for this begin_access.
+  EndAccessRange getEndAccesses() const;
 };
 
 /// Represents the end of an access scope.
@@ -1944,6 +1955,20 @@ class EndAccessInst : public UnaryInstructionBase<ValueKind::EndAccessInst> {
   }
 };
 
+struct BeginAccessInst::UseToEndAccess {
+  Optional<EndAccessInst *> operator()(Operand *use) const {
+    if (auto access = dyn_cast<EndAccessInst>(use->getUser())) {
+      return access;
+    } else {
+      return None;
+    }
+  }
+};
+
+inline auto BeginAccessInst::getEndAccesses() const -> EndAccessRange {
+  return EndAccessRange(getUses(), UseToEndAccess());
+}
+
 /// AssignInst - Represents an abstract assignment to a memory location, which
 /// may either be an initialization or a store sequence.  This is only valid in
 /// Raw SIL.

include/swift/SILOptimizer/PassManager/Passes.def

@@ -54,6 +54,8 @@ PASS(AADumper, "aa-dump",
      "Dump Alias Analysis over all Pairs")
 PASS(ABCOpt, "abcopts",
      "Array Bounds Check Optimization")
+PASS(AccessEnforcementSelection, "access-enforcement-selection",
+     "Access Enforcement Selection")
 PASS(AccessMarkerElimination, "access-marker-elim",
      "Access Marker Elimination.")
 PASS(AddressLowering, "address-lowering",

