[Attributor] Allow AAs to iterate on their own state

Future AAs might need to iterate their own state until they reach a
fixpoint. We do not want to forbid that but we want to avoid negative
effects or bugs once this happens. As a precaution, we now rerun an AA
that did not require outside information. If it does not change anymore
we are done, otherwise the AA needs to iterate some more.

diff --git a/llvm/lib/Transforms/IPO/Attributor.cpp b/llvm/lib/Transforms/IPO/Attributor.cpp
index 50facdb..416f554 100644 (file)
--- a/llvm/lib/Transforms/IPO/Attributor.cpp
+++ b/llvm/lib/Transforms/IPO/Attributor.cpp
@@ -2411,10 +2411,20 @@ ChangeStatus Attributor::updateAA(AbstractAttribute &AA) {
                      /* CheckBBLivenessOnly */ true))
     CS = AA.update(*this);
 
-  if (!AA.isQueryAA() && DV.empty()) {
-    // If the attribute did not query any non-fix information, the state
-    // will not change and we can indicate that right away.
-    AAState.indicateOptimisticFixpoint();
+  if (!AA.isQueryAA() && DV.empty() && !AA.getState().isAtFixpoint()) {
+    // If the AA did not rely on outside information but changed, we run it
+    // again to see if it found a fixpoint. Most AAs do but we don't require
+    // them to. Hence, it might take the AA multiple iterations to get to a
+    // fixpoint even if it does not rely on outside information, which is fine.
+    ChangeStatus RerunCS = ChangeStatus::UNCHANGED;
+    if (CS == ChangeStatus::CHANGED)
+      RerunCS = AA.update(*this);
+
+    // If the attribute did not change during the run or rerun, and it still did
+    // not query any non-fix information, the state will not change and we can
+    // indicate that right at this point.
+    if (RerunCS == ChangeStatus::UNCHANGED && !AA.isQueryAA() && DV.empty())
+      AAState.indicateOptimisticFixpoint();
   }
 
   if (!AAState.isAtFixpoint())