[Attributor] Guarantee getAAFor not to update AA in the manifestation…

… stage

If we query an AA with `Attributor::getAAFor` in `AbstractAttribute::manifest`, the AA may be updated.
This patch makes use of the phase flag in Attributor, and handle `getAAFor` behavior according to the flag.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D86635

llvm/include/llvm/Transforms/IPO/Attributor.h

@@ -969,14 +969,15 @@ struct Attributor {
   /// attribute. Using this after Attributor started running is restricted to
   /// only the Attributor itself. Initial seeding of AAs can be done via this
   /// function.
+  /// NOTE: ForceUpdate is ignored in any stage other than the update stage.
   template <typename AAType>
   const AAType &getOrCreateAAFor(const IRPosition &IRP,
                                  const AbstractAttribute *QueryingAA = nullptr,
                                  bool TrackDependence = false,
                                  DepClassTy DepClass = DepClassTy::OPTIONAL,
                                  bool ForceUpdate = false) {
     if (AAType *AAPtr = lookupAAFor<AAType>(IRP, QueryingAA, TrackDependence)) {
-      if (ForceUpdate)
+      if (ForceUpdate && Phase == AttributorPhase::UPDATE)
         updateAA(*AAPtr);
       return *AAPtr;
     }
@@ -1020,6 +1021,13 @@ struct Attributor {
       return AA;
     }
 
+    // If this is queried in the manifest stage, we force the AA to indicate
+    // pessimistic fixpoint immediately.
+    if (Phase == AttributorPhase::MANIFEST) {
+      AA.getState().indicatePessimisticFixpoint();
+      return AA;
+    }
+
     // Allow seeded attributes to declare dependencies.
     // Remember the seeding state.
     AttributorPhase OldPhase = Phase;

llvm/lib/Transforms/IPO/Attributor.cpp

@@ -1347,6 +1347,8 @@ ChangeStatus Attributor::run() {
 
 ChangeStatus Attributor::updateAA(AbstractAttribute &AA) {
   TimeTraceScope TimeScope(AA.getName() + "::updateAA");
+  assert(Phase == AttributorPhase::UPDATE &&
+         "We can update AA only in the update stage!");
 
   // Use a new dependence vector for this update.
   DependenceVector DV;