lib/SILOptimizer/Mandatory/AccessEnforcementSelection.cpp

@@ -0,0 +1,334 @@
+//===--- AccessEnforcementSelection.cpp - Select access enforcement -------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+///
+/// This pass eliminates 'unknown' access enforcement by selecting either
+/// static or dynamic enforcement.
+/// 
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "access-enforcement-selection"
+#include "swift/SIL/SILFunction.h"
+#include "swift/SILOptimizer/PassManager/Transforms.h"
+
+using namespace swift;
+
+static void setStaticEnforcement(BeginAccessInst *access) {
+  // TODO: delete if we're not using static enforcement?
+  access->setEnforcement(SILAccessEnforcement::Static);
+}
+
+static void setDynamicEnforcement(BeginAccessInst *access) {
+  // TODO: delete if we're not using dynamic enforcement?
+  access->setEnforcement(SILAccessEnforcement::Dynamic);
+}
+
+namespace {
+class SelectEnforcement {
+  AllocBoxInst *Box;
+
+  /// A state for tracking escape information about a variable.
+  /// StateMap only has entries for blocks for which the variable
+  /// has potentially escaped at exit.
+  struct State {
+    bool IsInWorklist = false;
+
+    // At least one of the following must be true.
+    bool HasEscape = false;
+    bool HasPotentiallyEscapedAtEntry = false;
+
+    // In a more advanced problem, this could easily be passed a State.
+    bool adjustForEscapeInPredecessor() {
+      bool updateSuccessors = false;
+
+      if (!HasPotentiallyEscapedAtEntry) {
+        HasPotentiallyEscapedAtEntry = true;
+        updateSuccessors = !HasEscape;
+      }
+
+      return updateSuccessors;
+    }
+  };
+  llvm::DenseMap<SILBasicBlock*, State> StateMap;
+
+  /// All the accesses in the function.
+  SmallVector<BeginAccessInst*, 8> Accesses;
+
+  /// All the escapes in the function.
+  SmallPtrSet<SILInstruction*, 8> Escapes;
+
+  /// A worklist we use for various purposes.
+  SmallVector<SILBasicBlock*, 8> Worklist;
+
+public:
+  SelectEnforcement(AllocBoxInst *box) : Box(box) {}
+
+  void run();
+
+private:
+  void analyzeUsesOfBox();
+  void analyzeProjection(ProjectBoxInst *projection);
+
+  void propagateEscapes();
+  void propagateEscapesFrom(SILBasicBlock *bb);
+
+  bool hasPotentiallyEscapedAt(SILInstruction *inst);
+  bool hasPotentiallyEscapedAtAnyReachableBlock(BeginAccessInst *access);
+
+  void updateAccesses();
+  void updateAccess(BeginAccessInst *access);
+};
+} // end anonymous namespace
+
+void SelectEnforcement::run() {
+  // Set up the data-flow problem.
+  analyzeUsesOfBox();
+
+  // Run the data-flow problem.
+  propagateEscapes();
+
+  // Update all the accesses.
+  updateAccesses();
+}
+
+void SelectEnforcement::analyzeUsesOfBox() {
+  // Collect accesses rooted off of projections.
+  for (auto use : Box->getUses()) {
+    auto user = use->getUser();
+
+    if (auto projection = dyn_cast<ProjectBoxInst>(user)) {
+      analyzeProjection(projection);
+      continue;
+    }
+      
+    // Ignore certain other uses that do not capture the value.
+    if (isa<StrongRetainInst>(user) ||
+        isa<StrongReleaseInst>(user) ||
+        isa<DestroyValueInst>(user) ||
+        isa<DeallocBoxInst>(user))
+      continue;
+
+    // Treat everything else as an escape:
+
+    // Add it to the escapes set.
+    Escapes.insert(user);
+
+    // 
+    auto userBB = user->getParent();
+    auto &state = StateMap[userBB];
+    if (!state.IsInWorklist) {
+      state.HasEscape = true;
+      state.IsInWorklist = true;
+      Worklist.push_back(userBB);
+    }
+    assert(state.HasEscape);
+    assert(state.IsInWorklist);
+  }
+
+  assert(!Accesses.empty() && "didn't find original access!");
+}
+
+void SelectEnforcement::analyzeProjection(ProjectBoxInst *projection) {
+  for (auto use : projection->getUses()) {
+    auto user = use->getUser();
+
+    // Collect accesses.
+    if (auto access = dyn_cast<BeginAccessInst>(user)) {
+      if (access->getEnforcement() == SILAccessEnforcement::Unknown)
+        Accesses.push_back(access);
+    }
+  }
+}
+
+void SelectEnforcement::propagateEscapes() {
+  while (!Worklist.empty()) {
+    auto bb = Worklist.pop_back_val();
+    auto it = StateMap.find(bb);
+    assert(it != StateMap.end() &&
+           "block was in worklist but doesn't have a tracking state");
+    auto &state = it->second;
+    assert(state.HasEscape || state.HasPotentiallyEscapedAtEntry);
+    state.IsInWorklist = false;
+    propagateEscapesFrom(bb);
+  }
+}
+
+/// Given that the box potentially escaped before we exited the
+/// given block, propagate that information to all of its successors.
+void SelectEnforcement::propagateEscapesFrom(SILBasicBlock *bb) {
+  assert(StateMap.count(bb));
+
+  // Iterate over the successors of the block.
+  for (SILBasicBlock *succ : bb->getSuccessors()) {
+    auto &succState = StateMap[succ];
+
+    // If updating the successor changes it in a way that will
