[SILOptimizer] Don't diagnose infinite recursion if a branch terminates (#19724)

This patch augments the infinite recursion checker to not warn if a
branch terminates, but still warns if a branch calls into something with
`@_semantics("arc.programtermination_point")`. This way, calling `fatalError`
doesn't disqualify you for the diagnostic, but calling `exit` does.

This also removes the warning workaround in the standard library, and
annotates the internal _assertionFailure functions as
`programtermination_point`s, so they get this treatment too.

Author: Harlan

docs/ARCOptimization.rst

@@ -373,7 +373,7 @@ Semantic Tags
 ARC takes advantage of certain semantic tags. This section documents these
 semantics and their meanings.
 
-arc.programtermination_point
+programtermination_point
 ----------------------------
 
 If this semantic tag is applied to a function, then we know that:
@@ -396,7 +396,7 @@ scope's lifetime may never end. This means that:
 
 1. While we can not ignore all such unreachable terminated blocks for ARC
 purposes for instance, if we sink a retain past a br into a non
-arc.programtermination_point block, we must sink the retain into the block.
+programtermination_point block, we must sink the retain into the block.
 
 2. If we are able to infer that an object's lifetime scope would never end due
 to the unreachable/no-return function, then we do not need to end the lifetime

include/swift/SIL/SILBasicBlock.h

@@ -370,9 +370,12 @@ public llvm::ilist_node<SILBasicBlock>, public SILAllocated<SILBasicBlock> {
   /// no-return apply or builtin.
   bool isNoReturn() const;
 
-  /// Returns true if this instruction only contains a branch instruction.
+  /// Returns true if this block only contains a branch instruction.
   bool isTrampoline() const;
 
+  /// Returns true if this block traps without any side effects.
+  bool isProgramTerminationPoint() const;
+
   /// Returns true if it is legal to hoist instructions into this block.
   ///
   /// Used by llvm::LoopInfo.

include/swift/SIL/SILInstruction.h

@@ -6645,6 +6645,9 @@ class TermInst : public NonValueInstruction {
   /// Returns true if this terminator exits the function.
   bool isFunctionExiting() const;
 
+  /// Returns true if this terminator terminates the program.
+  bool isProgramTerminating() const;
+
   TermKind getTermKind() const { return TermKind(getKind()); }
 };
 

include/swift/SILOptimizer/Analysis/ARCAnalysis.h

@@ -424,9 +424,6 @@ class ReleaseTracker {
 /// FinalRelease.
 bool getFinalReleasesForValue(SILValue Value, ReleaseTracker &Tracker);
 
-/// Match a call to a trap BB with no ARC relevant side effects.
-bool isARCInertTrapBB(const SILBasicBlock *BB);
-
 /// Get the two result values of the builtin "unsafeGuaranteed" instruction.
 ///
 /// Gets the (GuaranteedValue, Token) tuple from a call to "unsafeGuaranteed"

include/swift/SILOptimizer/Analysis/ProgramTerminationAnalysis.h

@@ -39,7 +39,7 @@ class ProgramTerminationFunctionInfo {
 public:
   ProgramTerminationFunctionInfo(const SILFunction *F) {
     for (const auto &BB : *F) {
-      if (!isARCInertTrapBB(&BB))
+      if (!BB.isProgramTerminationPoint())
         continue;
       ProgramTerminatingBlocks.insert(&BB);
     }

include/swift/Strings.h

@@ -77,8 +77,8 @@ constexpr static const char BUILTIN_TYPE_NAME_VEC[] = "Builtin.Vec";
 constexpr static const char BUILTIN_TYPE_NAME_SILTOKEN[] = "Builtin.SILToken";
 /// The name of the Builtin type for Word
 constexpr static const char BUILTIN_TYPE_NAME_WORD[] = "Builtin.Word";
-constexpr static StringLiteral SEMANTICS_ARC_PROGRAMTERMINATION_POINT =
-    "arc.programtermination_point";
+constexpr static StringLiteral SEMANTICS_PROGRAMTERMINATION_POINT =
+    "programtermination_point";
 } // end namespace swift
 
