[IR] Don't strip through pointer to vector of pointer bitcasts

When using stripPointerCasts() and getUnderlyingObject(), don't
strip through a bitcast from ptr to <1 x ptr>, which is not a
no-op pointer cast. Calling code is generally not prepared to
handle that situation, resulting in incorrect alias analysis
results for example.

Fixes #97600.

Author: nikic

llvm/lib/Analysis/ValueTracking.cpp

@@ -6403,9 +6403,10 @@ const Value *llvm::getUnderlyingObject(const Value *V, unsigned MaxLookup) {
       V = GEP->getPointerOperand();
     } else if (Operator::getOpcode(V) == Instruction::BitCast ||
                Operator::getOpcode(V) == Instruction::AddrSpaceCast) {
-      V = cast<Operator>(V)->getOperand(0);
-      if (!V->getType()->isPointerTy())
+      Value *NewV = cast<Operator>(V)->getOperand(0);
+      if (!NewV->getType()->isPointerTy())
         return V;
+      V = NewV;
     } else if (auto *GA = dyn_cast<GlobalAlias>(V)) {
       if (GA->isInterposable())
         return V;

llvm/lib/IR/Value.cpp

@@ -652,9 +652,10 @@ static const Value *stripPointerCastsAndOffsets(
       }
       V = GEP->getPointerOperand();
     } else if (Operator::getOpcode(V) == Instruction::BitCast) {
-      V = cast<Operator>(V)->getOperand(0);
-      if (!V->getType()->isPointerTy())
+      Value *NewV = cast<Operator>(V)->getOperand(0);
+      if (!NewV->getType()->isPointerTy())
         return V;
+      V = NewV;
     } else if (StripKind != PSK_ZeroIndicesSameRepresentation &&
                Operator::getOpcode(V) == Instruction::AddrSpaceCast) {
       // TODO: If we know an address space cast will not change the

llvm/test/Analysis/BasicAA/ptr-vector.ll

@@ -0,0 +1,12 @@
+; RUN: opt -print-all-alias-modref-info -passes=aa-eval -disable-output < %s 2>&1 | FileCheck %s
+
+; CHECK: MayAlias:	i8* %b, i8* %p
+; CHECK: Just Ref:  Ptr: i8* %p	<->  %v1p = call <1 x ptr> @llvm.masked.load.v1p0.p0(ptr %a, i32 8, <1 x i1> %c, <1 x ptr> poison)
+; CHECK: Just Ref:  Ptr: i8* %b	<->  %v1p = call <1 x ptr> @llvm.masked.load.v1p0.p0(ptr %a, i32 8, <1 x i1> %c, <1 x ptr> poison)
+define void @test(ptr %a, ptr %b, <1 x i1> %c) {
+  %v1p = call <1 x ptr> @llvm.masked.load.v1p0.p0(ptr %a, i32 8, <1 x i1> %c, <1 x ptr> poison)
+  %p = bitcast <1 x ptr> %v1p to ptr
+  load i8, ptr %p
+  store i8 0, ptr %b
+  ret void
+}