[llvm] [SCEV][LV] Invalidate LCSSA exit phis more thoroughly (PR #69909)

[Nikita Popov via llvm-commits]

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@@ -8408,6 +8408,35 @@ void ScalarEvolution::forgetValue(Value *V) {
   forgetMemoizedResults(ToForget);
 }
 
+void ScalarEvolution::forgetLcssaPhiWithNewPredecessor(PHINode *V) {
+  // If SCEV looked through a trivial LCSSA phi node, we might have SCEV's
+  // directly using a SCEVUnknown defined in the loop. After an extra
+  // predecessor is added, this is no longer valid. Find all SCEVUnknown's
+  // defined in the loop and invalidate any SCEV's making use of them.
+  if (isSCEVable(V->getType())) {
+    if (const SCEV *S = getExistingSCEV(V)) {
+      struct SCEVUnknownCollector {
+        SmallVector<const SCEV *, 8> Unknowns;
+
+        bool follow(const SCEV *S) {
+          // This could be restricted to unknowns defined in the loop.
+          if (isa<SCEVUnknown>(S))
+            Unknowns.push_back(S);
+          return true;
+        }
+        bool isDone() const { return false; }
+      };
+
+      SCEVUnknownCollector C;
+      visitAll(S, C);
+      forgetMemoizedResults(C.Unknowns);
+    }
+  }
+
+  // Also perform the normal invalidation.
+  forgetValue(V);
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nikic wrote:

Yes. forgetValue() will skip non-SCEVable itself. I've converted this into an early exit for the whole function.

https://github.com/llvm/llvm-project/pull/69909