[Const evaluator] Enable stepwise constant evaluator to skip

instructions without evaluating them while conservatively accounting
for the effects of the skipped instructions on the interpreter state.

Author: ravikandhadai

include/swift/AST/DiagnosticsSIL.def

@@ -395,6 +395,13 @@ NOTE(constexpr_witness_call_with_no_target_found, none,
 NOTE(constexpr_witness_call_found_here, none,
     "witness method call found here", ())
 
+NOTE(constexpr_unknown_control_flow_due_to_skip,none, "branch depends on "
+     "non-constant value produced by an unevaluated instructions", ())
+NOTE(constexpr_returned_by_unevaluated_instruction,none,
+     "return value of an unevaluated instruction is not a constant", ())
+NOTE(constexpr_mutated_by_unevaluated_instruction,none, "value mutable by an "
+    "unevaluated instruction is not a constant", ())
+
 ERROR(non_physical_addressof,none,
       "addressof only works with purely physical lvalues; "
       "use `withUnsafePointer` or `withUnsafeBytes` unless you're implementing "

include/swift/SIL/SILConstants.h

@@ -90,6 +90,14 @@ enum class UnknownReason {
   /// A top-level value has multiple writers. This is only relevant in the
   /// non-flow-sensitive evaluation mode,  which is used by #assert.
   MutipleTopLevelWriters,
+
+  /// Indicates the return value of an instruction that was not evaluated during
+  /// interpretation.
+  ReturnedByUnevaluatedInstruction,
+
+  /// Indicates that the value was possibly modified by an instruction
+  /// that was not evaluated during the interpretation.
+  MutatedByUnevaluatedInstruction,
 };
 
 /// An abstract class that exposes functions for allocating symbolic values.
@@ -416,6 +424,8 @@ class SymbolicValue {
   /// reason, we fall back to using the specified location.
   void emitUnknownDiagnosticNotes(SILLocation fallbackLoc);
 
+  bool isUnknownDueToUnevaluatedInstructions();
+
   /// Clone this SymbolicValue into the specified Allocator and return the new
   /// version. This only works for valid constants.
   SymbolicValue cloneInto(SymbolicValueAllocator &allocator) const;

include/swift/SILOptimizer/Utils/ConstExpr.h

@@ -85,6 +85,8 @@ class ConstExprEvaluator {
 
 /// A constant-expression evaluator that can be used to step through a control
 /// flow graph (SILFunction body) by evaluating one instruction at a time.
+/// This evaluator can also "skip" instructions without evaluating them and
+/// only track constant values of variables whose values could be computed.
 class ConstExprStepEvaluator {
 private:
   ConstExprEvaluator evaluator;
@@ -97,10 +99,6 @@ class ConstExprStepEvaluator {
   /// evaluation.
   SmallPtrSet<SILBasicBlock *, 8> visitedBlocks;
 
-  Optional<SymbolicValue>
-  incrementStepsAndCheckLimit(SILInstruction *inst,
-                              bool includeInInstructionLimit);
-
   ConstExprStepEvaluator(const ConstExprEvaluator &) = delete;
   void operator=(const ConstExprEvaluator &) = delete;
 
@@ -122,12 +120,61 @@ class ConstExprStepEvaluator {
   ///   Second element is None, if the evaluation is successful.
   ///   Otherwise, is an unknown symbolic value that contains the error.
   std::pair<Optional<SILBasicBlock::iterator>, Optional<SymbolicValue>>
-  evaluate(SILBasicBlock::iterator instI,
-           bool includeInInstructionLimit = true);
+  evaluate(SILBasicBlock::iterator instI);
+
+  /// Skip the instruction without evaluating it and conservatively account for
+  /// the effects of the instruction on the internal state. This operation
+  /// resets to an unknown symbolic value any portion of a
+  /// SymbolicValueMemoryObject that could possibly be mutated by the given
+  /// instruction. This function preserves the soundness of the interpretation.
+  /// \param instI instruction to be skipped.
+  /// \returns a pair where the first and second elements are defined as
+  /// follows:
+  ///   The first element, if is not None, is the iterator to the next
+  ///   instruction from the where the evaluation must continue.
+  ///   The first element is None if the next instruction from where the
+  ///   evaluation must continue cannot be determined.
+  ///   This would be the case if `instI` is a branch like a `condbr`.
+  ///
+  ///   Second element is None if skipping the instruction is successful.
+  ///   Otherwise, it is an unknown symbolic value containing the error.
+  std::pair<Optional<SILBasicBlock::iterator>, Optional<SymbolicValue>>
+  skipByMakingEffectsNonConstant(SILBasicBlock::iterator instI);
+
+  /// Try evaluating an instruction and if the evaluation fails, skip the
+  /// instruction and make it effects non constant. Note that it may not always
+  /// be possible to skip an instruction whose evaluation failed and
+  /// continue evalution (e.g. a conditional branch).
+  /// See `evaluate` and `skipByMakingEffectsNonConstant` functions for their
+  /// semantics.
+  /// \param instI instruction to be evaluated in the current interpreter state.
+  /// \returns a pair where the first and second elements are defined as
+  /// follows:
+  ///   The first element, if is not None, is the iterator to the next
+  ///   instruction from the where the evaluation must continue.
+  ///   The first element is None iff both `evaluate` and `skip` functions
+  ///   failed to determine the next instruction to continue evaluation from.
+  ///
+  ///   Second element is None if the evaluation is successful.
+  ///   Otherwise, it is an unknown symbolic value containing the error.
+  std::pair<Optional<SILBasicBlock::iterator>, Optional<SymbolicValue>>
+  tryEvaluateOrElseMakeEffectsNonConstant(SILBasicBlock::iterator instI);
 
