ValueTracking: Do not return nullptr from getUnderlyingObject (llvm#115258)

Fixup for 29a5c05. The failure case
should return the last value found.

Author: arsenm

llvm/lib/Analysis/ValueTracking.cpp

@@ -6723,9 +6723,10 @@ static bool isSameUnderlyingObjectInLoop(const PHINode *PN,
 const Value *llvm::getUnderlyingObject(const Value *V, unsigned MaxLookup) {
   for (unsigned Count = 0; MaxLookup == 0 || Count < MaxLookup; ++Count) {
     if (auto *GEP = dyn_cast<GEPOperator>(V)) {
-      V = GEP->getPointerOperand();
-      if (!V->getType()->isPointerTy()) // Only handle scalar pointer base.
-        return nullptr;
+      const Value *PtrOp = GEP->getPointerOperand();
+      if (!PtrOp->getType()->isPointerTy()) // Only handle scalar pointer base.
+        return V;
+      V = PtrOp;
     } else if (Operator::getOpcode(V) == Instruction::BitCast ||
                Operator::getOpcode(V) == Instruction::AddrSpaceCast) {
       Value *NewV = cast<Operator>(V)->getOperand(0);

llvm/test/Transforms/FunctionAttrs/vector-of-pointers-getunderlyingobject-crash.ll

@@ -0,0 +1,14 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -S -passes=function-attrs < %s | FileCheck %s
+
+define double @getUnderlyingObject_vector_ptr(<4 x i1> %arg0, <4 x i1> %arg1) {
+; CHECK-LABEL: define double @getUnderlyingObject_vector_ptr(
+; CHECK-SAME: <4 x i1> [[ARG0:%.*]], <4 x i1> [[ARG1:%.*]]) #[[ATTR0:[0-9]+]] {
+; CHECK-NEXT:    [[GATHER:%.*]] = tail call <4 x double> @llvm.masked.gather.v4f64.v4p0(<4 x ptr> getelementptr inbounds (i8, <4 x ptr> zeroinitializer, <4 x i64> splat (i64 8)), i32 0, <4 x i1> [[ARG0]], <4 x double> zeroinitializer)
+; CHECK-NEXT:    [[REDUCE_FADD:%.*]] = tail call double @llvm.vector.reduce.fadd.v4f64(double 0.000000e+00, <4 x double> [[GATHER]])
+; CHECK-NEXT:    ret double [[REDUCE_FADD]]
+;
+  %gather = tail call <4 x double> @llvm.masked.gather.v4f64.v4p0(<4 x ptr> getelementptr inbounds (i8, <4 x ptr> zeroinitializer, <4 x i64> <i64 8, i64 8, i64 8, i64 8>), i32 0, <4 x i1> %arg0, <4 x double> zeroinitializer)
+  %reduce.fadd = tail call double @llvm.vector.reduce.fadd.v4f64(double 0.000000e+00, <4 x double> %gather)
+  ret double %reduce.fadd
+}