Revert "[InstCombinePHI] Enhance PHI CSE to remove redundant phis" (llvm#164520)

Reverts llvm#163453

Causes crashes, see
llvm#163453 (comment)

Author: aeubanks

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -4866,89 +4866,6 @@ static Value *simplifySelectWithFCmp(Value *Cond, Value *T, Value *F,
   return nullptr;
 }
 
-/// Look for the following pattern and simplify %to_fold to %identicalPhi.
-/// Here %phi, %to_fold and %phi.next perform the same functionality as
-/// %identicalPhi and hence the select instruction %to_fold can be folded
-/// into %identicalPhi.
-///
-/// BB1:
-///   %identicalPhi = phi [ X, %BB0 ], [ %identicalPhi.next, %BB1 ]
-///   %phi = phi [ X, %BB0 ], [ %phi.next, %BB1 ]
-///   ...
-///   %identicalPhi.next = select %cmp, %val, %identicalPhi
-///                      (or select %cmp, %identicalPhi, %val)
-///   %to_fold = select %cmp2, %identicalPhi, %phi
-///   %phi.next = select %cmp, %val, %to_fold
-///             (or select %cmp, %to_fold, %val)
-///
-/// Prove that %phi and %identicalPhi are the same by induction:
-///
-/// Base case: Both %phi and %identicalPhi are equal on entry to the loop.
-/// Inductive case:
-/// Suppose %phi and %identicalPhi are equal at iteration i.
-/// We look at their values at iteration i+1 which are %phi.next and
-/// %identicalPhi.next. They would have become different only when %cmp is
-/// false and the corresponding values %to_fold and %identicalPhi differ
-/// (similar reason for the other "or" case in the bracket).
-///
-/// The only condition when %to_fold and %identicalPh could differ is when %cmp2
-/// is false and %to_fold is %phi, which contradicts our inductive hypothesis
-/// that %phi and %identicalPhi are equal. Thus %phi and %identicalPhi are
-/// always equal at iteration i+1.
-bool isSimplifierIdenticalPHI(PHINode &PN, PHINode &IdenticalPN) {
-  if (PN.getParent() != IdenticalPN.getParent())
-    return false;
-  if (PN.getNumIncomingValues() != 2)
-    return false;
-
-  // Check that only the backedge incoming value is different.
-  unsigned DiffVals = 0;
-  BasicBlock *DiffValBB = nullptr;
-  for (unsigned i = 0; i < 2; i++) {
-    BasicBlock *PredBB = PN.getIncomingBlock(i);
-    if (PN.getIncomingValueForBlock(PredBB) !=
-        IdenticalPN.getIncomingValueForBlock(PredBB)) {
-      DiffVals++;
-      DiffValBB = PredBB;
-    }
-  }
-  if (DiffVals != 1)
-    return false;
-  // Now check that the backedge incoming values are two select
-  // instructions with the same condition. Either their true
-  // values are the same, or their false values are the same.
-  auto *SI = dyn_cast<SelectInst>(PN.getIncomingValueForBlock(DiffValBB));
-  auto *IdenticalSI =
-      dyn_cast<SelectInst>(IdenticalPN.getIncomingValueForBlock(DiffValBB));
-  if (!SI || !IdenticalSI)
-    return false;
-  if (SI->getCondition() != IdenticalSI->getCondition())
-    return false;
-
-  SelectInst *SIOtherVal = nullptr;
-  Value *IdenticalSIOtherVal = nullptr;
-  if (SI->getTrueValue() == IdenticalSI->getTrueValue()) {
-    SIOtherVal = dyn_cast<SelectInst>(SI->getFalseValue());
-    IdenticalSIOtherVal = IdenticalSI->getFalseValue();
-  } else if (SI->getFalseValue() == IdenticalSI->getFalseValue()) {
-    SIOtherVal = dyn_cast<SelectInst>(SI->getTrueValue());
-    IdenticalSIOtherVal = IdenticalSI->getTrueValue();
-  } else {
-    return false;
-  }
-
-  // Now check that the other values in select, i.e., %to_fold and
-  // %identicalPhi, are essentially the same value.
-  if (!SIOtherVal || IdenticalSIOtherVal != &IdenticalPN)
-    return false;
-  if (!(SIOtherVal->getTrueValue() == &IdenticalPN &&
-        SIOtherVal->getFalseValue() == &PN) &&
-      !(SIOtherVal->getTrueValue() == &PN &&
-        SIOtherVal->getFalseValue() == &IdenticalPN))
-    return false;
-  return true;
-}
-
 /// Given operands for a SelectInst, see if we can fold the result.
 /// If not, this returns null.
 static Value *simplifySelectInst(Value *Cond, Value *TrueVal, Value *FalseVal,
@@ -5124,14 +5041,7 @@ static Value *simplifySelectInst(Value *Cond, Value *TrueVal, Value *FalseVal,
   std::optional<bool> Imp = isImpliedByDomCondition(Cond, Q.CxtI, Q.DL);
   if (Imp)
     return *Imp ? TrueVal : FalseVal;
-  // Look for same PHIs in the true and false values.
-  if (auto *TruePHI = dyn_cast<PHINode>(TrueVal))
-    if (auto *FalsePHI = dyn_cast<PHINode>(FalseVal)) {
-      if (isSimplifierIdenticalPHI(*TruePHI, *FalsePHI))
-        return FalseVal;
-      if (isSimplifierIdenticalPHI(*FalsePHI, *TruePHI))
-        return TrueVal;
-    }
+
   return nullptr;
 }