Index: llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
===================================================================
--- llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -7892,6 +7892,12 @@
   }
   case Instruction::ExtractValue:
     return TTI.getInstructionCost(I, TTI::TCK_RecipThroughput);
+  case Instruction::Alloca:
+    // We cannot easily widen alloca to a scalable alloca, as
+    // the result would need to be a vector of pointers.
+    if (VF.isScalable())
+      return InstructionCost::getInvalid();
+    LLVM_FALLTHROUGH;
   default:
     // This opcode is unknown. Assume that it is the same as 'mul'.
     return TTI.getArithmeticInstrCost(Instruction::Mul, VectorTy, CostKind);
Index: llvm/test/Transforms/LoopVectorize/AArch64/scalable-alloca.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/LoopVectorize/AArch64/scalable-alloca.ll
@@ -0,0 +1,31 @@
+; RUN: opt -S -loop-vectorize -mattr=+sve -mtriple aarch64-unknown-linux-gnu -force-vector-width=2 -scalable-vectorization=preferred -pass-remarks-analysis=loop-vectorize -pass-remarks-missed=loop-vectorize < %s 2>%t | FileCheck %s
+; RUN: FileCheck %s --check-prefix=CHECK-REMARKS < %t
+
+; CHECK-REMARKS: UserVF ignored because of invalid costs.
+; CHECK-REMARKS: Instruction with invalid costs prevented vectorization at VF=(vscale x 1, vscale x 2): alloca
+; CHECK-REMARKS: Instruction with invalid costs prevented vectorization at VF=(vscale x 1): store
+define void @alloca(i32** %vla, i64 %N) {
+; CHECK-LABEL: @alloca(
+; CHECK-NOT: <vscale x
+
+entry:
+  br label %for.body
+
+for.body:
+  %iv = phi i64 [ %iv.next, %for.body ], [ 0, %entry ]
+  %alloca = alloca i32, align 16
+  %arrayidx = getelementptr inbounds i32*, i32** %vla, i64 %iv
+  store i32* %alloca, i32** %arrayidx, align 8
+  %iv.next = add nuw nsw i64 %iv, 1
+  %exitcond.not = icmp eq i64 %iv.next, %N
+  br i1 %exitcond.not, label %for.end, label %for.body, !llvm.loop !0
+
+for.end:
+  call void @foo(i32** nonnull %vla)
+  ret void
+}
+
+declare void @foo(i32**)
+
+!0 = !{!0, !1}
+!1 = !{!"llvm.loop.vectorize.scalable.enable", i1 true}