[llvm] [ValueTracking] isNonZero trunc of sub of ptr2int's with recursive GEP where pointers are limited to a 32bit alloc. (PR #84933)

[via llvm-commits]

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@@ -2644,6 +2649,81 @@ static bool isKnownNonZeroFromOperator(const Operator *I,
     // ext X != 0 if X != 0.
     return isKnownNonZero(I->getOperand(0), Q, Depth);
 
+  case Instruction::Trunc:
+    // trunc(operand) is KnownNonZero if lower order bits of operand is a
+    // KnownNonZero. Handle scenarios like trunc i64 (sub(ptr2int, ptr2int)) to
+    // i32 / trunc i64 (lshr(sub(ptr2int, ptr2int))) to i32, where the ptrs are
+    // from same object and one of the pointers is a recursive GEP. And either
+    // the objectSize is known OR is indicative via a compiler flag, which
+    // suggests objectSize<4G.
+    Value *A, *B;
+    if (match(I->getOperand(0), m_Sub(m_PtrToIntSameSize(Q.DL, m_Value(A)),
+                                      m_PtrToIntSameSize(Q.DL, m_Value(B)))) ||
+        match(I->getOperand(0),
+              m_LShr(m_Sub(m_PtrToIntSameSize(Q.DL, m_Value(A)),
+                           m_PtrToIntSameSize(Q.DL, m_Value(B))),
+                     m_ConstantInt()))) {
+      // Check for a specific pattern where A is recursive GEP with a PHI ptr
+      // with incoming values as (Start,Step) and Start==B.
+      auto *GEPA = dyn_cast<GEPOperator>(A);
+      if (!GEPA) {
+        GEPA = dyn_cast<GEPOperator>(B);
+        std::swap(A, B);
+      }
+      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;
+      Value *Step = const_cast<Value *>(A);
+      if (PN->getIncomingValue(0) == Step)
+        Start = PN->getIncomingValue(1);
+      else if (PN->getIncomingValue(1) == Step)
+        Start = PN->getIncomingValue(0);
+      else
+        return false;
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goldsteinn wrote:

The return false should probably be `break` as we might be able to get this w/ `computeKnownBits`.

https://github.com/llvm/llvm-project/pull/84933