Revert "[SCEV] Replace IsAvailableOnEntry with block disposition"

This reverts commit 103fc0f.
Causes a clang crash in ChromeOS builds.
Testcase provided at D149344.

Author: Manoj Gupta

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -5907,6 +5907,90 @@ const SCEV *ScalarEvolution::createAddRecFromPHI(PHINode *PN) {
   return nullptr;
 }
 
+// Checks if the SCEV S is available at BB.  S is considered available at BB
+// if S can be materialized at BB without introducing a fault.
+static bool IsAvailableOnEntry(const Loop *L, DominatorTree &DT, const SCEV *S,
+                               BasicBlock *BB) {
+  struct CheckAvailable {
+    bool TraversalDone = false;
+    bool Available = true;
+
+    const Loop *L = nullptr;  // The loop BB is in (can be nullptr)
+    BasicBlock *BB = nullptr;
+    DominatorTree &DT;
+
+    CheckAvailable(const Loop *L, BasicBlock *BB, DominatorTree &DT)
+      : L(L), BB(BB), DT(DT) {}
+
+    bool setUnavailable() {
+      TraversalDone = true;
+      Available = false;
+      return false;
+    }
+
+    bool follow(const SCEV *S) {
+      switch (S->getSCEVType()) {
+      case scConstant:
+      case scVScale:
+      case scPtrToInt:
+      case scTruncate:
+      case scZeroExtend:
+      case scSignExtend:
+      case scAddExpr:
+      case scMulExpr:
+      case scUMaxExpr:
+      case scSMaxExpr:
+      case scUMinExpr:
+      case scSMinExpr:
+      case scSequentialUMinExpr:
+        // These expressions are available if their operand(s) is/are.
+        return true;
+
+      case scAddRecExpr: {
+        // We allow add recurrences that are on the loop BB is in, or some
+        // outer loop.  This guarantees availability because the value of the
+        // add recurrence at BB is simply the "current" value of the induction
+        // variable.  We can relax this in the future; for instance an add
+        // recurrence on a sibling dominating loop is also available at BB.
+        const auto *ARLoop = cast<SCEVAddRecExpr>(S)->getLoop();
+        if (L && (ARLoop == L || ARLoop->contains(L)))
+          return true;
+
+        return setUnavailable();
+      }
+
+      case scUnknown: {
+        // For SCEVUnknown, we check for simple dominance.
+        const auto *SU = cast<SCEVUnknown>(S);
+        Value *V = SU->getValue();
+
+        if (isa<Argument>(V))
+          return false;
+
+        if (isa<Instruction>(V) && DT.dominates(cast<Instruction>(V), BB))
+          return false;
+
+        return setUnavailable();
+      }
+
+      case scUDivExpr:
+      case scCouldNotCompute:
+        // We do not try to smart about these at all.
+        return setUnavailable();
+      }
+      llvm_unreachable("Unknown SCEV kind!");
+    }
+
+    bool isDone() { return TraversalDone; }
+  };
+
+  CheckAvailable CA(L, BB, DT);
+  SCEVTraversal<CheckAvailable> ST(CA);
+
+  ST.visitAll(S);
+  return CA.Available;
+}
+
 // Try to match a control flow sequence that branches out at BI and merges back
 // at Merge into a "C ? LHS : RHS" select pattern.  Return true on a successful
 // match.
@@ -5944,6 +6028,8 @@ const SCEV *ScalarEvolution::createNodeFromSelectLikePHI(PHINode *PN) {
   auto IsReachable =
       [&](BasicBlock *BB) { return DT.isReachableFromEntry(BB); };
   if (PN->getNumIncomingValues() == 2 && all_of(PN->blocks(), IsReachable)) {
+    const Loop *L = LI.getLoopFor(PN->getParent());
+
     // Try to match
     //
     //  br %cond, label %left, label %right
@@ -5964,8 +6050,8 @@ const SCEV *ScalarEvolution::createNodeFromSelectLikePHI(PHINode *PN) {
 
     if (BI && BI->isConditional() &&
         BrPHIToSelect(DT, BI, PN, Cond, LHS, RHS) &&
-        properlyDominates(getSCEV(LHS), PN->getParent()) &&
-        properlyDominates(getSCEV(RHS), PN->getParent()))
+        IsAvailableOnEntry(L, DT, getSCEV(LHS), PN->getParent()) &&
+        IsAvailableOnEntry(L, DT, getSCEV(RHS), PN->getParent()))
       return createNodeForSelectOrPHI(PN, Cond, LHS, RHS);
   }
 

llvm/test/Analysis/ScalarEvolution/logical-operations.ll

@@ -427,7 +427,7 @@ define ptr @tautological_select_like_phi(i32 %tc) {
 ; CHECK-NEXT:    %iv = phi i32 [ 0, %entry ], [ %iv.next, %latch ]
 ; CHECK-NEXT:    --> {0,+,1}<nuw><nsw><%loop> U: [0,101) S: [0,101) Exits: 100 LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %r = phi ptr [ @constant, %truebb ], [ @another_constant, %falsebb ]
-; CHECK-NEXT:    --> @constant U: [0,-3) S: [-9223372036854775808,9223372036854775805) Exits: @constant LoopDispositions: { %loop: Invariant }
+; CHECK-NEXT:    --> %r U: [0,-3) S: [-9223372036854775808,9223372036854775805) Exits: <<Unknown>> LoopDispositions: { %loop: Variant }
 ; CHECK-NEXT:    %iv.next = add i32 %iv, 1
 ; CHECK-NEXT:    --> {1,+,1}<nuw><nsw><%loop> U: [1,102) S: [1,102) Exits: 101 LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:  Determining loop execution counts for: @tautological_select_like_phi