[AMDGPU] Save VGPR of whole wave when spillingdev-main-update

Spilling SGPRs to scratch uses a temporary VGPR. LLVM currently cannot
determine if a VGPR is used in other lanes or not, so we need to save
all lanes of the VGPR. We even need to save the VGPR if it is marked as
dead.

The generated code depends on two things:
- Can we scavenge an SGPR to save EXEC?
- And can we scavenge a VGPR?

If we can scavenge an SGPR, we
- save EXEC into the SGPR
- set the needed lane mask
- save the temporary VGPR
- write the spilled SGPR into VGPR lanes
- save the VGPR again to the target stack slot
- restore the VGPR
- restore EXEC

If we were not able to scavenge an SGPR, we do the same operations, but
everytime the temporary VGPR is written to memory, we
- write VGPR to memory
- flip exec (s_not exec, exec)
- write VGPR again (previously inactive lanes)

Surprisingly often, we are able to scavenge an SGPR, even though we are
at the brink of running out of SGPRs.
Scavenging a VGPR does not have a great effect (saves three instructions
if no SGPR was scavenged), but we need to know if the VGPR we use is
live before or not, otherwise the machine verifier complains.

Differential Revision: https://reviews.llvm.org/D96336

1 files changed, 2 insertions, 10 deletions

diff --git a/llvm/lib/Target/AMDGPU/SIFrameLowering.cpp b/llvm/lib/Target/AMDGPU/SIFrameLowering.cpp
index 3a956f3e7cd9..d0fd59a4e9c9 100644
--- a/llvm/lib/Target/AMDGPU/SIFrameLowering.cpp
+++ b/llvm/lib/Target/AMDGPU/SIFrameLowering.cpp
@@ -1237,16 +1237,8 @@ void SIFrameLowering::processFunctionBeforeFrameFinalized(
   if (!allStackObjectsAreDead(MFI)) {
     assert(RS && "RegScavenger required if spilling");
 
-    if (FuncInfo->isEntryFunction()) {
-      int ScavengeFI = MFI.CreateFixedObject(
-        TRI->getSpillSize(AMDGPU::SGPR_32RegClass), 0, false);
-      RS->addScavengingFrameIndex(ScavengeFI);
-    } else {
-      int ScavengeFI = MFI.CreateStackObject(
-          TRI->getSpillSize(AMDGPU::SGPR_32RegClass),
-          TRI->getSpillAlign(AMDGPU::SGPR_32RegClass), false);
-      RS->addScavengingFrameIndex(ScavengeFI);
-    }
+    // Add an emergency spill slot
+    RS->addScavengingFrameIndex(FuncInfo->getScavengeFI(MFI, *TRI));
   }
 }