[MIR] Provide a default alignment for stack objects

[thegameg]

Summary

Currently, stack objects without alignment cause div by 0 in estimateStackSize.

See PR36459.

[dsanders]

lib/CodeGen/MIRParser/MIRParser.cpp

574–578

This isn't correct since both of these are the alignment of the stack frame itself, not a default alignment of the objects in the frame. You're right that alloca can choose an alignment when it isn't known (it's worth mentioning that this isn't true of other missing alignment information). However, the chosen alignment is necessarily a target decision. Aside from varying between targets, it can be also be context sensitive and varies between types and other things (e.g. fixed location stack frame objects generally don't get to chose, they're specified by the ABI).

Most importantly though, serializing+deserializing via MIR shouldn't change the alignment. With this code present, we would get different behaviour if we pass MIR from one pass to the next directly compared to if we write it to an intermediate file and read it back. I think we should fix AArch64 to account for missing alignment information instead.

[thegameg]

lib/CodeGen/MIRParser/MIRParser.cpp

574–578

Thanks. I agree this is not ideal. I believe none of the backends actually generate MIR with missing alignment information, I just triggered this when I forgot to include it.

I guess the right thing to do here is making the alignment non-optional on all stack objects?

lib/CodeGen/MIRParser/MIRParser.cpp

 #include "llvm/CodeGen/GlobalISel/RegisterBank.h"
 #include "llvm/CodeGen/GlobalISel/RegisterBankInfo.h"
 #include "llvm/CodeGen/MIRYamlMapping.h"
 #include "llvm/CodeGen/MachineConstantPool.h"
 #include "llvm/CodeGen/MachineFrameInfo.h"
 #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"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/ValueSymbolTable.h"
 #include "llvm/Support/LineIterator.h"
bool MIRParserImpl::setupRegisterInfo(const PerFunctionMIParsingState &PFS,
   }
 
   // FIXME: This is a temporary workaround until the reserved registers can be
   // serialized.
   MRI.freezeReservedRegs(MF);
   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;
   MachineFrameInfo &MFI = MF.getFrameInfo();
   const Function &F = MF.getFunction();
   const yaml::MachineFrameInfo &YamlMFI = YamlMF.FrameInfo;
   MFI.setFrameAddressIsTaken(YamlMFI.IsFrameAddressTaken);
   MFI.setReturnAddressIsTaken(YamlMFI.IsReturnAddressTaken);
if (!Name.Value.empty()) {
                      "alloca instruction named '" + Name.Value +
                          "' isn't defined in the function '" + F.getName() +
                          "'");
     }
     if (Object.Type == yaml::MachineStackObject::VariableSized)
       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);
 
     if (!PFS.StackObjectSlots.insert(std::make_pair(Object.ID.Value, ObjectIdx))
              .second)
       return error(Object.ID.SourceRange.Start,
                    Twine("redefinition of stack object '%stack.") +

test/CodeGen/MIR/AArch64/stack-align-zero.mir

+# 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
+...