 #endif // SWIFT_STRINGS_H

lib/SIL/SILBasicBlock.cpp

@@ -24,6 +24,7 @@
 #include "swift/SIL/SILFunction.h"
 #include "swift/SIL/SILInstruction.h"
 #include "swift/SIL/SILModule.h"
+#include "swift/Strings.h"
 using namespace swift;
 
 //===----------------------------------------------------------------------===//
@@ -336,6 +337,80 @@ bool SILBasicBlock::isEntry() const {
   return this == &*getParent()->begin();
 }
 
+//===----------------------------------------------------------------------===//
+//                   Program Termination Point Analysis
+//===----------------------------------------------------------------------===//
+
+static bool ignorableApplyInstInUnreachableBlock(const ApplyInst *AI) {
+  const auto *Fn = AI->getReferencedFunction();
+  if (!Fn)
+    return false;
+
+  return Fn->hasSemanticsAttr(SEMANTICS_PROGRAMTERMINATION_POINT);
+}
+
+static bool ignorableBuiltinInstInUnreachableBlock(const BuiltinInst *BI) {
+  const BuiltinInfo &BInfo = BI->getBuiltinInfo();
+  if (BInfo.ID == BuiltinValueKind::CondUnreachable)
+    return true;
+
+  const IntrinsicInfo &IInfo = BI->getIntrinsicInfo();
+  if (IInfo.ID == llvm::Intrinsic::trap)
+    return true;
+
+  return false;
+}
+
+// Match a call to a basic block that's going to terminate
+// with no side effects.
+bool SILBasicBlock::isProgramTerminationPoint() const {
+  // Do a quick check at the beginning to make sure that our terminator is
+  // actually an unreachable. This ensures that in many cases this function will
+  // exit early and quickly.
+  auto II = rbegin();
+  if (!isa<UnreachableInst>(*II))
+    return false;
+
+  auto IE = rend();
+  while (II != IE) {
+    // Ignore any instructions without side effects.
+    if (!II->mayHaveSideEffects()) {
+      ++II;
+      continue;
+    }
+
+    // Ignore cond fail.
+    if (isa<CondFailInst>(*II)) {
+      ++II;
+      continue;
+    }
+
+    // Check for apply insts that we can ignore.
+    if (auto *AI = dyn_cast<ApplyInst>(&*II)) {
+      if (ignorableApplyInstInUnreachableBlock(AI)) {
+        ++II;
+        continue;
+      }
+    }
+
+    // Check for builtins that we can ignore.
+    if (auto *BI = dyn_cast<BuiltinInst>(&*II)) {
+      if (ignorableBuiltinInstInUnreachableBlock(BI)) {
+        ++II;
+        continue;
+      }
+    }
+
+    // If we can't ignore the instruction, return false.
+    return false;
+  }
+
+  // Otherwise, we have an unreachable and every instruction is inert in an
+  // unreachable BB.
+  return true;
+}
+
+
 /// Declared out of line so we can have a declaration of SILArgument.
 PhiArgumentArrayRef SILBasicBlock::getPhiArguments() const {
   return PhiArgumentArrayRef(getArguments(), [](SILArgument *arg) {

lib/SIL/SILInstructions.cpp

@@ -1061,6 +1061,30 @@ bool TermInst::isFunctionExiting() const {
   llvm_unreachable("Unhandled TermKind in switch.");
 }
 
