diff --git a/llvm/lib/Target/AArch64/AArch64StackTagging.cpp b/llvm/lib/Target/AArch64/AArch64StackTagging.cpp
index 75c7dd944b467..35a0aa0ef7df3 100644
--- a/llvm/lib/Target/AArch64/AArch64StackTagging.cpp
+++ b/llvm/lib/Target/AArch64/AArch64StackTagging.cpp
@@ -586,8 +586,7 @@ bool AArch64StackTagging::runOnFunction(Function &Fn) {
                                    ClMaxLifetimes);
     if (StandardLifetime) {
       IntrinsicInst *Start = Info.LifetimeStart[0];
-      uint64_t Size =
-          cast<ConstantInt>(Start->getArgOperand(0))->getZExtValue();
+      uint64_t Size = *Info.AI->getAllocationSize(*DL);
       Size = alignTo(Size, kTagGranuleSize);
       tagAlloca(AI, Start->getNextNode(), TagPCall, Size);