[llvm] [ValueTracking] isNonEqual Pointers with with a recursive GEP (PR #70459)

[Nikita Popov via llvm-commits]

================
@@ -3141,6 +3141,60 @@ static bool isNonEqualSelect(const Value *V1, const Value *V2, unsigned Depth,
          isKnownNonEqual(SI1->getFalseValue(), V2, Depth + 1, Q);
 }
 
+// Check to see if A is both a GEP and is the incoming value for a PHI in the
+// loop, and B is either a ptr or another GEP. If the PHI has 2 incoming values,
+// one of them being the recursive GEP A and the other a ptr at same base and at
+// the same/higher offset than B we are only incrementing the pointer further in
+// loop if offset of recursive GEP is greater than 0.
+static bool isNonEqualPointersWithRecursiveGEP(const Value *A, const Value *B,
+                                               unsigned Depth,
+                                               const SimplifyQuery &Q) {
+  if (!A->getType()->isPointerTy() || !B->getType()->isPointerTy())
+    return false;
+
+  auto *GEPA = dyn_cast<GEPOperator>(A);
+  if (!GEPA || GEPA->getNumIndices() != 1 || !isa<Constant>(GEPA->idx_begin()))
+    return false;
+
+  // Handle 2 incoming PHI values with one being a recursive GEP.
+  auto *PN = dyn_cast<PHINode>(GEPA->getPointerOperand());
+  if (!PN || PN->getNumIncomingValues() != 2)
+    return false;
+
+  // Search for the recursive GEP as an incoming operand, and record that as
+  // Step.
+  Value *Start = nullptr;
+  auto *Step = A;
+  if (PN->getIncomingValue(0) == Step)
+    Start = PN->getIncomingValue(1);
+  else if (PN->getIncomingValue(1) == Step)
+    Start = PN->getIncomingValue(0);
+  else
+    return false;
+
+  // Other incoming node base should match the B base.
+  // StartOffset >= OffsetB && StepOffset > 0?
+  // StartOffset <= OffsetB && StepOffset < 0?
+  // Is non-equal if above are true.
+  // We use stripAndAccumulateInBoundsConstantOffsets to restrict the
+  // optimisation to inbounds GEPs only.
+  APInt StartOffset(Q.DL.getIndexTypeSizeInBits(Start->getType()), 0);
+  Start = Start->stripAndAccumulateInBoundsConstantOffsets(Q.DL, StartOffset);
+  APInt StepOffset(Q.DL.getIndexTypeSizeInBits(Step->getType()), 0);
+  Step = Step->stripAndAccumulateInBoundsConstantOffsets(Q.DL, StepOffset);
+
+  // Check if Base Pointer of Step matches the PHI.
+  if (Step != PN)
+    return false;
+  APInt OffsetB(Q.DL.getIndexTypeSizeInBits(B->getType()), 0);
+  B = B->stripAndAccumulateInBoundsConstantOffsets(Q.DL, OffsetB);
+  if (Start == B && ((StartOffset.sge(OffsetB) && StepOffset.sgt(0)) ||
----------------
nikic wrote:

```suggestion
  if (Start == B && ((StartOffset.sge(OffsetB) && StepOffset.isStrictlyPositive())) ||
```

https://github.com/llvm/llvm-project/pull/70459