+bool TermInst::isProgramTerminating() const {
+  switch (getTermKind()) {
+  case TermKind::BranchInst:
+  case TermKind::CondBranchInst:
+  case TermKind::SwitchValueInst:
+  case TermKind::SwitchEnumInst:
+  case TermKind::SwitchEnumAddrInst:
+  case TermKind::DynamicMethodBranchInst:
+  case TermKind::CheckedCastBranchInst:
+  case TermKind::CheckedCastValueBranchInst:
+  case TermKind::CheckedCastAddrBranchInst:
+  case TermKind::TryApplyInst:
+  case TermKind::YieldInst:
+  case TermKind::ReturnInst:
+  case TermKind::ThrowInst:
+  case TermKind::UnwindInst:
+    return false;
+  case TermKind::UnreachableInst:
+    return true;
+  }
+
+  llvm_unreachable("Unhandled TermKind in switch.");
+}
+
 TermInst::SuccessorBlockArgumentsListTy
 TermInst::getSuccessorBlockArguments() const {
   function_ref<PhiArgumentArrayRef(const SILSuccessor &)> op;

lib/SILOptimizer/Analysis/ARCAnalysis.cpp

@@ -1087,78 +1087,6 @@ bool swift::getFinalReleasesForValue(SILValue V, ReleaseTracker &Tracker) {
   return true;
 }
 
-//===----------------------------------------------------------------------===//
-//                            Leaking BB Analysis
-//===----------------------------------------------------------------------===//
-
-static bool ignorableApplyInstInUnreachableBlock(const ApplyInst *AI) {
-  const auto *Fn = AI->getReferencedFunction();
-  if (!Fn)
-    return false;
-
-  return Fn->hasSemanticsAttr("arc.programtermination_point");
-}
-
-static bool ignorableBuiltinInstInUnreachableBlock(const BuiltinInst *BI) {
-  const BuiltinInfo &BInfo = BI->getBuiltinInfo();
-  if (BInfo.ID == BuiltinValueKind::CondUnreachable)
-    return true;
-
-  const IntrinsicInfo &IInfo = BI->getIntrinsicInfo();
-  if (IInfo.ID == llvm::Intrinsic::trap)
-    return true;
-
-  return false;
-}
-
-/// Match a call to a trap BB with no ARC relevant side effects.
-bool swift::isARCInertTrapBB(const SILBasicBlock *BB) {
-  // Do a quick check at the beginning to make sure that our terminator is
-  // actually an unreachable. This ensures that in many cases this function will
-  // exit early and quickly.
-  auto II = BB->rbegin();
-  if (!isa<UnreachableInst>(*II))
-    return false;
-
-  auto IE = BB->rend();
-  while (II != IE) {
-    // Ignore any instructions without side effects.
-    if (!II->mayHaveSideEffects()) {
-      ++II;
-      continue;
-    }
-
-    // Ignore cond fail.
-    if (isa<CondFailInst>(*II)) {
-      ++II;
-      continue;
-    }
-
-    // Check for apply insts that we can ignore.
-    if (auto *AI = dyn_cast<ApplyInst>(&*II)) {
-      if (ignorableApplyInstInUnreachableBlock(AI)) {
-        ++II;
-        continue;
-      }
-    }
-
-    // Check for builtins that we can ignore.
-    if (auto *BI = dyn_cast<BuiltinInst>(&*II)) {
-      if (ignorableBuiltinInstInUnreachableBlock(BI)) {
-        ++II;
-        continue;
-      }
-    }
-
-    // If we can't ignore the instruction, return false.
-    return false;
-  }
-
-  // Otherwise, we have an unreachable and every instruction is inert from an
-  // ARC perspective in an unreachable BB.
-  return true;
-}
-
 //===----------------------------------------------------------------------===//
 //             Analysis of builtin "unsafeGuaranteed" instructions
 //===----------------------------------------------------------------------===//

lib/SILOptimizer/Analysis/SideEffectAnalysis.cpp

@@ -17,6 +17,7 @@
 #include "swift/SILOptimizer/Analysis/BasicCalleeAnalysis.h"
 #include "swift/SILOptimizer/Analysis/FunctionOrder.h"
 #include "swift/SILOptimizer/PassManager/PassManager.h"
+#include "swift/Strings.h"
 
 using namespace swift;
 
