[llvm] [AVR] Support stack realignment (PR #187129)

[Patryk Wychowaniec via llvm-commits]

================
@@ -222,6 +213,32 @@ void AVRFrameLowering::emitEpilogue(MachineFunction &MF,
   restoreStatusRegister(MF, MBB);
 }
 
+StackOffset AVRFrameLowering::getFrameIndexReference(const MachineFunction &MF,
+                                                     int FI,
+                                                     Register &FrameReg) const {
+  int64_t Offset;
+  const MachineFrameInfo &MFI = MF.getFrameInfo();
+
+  switch (MFI.getStackID(FI)) {
+  case TargetStackID::Default:
+    Offset = MFI.getObjectOffset(FI) + MFI.getOffsetAdjustment() +
+             MFI.getStackSize() - getOffsetOfLocalArea() + 1;
+
+    assert(Offset > 0);
+    break;
+
+  case TargetStackID::AvrAlign:
+    Offset = MFI.getObjectOffset(FI);
----------------
Patryk27 wrote:

I kiinda see what you mean, but in some sense the variables _do_ end up in two different spaces - that's because all variables that belong to the `AvrAlign` stack end up addressed via a different register:

```cpp
void fun() {
  // Will be addressed using register Y:
  int a;
  int b;

  // Will be addressed using a dynamically allocated, stack-aligned register:
  _Alignas(4) int c;
  _Alignas(4) int d;

  /* ... */
}
```

I'm not saying we _have to_ implement it the way I did, but if everything you say is "lol no", then it doesn't really help me. If I had twenty implementation ideas, I would've written them down here.

https://github.com/llvm/llvm-project/pull/187129