[DebugInfo][LSR] Add more stringent checks on IV selection and salvage

attempts

Prevent the selection of IVs that have a SCEV containing an undef. Also
prevent salvaging attempts for values for which a SCEV could not be
created by ScalarEvolution and have only SCEVUknown.

Reviewed by: Orlando

Differential Revision: https://reviews.llvm.org/D111810

Author: Chris Jackson

llvm/include/llvm/Analysis/ScalarEvolution.h

@@ -537,6 +537,9 @@ class ScalarEvolution {
   /// Notify this ScalarEvolution that \p User directly uses SCEVs in \p Ops.
   void registerUser(const SCEV *User, ArrayRef<const SCEV *> Ops);
 
+  /// Return true if the SCEV expression contains an undef value.
+  bool containsUndefs(const SCEV *S) const;
+
   /// Return a SCEV expression for the full generality of the specified
   /// expression.
   const SCEV *getSCEV(Value *V);

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -12375,7 +12375,7 @@ SCEVAddRecExpr::getPostIncExpr(ScalarEvolution &SE) const {
 }
 
 // Return true when S contains at least an undef value.
-static inline bool containsUndefs(const SCEV *S) {
+bool ScalarEvolution::containsUndefs(const SCEV *S) const {
   return SCEVExprContains(S, [](const SCEV *S) {
     if (const auto *SU = dyn_cast<SCEVUnknown>(S))
       return isa<UndefValue>(SU->getValue());

llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp

@@ -6236,7 +6236,8 @@ DbgRewriteSalvageableDVIs(llvm::Loop *L, ScalarEvolution &SE,
 }
 
 /// Identify and cache salvageable DVI locations and expressions along with the
-/// corresponding SCEV(s). Also ensure that the DVI is not deleted before
+/// corresponding SCEV(s). Also ensure that the DVI is not deleted between
+/// cacheing and salvaging.
 static void
 DbgGatherSalvagableDVI(Loop *L, ScalarEvolution &SE,
                        SmallVector<DVIRecoveryRec, 2> &SalvageableDVISCEVs,
@@ -6257,6 +6258,16 @@ DbgGatherSalvagableDVI(Loop *L, ScalarEvolution &SE,
           !SE.isSCEVable(DVI->getVariableLocationOp(0)->getType()))
         continue;
 
+      // SCEVUnknown wraps an llvm::Value, it does not have a start and stride.
+      // Therefore no translation to DIExpression is performed.
+      const SCEV *S = SE.getSCEV(DVI->getVariableLocationOp(0));
+      if (isa<SCEVUnknown>(S))
+        continue;
+
+      // Avoid wasting resources generating an expression containing undef.
+      if (SE.containsUndefs(S))
+        continue;
+
       SalvageableDVISCEVs.push_back(
           {DVI, DVI->getExpression(), DVI->getRawLocation(),
            SE.getSCEV(DVI->getVariableLocationOp(0))});
@@ -6270,33 +6281,32 @@ DbgGatherSalvagableDVI(Loop *L, ScalarEvolution &SE,
 /// surviving subsequent transforms.
 static llvm::PHINode *GetInductionVariable(const Loop &L, ScalarEvolution &SE,
                                            const LSRInstance &LSR) {
-  // For now, just pick the first IV generated and inserted. Ideally pick an IV
-  // that is unlikely to be optimised away by subsequent transforms.
+
+  auto IsSuitableIV = [&](PHINode *P) {
+    if (!SE.isSCEVable(P->getType()))
+      return false;
+    if (const SCEVAddRecExpr *Rec = dyn_cast<SCEVAddRecExpr>(SE.getSCEV(P)))
+      return Rec->isAffine() && !SE.containsUndefs(SE.getSCEV(P));
+    return false;
+  };
+
+  // For now, just pick the first IV that was generated and inserted by
+  // ScalarEvolution. Ideally pick an IV that is unlikely to be optimised away
+  // by subsequent transforms.
   for (const WeakVH &IV : LSR.getScalarEvolutionIVs()) {
     if (!IV)
       continue;
 
-    assert(isa<PHINode>(&*IV) && "Expected PhI node.");
-    if (SE.isSCEVable((*IV).getType())) {
-      PHINode *Phi = dyn_cast<PHINode>(&*IV);
-      LLVM_DEBUG(dbgs() << "scev-salvage: IV : " << *IV
-                        << "with SCEV: " << *SE.getSCEV(Phi) << "\n");
-      return Phi;
-    }
-  }
+    // There should only be PHI node IVs.
+    PHINode *P = cast<PHINode>(&*IV);
 
-  for (PHINode &Phi : L.getHeader()->phis()) {
-    if (!SE.isSCEVable(Phi.getType()))
-      continue;
-
-    const llvm::SCEV *PhiSCEV = SE.getSCEV(&Phi);
-    if (const llvm::SCEVAddRecExpr *Rec = dyn_cast<SCEVAddRecExpr>(PhiSCEV))
-      if (!Rec->isAffine())
-        continue;
+    if (IsSuitableIV(P))
+      return P;
+  }
 
-    LLVM_DEBUG(dbgs() << "scev-salvage: Selected IV from loop header: " << Phi
-                      << " with SCEV: " << *PhiSCEV << "\n");
-    return &Phi;
+  for (PHINode &P : L.getHeader()->phis()) {
+    if (IsSuitableIV(&P))
+      return &P;
   }
   return nullptr;
 }