LifetimeDependenceDefUseAddressWalker: avoid infinite recursion.

This utility is used by DependentAddressUseDefWalker which now conservatively
follows all possible uses. This could result in the same address being reached
multiple times during a def-use walk. Ensure that we don't infinitely recurse.

There is no small test case for this, but the fix is trivial and standard
practice for such walkers, and this is hit quickly in real usage, so there is no
danger of it regressing.

Fixes rdar://150403948 ([nonescapable] Infinite recursion compiler crash in
lifetime dependence checking)

Author: atrick

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/LifetimeDependenceInsertion.swift

@@ -424,6 +424,10 @@ struct VariableIntroducerUseDefWalker : LifetimeDependenceUseDefValueWalker, Lif
     visitedValues.insert(value)
   }
 
+  mutating func needWalk(for address: Value) -> Bool {
+    visitedValues.insert(address)
+  }
+
   mutating func walkUp(newLifetime: Value) -> WalkResult {
     if newLifetime.type.isAddress {
       return walkUp(address: newLifetime)

SwiftCompilerSources/Sources/Optimizer/Utilities/LifetimeDependenceUtils.swift

@@ -817,7 +817,10 @@ extension LifetimeDependenceDefUseWalker {
   }
 
   private mutating func walkDownAddressUses(of address: Value) -> WalkResult {
-    address.uses.ignoreTypeDependence.walk {
+    if !needWalk(for: address) {
+      return .continueWalk
+    }
+    return address.uses.ignoreTypeDependence.walk {
       return classifyAddress(operand: $0)
     }
   }
@@ -1199,6 +1202,8 @@ protocol LifetimeDependenceUseDefAddressWalker {
   // ignored.
   var isTrivialScope: Bool { get }
 
+  mutating func needWalk(for address: Value) -> Bool
+
   mutating func addressIntroducer(_ address: Value, access: AccessBaseAndScopes) -> WalkResult
 
   // The 'newLifetime' value is not forwarded to its uses. It may be a non-address or an address.
@@ -1221,6 +1226,9 @@ extension LifetimeDependenceUseDefAddressWalker {
   }
 
   mutating func walkUpDefault(address: Value, access: AccessBaseAndScopes) -> WalkResult {
+    if !needWalk(for: address) {
+      return .continueWalk
+    }
     if let beginAccess = access.innermostAccess {
       // Skip the access scope for unsafe[Mutable]Address. Treat it like a projection of 'self' rather than a separate
       // variable access.
@@ -1333,6 +1341,10 @@ struct LifetimeDependenceRootWalker : LifetimeDependenceUseDefValueWalker, Lifet
     visitedValues.insert(value)
   }
 
+  mutating func needWalk(for address: Value) -> Bool {
+    visitedValues.insert(address)
+  }
+
   mutating func walkUp(newLifetime: Value) -> WalkResult {
     if newLifetime.type.isAddress {
       return walkUp(address: newLifetime)