   Optional<SymbolicValue> lookupConstValue(SILValue value);
 
   bool isKnownFunction(SILFunction *fun);
+
+  /// Returns true if and only if `errorVal` denotes an error that requires
+  /// aborting interpretation and returning the error. Skipping an instruction
+  /// that produces such errors is not a valid behavior.
+  bool isFailStopError(SymbolicValue errorVal);
+
+  /// Return the number of instructions evaluated for the last `evaluate`
+  /// operation. This could be used by the clients to limit the number of
+  /// instructions that should be evaluated by the step-wise evaluator.
+  /// Note that 'skipByMakingEffectsNonConstant' operation is not considered
+  /// as an evaluation.
+  unsigned instructionsEvaluatedByLastEvaluation() { return stepsEvaluated; }
 };
 
 } // end namespace swift

lib/SIL/SILConstants.cpp

@@ -568,6 +568,12 @@ SymbolicValue SymbolicValue::lookThroughSingleElementAggregates() const {
   }
 }
 
+bool SymbolicValue::isUnknownDueToUnevaluatedInstructions() {
+  auto unknownReason = getUnknownReason();
+  return (unknownReason == UnknownReason::ReturnedByUnevaluatedInstruction ||
+          unknownReason == UnknownReason::MutatedByUnevaluatedInstruction);
+}
+
 /// Given that this is an 'Unknown' value, emit diagnostic notes providing
 /// context about what the problem is. Specifically, point to interesting
 /// source locations and function calls in the call stack.
@@ -676,6 +682,12 @@ void SymbolicValue::emitUnknownDiagnosticNotes(SILLocation fallbackLoc) {
     if (emitTriggerLocInDiag)
       diagnose(ctx, *triggerLoc, diag::constexpr_witness_call_found_here);
     return;
+  case UnknownReason::ReturnedByUnevaluatedInstruction:
+    diagnose(ctx, diagLoc, diag::constexpr_returned_by_unevaluated_instruction);
+    break;
+  case UnknownReason::MutatedByUnevaluatedInstruction:
+    diagnose(ctx, diagLoc, diag::constexpr_mutated_by_unevaluated_instruction);
+    break;
   }
   // TODO: print the call-stack in a controlled way if needed.
 }

lib/SILOptimizer/UtilityPasses/ConstantEvaluatorTester.cpp

@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 #define DEBUG_TYPE "sil-constant-evaluation-tester"
+#include "swift/AST/DiagnosticsSIL.h"
 #include "swift/SIL/SILConstants.h"
 #include "swift/SILOptimizer/PassManager/Passes.h"
 #include "swift/SILOptimizer/PassManager/Transforms.h"
@@ -20,6 +21,12 @@ using namespace swift;
 
 namespace {
 
+template <typename... T, typename... U>
+static InFlightDiagnostic diagnose(ASTContext &Context, SourceLoc loc,
+                                   Diag<T...> diag, U &&... args) {
+  return Context.Diags.diagnose(loc, diag, std::forward<U>(args)...);
+}
+
 /// A compiler pass for testing constant evaluator in the step-wise evaluation
 /// mode. The pass evaluates SIL functions whose names start with "interpret"
 /// and outputs the constant value returned by the function or diagnostics if
@@ -31,6 +38,18 @@ class ConstantEvaluatorTester : public SILFunctionTransform {
     return fun->getName().startswith("interpret");
   }
 
+  bool shouldSkipInstruction(SILInstruction *inst) {
+    auto *applyInst = dyn_cast<ApplyInst>(inst);
+    if (!applyInst)
+      return false;
+
+    auto *callee = applyInst->getReferencedFunction();
+    if (!callee)
+      return false;
+
+    return callee->getName().startswith("skip");
+  }
+
   void run() override {
     SILFunction *fun = getFunction();
 
