Index: lib/CodeGen/MIRParser/MIRParser.cpp
===================================================================
--- lib/CodeGen/MIRParser/MIRParser.cpp
+++ lib/CodeGen/MIRParser/MIRParser.cpp
@@ -28,6 +28,7 @@
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineModuleInfo.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/CodeGen/TargetFrameLowering.h"
 #include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/DebugInfo.h"
 #include "llvm/IR/DiagnosticInfo.h"
@@ -562,6 +563,21 @@
   return Error;
 }
 
+static unsigned GetStackObjectAlignment(const yaml::MachineStackObject &Object,
+                                        const MachineFunction &MF) {
+  const Function &F = MF.getFunction();
+  // If set in the YAML, use that one.
+  unsigned Alignment = Object.Alignment;
+  if (Alignment)
+    return Alignment;
+
+  // Try to get the default stack alignment from the IR function attrs.
+  if (F.hasFnAttribute(Attribute::StackAlignment))
+    return F.getFnStackAlignment();
+  else
+    return MF.getSubtarget().getFrameLowering()->getStackAlignment();
+}
+
 bool MIRParserImpl::initializeFrameInfo(PerFunctionMIParsingState &PFS,
                                         const yaml::MachineFunction &YamlMF) {
   MachineFunction &MF = PFS.MF;
@@ -640,7 +656,7 @@
       ObjectIdx = MFI.CreateVariableSizedObject(Object.Alignment, Alloca);
     else
       ObjectIdx = MFI.CreateStackObject(
-          Object.Size, Object.Alignment,
+          Object.Size, GetStackObjectAlignment(Object, MF),
           Object.Type == yaml::MachineStackObject::SpillSlot, Alloca);
     MFI.setObjectOffset(ObjectIdx, Object.Offset);
     MFI.setStackID(ObjectIdx, Object.StackID);
Index: test/CodeGen/MIR/AArch64/stack-align-zero.mir
===================================================================
--- /dev/null
+++ test/CodeGen/MIR/AArch64/stack-align-zero.mir
@@ -0,0 +1,12 @@
+# RUN: llc -run-pass=none -o - -mtriple=aarch64-- %s | FileCheck %s
+# Check that the stack object alignment is never 0, even if it's not specified.
+---
+name:            foo
+tracksRegLiveness: true
+stack:
+  - { id : 0, size: 8 }
+# CHECK: - { id: 0, name: '', type: default, offset: 0, size: 8, alignment: 16
+body:             |
+  bb.0:
+    RET_ReallyLR
+...