+    // require us to update its successors, add it to the worklist.
+    if (succState.adjustForEscapeInPredecessor()) {
+      if (!succState.IsInWorklist) {
+        succState.IsInWorklist = true;
+        Worklist.push_back(succ);
+      }
+    }
+  }
+}
+
+bool SelectEnforcement::hasPotentiallyEscapedAt(SILInstruction *point) {
+  auto bb = point->getParent();
+
+  // If we're not tracking anything for the whole block containing
+  // the instruction, we're done; it hasn't escaped here.
+  auto it = StateMap.find(bb);
+  if (it == StateMap.end())
+    return false;
+
+  // If the tracking information says there are escapes before entry,
+  // we're done; it has potentially escaped.
+  const auto &state = it->second;
+  if (state.HasPotentiallyEscapedAtEntry)
+    return true;
+
+  // Okay, there must be an escape within this block.
+  assert(state.HasEscape);
+  for (auto ii = point->getIterator(), ie = bb->begin(); ii != ie; ) {
+    auto inst = &*--ii;
+
+    // Maybe just record the first escape in the block and see if we
+    // come after it?
+    if (Escapes.count(inst))
+      return true;
+  }
+
+  return false;
+}
+
+bool SelectEnforcement::hasPotentiallyEscapedAtAnyReachableBlock(
+                                                      BeginAccessInst *access) {
+  // Fast path: we're not tracking any escapes.  (But the box should
+  // probably have been promoted to the stack in this case.)
+  if (StateMap.empty())
+    return false;
+
+  auto bb = access->getParent();
+
+  assert(Worklist.empty());
+  SmallPtrSet<SILBasicBlock*, 8> visited;
+  visited.insert(bb);
+  Worklist.push_back(bb);
+
+  while (!Worklist.empty()) {
+    bb = Worklist.pop_back_val();
+    assert(visited.count(bb));
+
+    // If we're tracking information for this block, there's an escape.
+    if (StateMap.count(bb))
+      return true;
+
+    // Add all reachable successors.
+    for (SILBasicBlock *succ : bb->getSuccessors()) {
+      if (visited.insert(succ).second)
+        Worklist.push_back(succ);
+    }
+  }
+
+  // No reachable block has an escape.
+  return false;
+}
+
+void SelectEnforcement::updateAccesses() {
+  for (auto access : Accesses) {
+    updateAccess(access);
+  }
+}
+
+void SelectEnforcement::updateAccess(BeginAccessInst *access) {
+  assert(access->getEnforcement() == SILAccessEnforcement::Unknown);
+
+  // Check whether the variable escaped before any of the end_accesses.
+  bool anyDynamic = false;
+  bool hasEndAccess = false;
+  for (auto endAccess : access->getEndAccesses()) {
+    hasEndAccess = true;
+    if (hasPotentiallyEscapedAt(endAccess)) {
+      anyDynamic = true;
+      break;
+    }
+  }
+
+  // If so, make the access dynamic.
+  if (anyDynamic)
+    return setDynamicEnforcement(access);
+
+  // Otherwise, if there are no end_access instructions,
+  // check the terminators of every reachable block.
+  if (!hasEndAccess) {
+    if (hasPotentiallyEscapedAtAnyReachableBlock(access))
+      return setDynamicEnforcement(access);
+  }
+
+  // Otherwise, use static enforcement.
+  setStaticEnforcement(access);
+}
+
+namespace {
+
+/// The pass.
+struct AccessEnforcementSelection : SILFunctionTransform {
+  void run() override {
+    for (auto &bb : *getFunction()) {
+      for (auto ii = bb.begin(), ie = bb.end(); ii != ie; ) {
+        SILInstruction *inst = &*ii;
+        ++ii;
+
+        if (auto access = dyn_cast<BeginAccessInst>(inst)) {
+          handleAccess(access);
+        }
+      }
+    }
+  }
+
+  void handleAccess(BeginAccessInst *access) {
+    if (access->getEnforcement() != SILAccessEnforcement::Unknown)
+      return;
+
+    auto address = access->getOperand();
+    assert(!isa<MarkUninitializedInst>(address) &&
+           "pass should be run after definite initialization");
+
+    if (auto box = dyn_cast<ProjectBoxInst>(address)) {
+      return handleAccessToBox(access, box);
+    }
+
+    // If we're not accessing a box, we must've lowered to
+    // a stack element.
+    assert(isa<AllocStackInst>(address));
+    setStaticEnforcement(access);
+  }
+
+  void handleAccessToBox(BeginAccessInst *access, ProjectBoxInst *projection) {
+    // If we didn't allocate the box, assume that we need to use
+    // dynamic enforcement.
+    // TODO: use static enforcement in certain provable cases.
+    auto box = dyn_cast<AllocBoxInst>(projection->getOperand());
+    if (!box) {
+      setDynamicEnforcement(access);
+      return;
+    }
+
+    SelectEnforcement(box).run();
+  }
+};
+
+} // end anonymous namespace
+
+SILTransform *swift::createAccessEnforcementSelection() {
+  return new AccessEnforcementSelection();
+}

lib/SILOptimizer/Mandatory/CMakeLists.txt

@@ -1,4 +1,5 @@
 set(MANDATORY_SOURCES
+  Mandatory/AccessEnforcementSelection.cpp
   Mandatory/AccessMarkerElimination.cpp
   Mandatory/AddressLowering.cpp
   Mandatory/DefiniteInitialization.cpp