@@ -52,6 +71,7 @@ class ConstantEvaluatorTester : public SILFunctionTransform {
         if (!returnVal) {
           llvm::errs() << "Returns unknown"
                        << "\n";
+          break;
         }
         llvm::errs() << "Returns " << returnVal.getValue() << "\n";
         break;
@@ -60,15 +80,33 @@ class ConstantEvaluatorTester : public SILFunctionTransform {
       Optional<SILBasicBlock::iterator> nextInstOpt;
       Optional<SymbolicValue> errorVal;
 
-      std::tie(nextInstOpt, errorVal) = stepEvaluator.evaluate(currI);
-      if (errorVal.hasValue()) {
-        // Diagnose the error.
-        assert(errorVal->getKind() == SymbolicValue::Unknown);
+      // If the instruction is marked as skip, skip it and make its effects
+      // non-constant. Otherwise, try evaluating the instruction and if the
+      // evaluation fails due to a previously skipped instruction,
+      // skip the current instruction.
+      if (shouldSkipInstruction(inst)) {
+        std::tie(nextInstOpt, errorVal) =
+            stepEvaluator.skipByMakingEffectsNonConstant(currI);
+      } else {
+        std::tie(nextInstOpt, errorVal) =
+            stepEvaluator.tryEvaluateOrElseMakeEffectsNonConstant(currI);
+      }
+
+      // Diagnose errors in the evaluation. Unknown symbolic values produced
+      // by skipping instructions are not considered errors.
+      if (errorVal.hasValue() &&
+          !errorVal->isUnknownDueToUnevaluatedInstructions()) {
+        errorVal->emitUnknownDiagnosticNotes(inst->getLoc());
+        break;
+      }
+
+      if (!nextInstOpt) {
+        diagnose(fun->getASTContext(), inst->getLoc().getSourceLoc(),
+                 diag::constexpr_unknown_control_flow_due_to_skip);
         errorVal->emitUnknownDiagnosticNotes(inst->getLoc());
         break;
       }
 
-      assert(nextInstOpt);
       currI = nextInstOpt.getValue();
     }
   }

lib/SILOptimizer/Utils/ConstExpr.cpp

@@ -278,8 +278,11 @@ SymbolicValue ConstExprFunctionState::computeConstantValue(SILValue value) {
 
     for (unsigned i = 0, e = inst->getNumOperands(); i != e; ++i) {
       auto val = getConstantValue(inst->getOperand(i));
-      if (!val.isConstant())
+      if (!val.isConstant() && !val.isUnknownDueToUnevaluatedInstructions())
         return val;
+      // Unknown values due to unevaluated instructions can be assigned to
+      // struct properties as they are not indicative of a fatal error or
+      // trap.
       elts.push_back(val);
     }
 
@@ -1528,28 +1531,132 @@ ConstExprStepEvaluator::ConstExprStepEvaluator(SymbolicValueAllocator &alloc,
 
