[AMDGPU] Don't fix the scavenge slot at offset 0 (#79136)

At the moment, the emergency spill slot is a fixed object for entry
functions and chain functions, and a regular stack object otherwise.
This patch adopts the latter behaviour for entry/chain functions too. It
seems this was always the intention [1] and it will also save us a bit
of stack space in cases where the first stack object has a large
alignment.

[1]
https://github.com/llvm/llvm-project/commit/34c8b835b16fb3879f1b9770e91df21883356bb6

1 files changed, 4 insertions, 8 deletions

diff --git a/llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp b/llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp
index b94d143..52d6fe6 100644
--- a/llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp
+++ b/llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp
@@ -552,14 +552,10 @@ int SIMachineFunctionInfo::getScavengeFI(MachineFrameInfo &MFI,
                                          const SIRegisterInfo &TRI) {
   if (ScavengeFI)
     return *ScavengeFI;
-  if (isBottomOfStack()) {
-    ScavengeFI = MFI.CreateFixedObject(
-        TRI.getSpillSize(AMDGPU::SGPR_32RegClass), 0, false);
-  } else {
-    ScavengeFI = MFI.CreateStackObject(
-        TRI.getSpillSize(AMDGPU::SGPR_32RegClass),
-        TRI.getSpillAlign(AMDGPU::SGPR_32RegClass), false);
-  }
+
+  ScavengeFI =
+      MFI.CreateStackObject(TRI.getSpillSize(AMDGPU::SGPR_32RegClass),
+                            TRI.getSpillAlign(AMDGPU::SGPR_32RegClass), false);
   return *ScavengeFI;
 }