@@ -321,7 +322,7 @@ FunctionSideEffectFlags *FunctionSideEffects::getEffectsOn(SILValue Addr) {
 // Return true if the given function has defined effects that were successfully
 // recorded in this FunctionSideEffects object.
 bool FunctionSideEffects::setDefinedEffects(SILFunction *F) {
-  if (F->hasSemanticsAttr("arc.programtermination_point")) {
+  if (F->hasSemanticsAttr(SEMANTICS_PROGRAMTERMINATION_POINT)) {
     Traps = true;
     return true;
   }

lib/SILOptimizer/Mandatory/DiagnoseInfiniteRecursion.cpp

@@ -22,10 +22,12 @@
 #include "swift/SIL/CFG.h"
 #include "swift/SIL/SILArgument.h"
 #include "swift/SIL/SILInstruction.h"
+#include "swift/SILOptimizer/Analysis/ARCAnalysis.h"
 #include "swift/SILOptimizer/Analysis/PostOrderAnalysis.h"
 #include "swift/SILOptimizer/PassManager/Passes.h"
 #include "swift/SILOptimizer/PassManager/Transforms.h"
 #include "swift/SILOptimizer/Utils/Devirtualize.h"
+#include "swift/Strings.h"
 #include "llvm/ADT/PostOrderIterator.h"
 #include "llvm/ADT/iterator_range.h"
 #include "llvm/Support/Debug.h"
@@ -109,6 +111,14 @@ static bool hasInfinitelyRecursiveApply(SILFunction *targetFn) {
   while (!workList.empty()) {
     SILBasicBlock *curBlock = workList.pop_back_val();
 
+    // If the block is "program terminating", meaning it traps without relevant
+    // side effects, then skip it.
+    // Note that this check happens _before_ the recursion check. This is
+    // deliberate, as we want to ignore recursive calls inside a program
+    // termination point.
+    if (curBlock->isProgramTerminationPoint())
+      continue;
+
     // We're looking for functions that are recursive on _all_ paths. If this
     // block is recursive, mark that we found recursion and check the next
     // block in the work list.
@@ -119,7 +129,8 @@ static bool hasInfinitelyRecursiveApply(SILFunction *targetFn) {
 
     // If this block doesn't have a recursive call, and it exits the function,
     // then we know the function is not infinitely recursive.
-    if (curBlock->getTerminator()->isFunctionExiting())
+    TermInst *terminator = curBlock->getTerminator();
+    if (terminator->isFunctionExiting() || terminator->isProgramTerminating())
       return false;
 
     // Otherwise, push the successors onto the stack if we haven't visited them.

lib/SILOptimizer/Transforms/ARCCodeMotion.cpp

@@ -1074,7 +1074,7 @@ static void eliminateRetainsPrecedingProgramTerminationPoints(SILFunction *f) {
     if (auto apply = FullApplySite::isa(&*iter)) {
       SILFunction *callee = apply.getCalleeFunction();
       if (!callee ||
-          !callee->hasSemanticsAttr(SEMANTICS_ARC_PROGRAMTERMINATION_POINT)) {
+          !callee->hasSemanticsAttr(SEMANTICS_PROGRAMTERMINATION_POINT)) {
         continue;
       }
     } else {

lib/SILOptimizer/Transforms/SimplifyCFG.cpp

@@ -3515,7 +3515,7 @@ bool SimplifyCFG::simplifyProgramTerminationBlock(SILBasicBlock *BB) {
   // use the analysis is because the CFG is likely to be invalidated right
   // after this pass, o we do not really get the benefit of reusing the
   // computation for the next iteration of the pass.
-  if (!isARCInertTrapBB(BB))
+  if (!BB->isProgramTerminationPoint())
     return false;
 
   // This is going to be the last basic block this program is going to execute