[InstCombinePHI] Enhance PHI CSE to remove redundant phis

Enhanced PHI CSE to eliminate redundant PHIs, which could clean up
the IR and open up opportunities for other passes such as loop
vectorization.

Author: CongzheUalberta

llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp

@@ -1621,11 +1621,90 @@ Instruction *InstCombinerImpl::visitPHINode(PHINode &PN) {
     // Note that even though we've just canonicalized this PHI, due to the
     // worklist visitation order, there are no guarantess that *every* PHI
     // has been canonicalized, so we can't just compare operands ranges.
-    if (!PN.isIdenticalToWhenDefined(&IdenticalPN))
-      continue;
-    // Just use that PHI instead then.
-    ++NumPHICSEs;
-    return replaceInstUsesWith(PN, &IdenticalPN);
+    if (PN.isIdenticalToWhenDefined(&IdenticalPN)) {
+      // Just use that PHI instead then.
+      ++NumPHICSEs;
+      return replaceInstUsesWith(PN, &IdenticalPN);
+    }
+
+    // Look for the following pattern and do PHI CSE to clean up the
+    // redundant %phi. Here %phi, %1 and %phi.next perform the same
+    // functionality as %identicalPhi and hence %phi can be eliminated.
+    //
+    // BB1:
+    //   %identicalPhi = phi [ X, %BB0 ], [ %identicalPhi.next, %BB1 ]
+    //   %phi = phi [ X, %BB0 ], [ %phi.next, %BB1 ]
+    //   ...
+    //   %identicalPhi.next = select %cmp, %val, %identicalPhi
+    //   %1 = select %cmp2, %identicalPhi, float %phi
+    //   %phi.next = select %cmp, %val, %1
+    //
+    // 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 %1 and %identicalPhi differ.
+    //
+    // The only condition when %1 and %identicalPh could differ is when %cmp2
+    // is false and %1 is %phi, which contradicts our inductive hypothesis
+    // that %phi and %identicalPhi are equal. Thus %phi and %identicalPhi are
+    // always equal at iteration i+1.
+
+    if (PN.getNumIncomingValues() == 2 && PN.getNumUses() == 1) {
+      unsigned diffVals = 0;
+      unsigned diffValIdx = 0;
+      // Check that only the backedge incoming value is different.
+      for (unsigned i = 0; i < 2; i++) {
+        if (PN.getIncomingValue(i) != IdenticalPN.getIncomingValue(i)) {
+          diffVals++;
+          diffValIdx = i;
+        }
+      }
+      BasicBlock *CurBB = PN.getParent();
+      if (diffVals == 2 || PN.getIncomingBlock(diffValIdx) != CurBB)
+        continue;
+      // Now check that the backedge incoming values are two select
+      // instructions that are in the same BB, and have the same condition,
+      // true value.
+      auto *Val = PN.getIncomingValue(diffValIdx);
+      auto *IdenticalVal = IdenticalPN.getIncomingValue(diffValIdx);
+      if (!isa<SelectInst>(Val) || !isa<SelectInst>(IdenticalVal))
+        continue;
+
+      auto *SI = cast<SelectInst>(Val);
+      auto *IdenticalSI = cast<SelectInst>(IdenticalVal);
+      if (SI->getParent() != CurBB || IdenticalSI->getParent() != CurBB)
+        continue;
+      if (SI->getCondition() != IdenticalSI->getCondition() ||
+          SI->getTrueValue() != IdenticalSI->getTrueValue())
+        continue;
+
+      // Now check that the false values, i.e., %1 and %identicalPhi,
+      // are essentially the same value within the same BB.
+      auto SameSelAndPhi = [&](SelectInst *SI, PHINode *IdenticalPN,
+                               PHINode *PN) {
+        if (SI->getTrueValue() == IdenticalPN) {
+          return SI->getFalseValue() == PN;
+        }
+        return false;
+      };
+      auto *FalseVal = SI->getFalseValue();
+      auto *IdenticalSIFalseVal =
+          dyn_cast<PHINode>(IdenticalSI->getFalseValue());
+      if (!isa<SelectInst>(FalseVal) || !IdenticalSIFalseVal ||
+          IdenticalSIFalseVal != &IdenticalPN)
+        continue;
+      auto *FalseValSI = cast<SelectInst>(FalseVal);
+      if (FalseValSI->getParent() != CurBB ||
+          !SameSelAndPhi(FalseValSI, &IdenticalPN, &PN))
+        continue;
+
+      ++NumPHICSEs;
+      return replaceInstUsesWith(PN, &IdenticalPN);
+    }
   }
 
   // If this is an integer PHI and we know that it has an illegal type, see if