[llvm] [AMDGPU] Restore SP correctly in functions with dynamic allocas (PR #122743)

[via llvm-commits]

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@@ -525,8 +525,11 @@ Register SIRegisterInfo::getFrameRegister(const MachineFunction &MF) const {
 bool SIRegisterInfo::hasBasePointer(const MachineFunction &MF) const {
   // When we need stack realignment, we can't reference off of the
   // stack pointer, so we reserve a base pointer.
+  // For functions with dynamically sized stack objects, we need to reference
+  // off the base pointer in the epilog to restore the stack frame.
   const MachineFrameInfo &MFI = MF.getFrameInfo();
-  return MFI.getNumFixedObjects() && shouldRealignStack(MF);
+  return (MFI.getNumFixedObjects() && shouldRealignStack(MF)) ||
+         MFI.hasVarSizedObjects();
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easyonaadit wrote:

If the function doesn't access any incoming parameters, BP will not be setup.
For a function with an over-aligned dynamic alloca, I couldn't find a way to reverse the calculations done in the prolog without using the base pointer (I have another idea but it would require heavy changes in PEI) 

https://github.com/llvm/llvm-project/pull/122743