 ConstExprStepEvaluator::~ConstExprStepEvaluator() { delete internalState; }
 
-Optional<SymbolicValue> ConstExprStepEvaluator::incrementStepsAndCheckLimit(
-    SILInstruction *inst, bool includeInInstructionLimit) {
-  if (includeInInstructionLimit && ++stepsEvaluated > ConstExprLimit) {
-    // Note that there is no call stack to associate with the unknown value
-    // as the interpreter is at the top-level here.
-    return SymbolicValue::getUnknown(inst, UnknownReason::TooManyInstructions,
-                                     {}, evaluator.getAllocator());
+std::pair<Optional<SILBasicBlock::iterator>, Optional<SymbolicValue>>
+ConstExprStepEvaluator::evaluate(SILBasicBlock::iterator instI) {
+  // Reset `stepsEvaluated` to zero.
+  stepsEvaluated = 0;
+  return internalState->evaluateInstructionAndGetNext(instI, visitedBlocks);
+}
+
+std::pair<Optional<SILBasicBlock::iterator>, Optional<SymbolicValue>>
+ConstExprStepEvaluator::skipByMakingEffectsNonConstant(
+    SILBasicBlock::iterator instI) {
+  SILInstruction *inst = &(*instI);
+
+  // Set all constant state that could be mutated by the instruction
+  // to an unknown symbolic value.
+  for (auto &operand : inst->getAllOperands()) {
+    auto constValOpt = lookupConstValue(operand.get());
+    if (!constValOpt) {
+      continue;
+    }
+    auto constVal = constValOpt.getValue();
+    auto constKind = constVal.getKind();
+
+    // Skip can only be invoked on value types or addresses of value types.
+    // Note that adding a new kind of symbolic value may require handling its
+    // side-effects, especially if that symbolic value does not represent a
+    // value type.
+    assert(constKind == SymbolicValue::Address ||
+           constKind == SymbolicValue::Unknown ||
+           constKind == SymbolicValue::Metatype ||
+           constKind == SymbolicValue::Function ||
+           constKind == SymbolicValue::Integer ||
+           constKind == SymbolicValue::String ||
+           constKind == SymbolicValue::Aggregate ||
+           constKind == SymbolicValue::Enum ||
+           constKind == SymbolicValue::EnumWithPayload ||
+           constKind == SymbolicValue::UninitMemory);
+
+    if (constKind != SymbolicValue::Address) {
+      continue;
+    }
+
+    // If the address is only used @in_guaranteed or @in_constant, there
+    // can be no mutation through this address. Therefore, ignore it.
+    if (ApplyInst *applyInst = dyn_cast<ApplyInst>(inst)) {
+      ApplySite applySite(applyInst);
+      SILArgumentConvention convention =
+        applySite.getArgumentConvention(operand);
+      if (convention == SILArgumentConvention::Indirect_In_Guaranteed ||
+          convention == SILArgumentConvention::Indirect_In_Constant) {
+        continue;
+      }
+    }
+
+    // Write an unknown value into the address.
+    SmallVector<unsigned, 4> accessPath;
+    auto *memoryObject = constVal.getAddressValue(accessPath);
+    auto unknownValue = SymbolicValue::getUnknown(
+        inst, UnknownReason::MutatedByUnevaluatedInstruction, {},
+        evaluator.getAllocator());
+
+    auto memoryContent = memoryObject->getValue();
+    if (memoryContent.getKind() == SymbolicValue::Aggregate) {
+      memoryObject->setIndexedElement(accessPath, unknownValue,
+                                      evaluator.getAllocator());
+    } else {
+      memoryObject->setValue(unknownValue);
+    }
+  }
+
+  // Map the results of this instruction to unknown values.
+  for (auto result : inst->getResults()) {
+    internalState->setValue(
+        result, SymbolicValue::getUnknown(
+                    inst, UnknownReason::ReturnedByUnevaluatedInstruction, {},
+                    evaluator.getAllocator()));
+  }
+
+  // If we have a next instruction in the basic block return it.
+  // Otherwise, return None for the next instruction.
+  // Note that we can find the next instruction in the case of unconditional
+  // branches. But, there is no real need to do that as of now.
+  if (!isa<TermInst>(inst)) {
+    return {++instI, None};
+  }
+  return {None, None};
+}
+
+bool ConstExprStepEvaluator::isFailStopError(SymbolicValue errorVal) {
+  assert(errorVal.isUnknown());
+
+  switch (errorVal.getUnknownReason()) {
+  case UnknownReason::TooManyInstructions:
+  case UnknownReason::Loop:
+  case UnknownReason::Overflow:
+  case UnknownReason::Trap:
+    return true;
+  default:
+    return false;
   }
-  return None;
 }
 
 std::pair<Optional<SILBasicBlock::iterator>, Optional<SymbolicValue>>
-ConstExprStepEvaluator::evaluate(SILBasicBlock::iterator instI,
-                                 bool includeInInstructionLimit) {
-  // Diagnose whether evaluating this instruction exceeds the instruction
-  // limit, unless asked to not include this instruction in the limit.
-  auto limitError =
-      incrementStepsAndCheckLimit(&(*instI), includeInInstructionLimit);
-  if (limitError) {
-    return {None, limitError.getValue()};
+ConstExprStepEvaluator::tryEvaluateOrElseMakeEffectsNonConstant(
+    SILBasicBlock::iterator instI) {
+
+  auto evaluateResult = evaluate(instI);
+  Optional<SILBasicBlock::iterator> nextI = evaluateResult.first;
+  Optional<SymbolicValue> errorVal = evaluateResult.second;
+
+  if (!errorVal) {
+    assert(nextI);
+    return evaluateResult;
   }
-  return internalState->evaluateInstructionAndGetNext(instI, visitedBlocks);
+  assert(!nextI);
+
+  if (isFailStopError(*errorVal)) {
+    return evaluateResult;
+  }
+
+  // Evaluation cannot fail on unconditional branches.
+  assert(!isa<BranchInst>(&(*instI)));
+
+  // Since the evaluation has failed, make the effects of this instruction
+  // unknown.
+  auto result = skipByMakingEffectsNonConstant(instI);
+  return {result.first, errorVal};
 }
 
 Optional<SymbolicValue>