[llvm] [InstCombinePHI] Enhance PHI CSE to remove redundant phis (PR #163453)

[Nikita Popov via llvm-commits]

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@@ -4866,6 +4866,89 @@ 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) !=
----------------
nikic wrote:

To clarify, I'm referring to `PN.getIncomingValueForBlock(PredBB)` here. `IdenticalPN.getIncomingValueForBlock(PredBB)` needs `getIncomingValueForBlock()` because it can have different phi argument order.

https://github.com/llvm/llvm-project/pull/163453