[PATCH] D43997: [SCEV][NFC] Smarter implementation of isAvailableAtLoopEntry

[Sanjoy Das via Phabricator via llvm-commits]

sanjoy requested changes to this revision.
sanjoy added inline comments.
This revision now requires changes to proceed.


================
Comment at: lib/Analysis/ScalarEvolution.cpp:2237
 bool ScalarEvolution::isAvailableAtLoopEntry(const SCEV *S, const Loop *L) {
-  if (!isLoopInvariant(S, L))
-    return false;
-  // If a value depends on a SCEVUnknown which is defined after the loop, we
-  // conservatively assume that we cannot calculate it at the loop's entry.
-  struct FindDominatedSCEVUnknown {
-    bool Found = false;
-    const Loop *L;
-    DominatorTree &DT;
-    LoopInfo &LI;
-
-    FindDominatedSCEVUnknown(const Loop *L, DominatorTree &DT, LoopInfo &LI)
-        : L(L), DT(DT), LI(LI) {}
-
-    bool checkSCEVUnknown(const SCEVUnknown *SU) {
-      if (auto *I = dyn_cast<Instruction>(SU->getValue())) {
-        if (DT.dominates(L->getHeader(), I->getParent()))
-          Found = true;
-        else
-          assert(DT.dominates(I->getParent(), L->getHeader()) &&
-                 "No dominance relationship between SCEV and loop?");
-      }
-      return false;
-    }
-
-    bool follow(const SCEV *S) {
-      switch (static_cast<SCEVTypes>(S->getSCEVType())) {
-      case scConstant:
-        return false;
-      case scAddRecExpr:
-      case scTruncate:
-      case scZeroExtend:
-      case scSignExtend:
-      case scAddExpr:
-      case scMulExpr:
-      case scUMaxExpr:
-      case scSMaxExpr:
-      case scUDivExpr:
-        return true;
-      case scUnknown:
-        return checkSCEVUnknown(cast<SCEVUnknown>(S));
-      case scCouldNotCompute:
-        llvm_unreachable("Attempt to use a SCEVCouldNotCompute object!");
-      }
-      return false;
-    }
-
-    bool isDone() { return Found; }
-  };
-
-  FindDominatedSCEVUnknown FSU(L, DT, LI);
-  SCEVTraversal<FindDominatedSCEVUnknown> ST(FSU);
-  ST.visitAll(S);
-  return !FSU.Found;
+  return isLoopInvariant(S, L) && properlyDominates(S, L->getHeader());
 }
----------------
Looks like `properlyDominates` is not transitive when this used to be transitive?

(It could be that `properlyDominates` is incorrect itself and needs to be transitive too - if yes, let's make that change first.)


https://reviews.llvm.org/D43997