AMDGPU: Don't use stack space for SGPR->VGPR spills

Before frame offsets are calculated, try to eliminate the
frame indexes used by SGPR spills. Then we can delete them
after.

I think for now we can be sure that no other instruction
will be re-using the same frame indexes. It should be easy
to notice if this assumption ever breaks since everything
asserts if it tries to use a dead frame index later.

The unused emergency stack slot seems to still be left behind,
so an additional 4 bytes is still wasted.

llvm-svn: 295753

1 files changed, 51 insertions, 42 deletions

diff --git a/llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp b/llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp
index ecd46b95ca6..efae3382f49 100644
--- a/llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp
+++ b/llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp
@@ -20,12 +20,6 @@
 
 using namespace llvm;
 
-static cl::opt<bool> EnableSpillSGPRToVGPR(
-  "amdgpu-spill-sgpr-to-vgpr",
-  cl::desc("Enable spilling VGPRs to SGPRs"),
-  cl::ReallyHidden,
-  cl::init(true));
-
 SIMachineFunctionInfo::SIMachineFunctionInfo(const MachineFunction &MF)
   : AMDGPUMachineFunction(MF),
     TIDReg(AMDGPU::NoRegister),
@@ -193,45 +187,60 @@ unsigned SIMachineFunctionInfo::addPrivateMemoryPtr(const SIRegisterInfo &TRI) {
   return PrivateMemoryPtrUserSGPR;
 }
 
-SIMachineFunctionInfo::SpilledReg SIMachineFunctionInfo::getSpilledReg (
-                                                       MachineFunction *MF,
-                                                       unsigned FrameIndex,
-                                                       unsigned SubIdx) {
-  if (!EnableSpillSGPRToVGPR)
-    return SpilledReg();
-
-  const SISubtarget &ST = MF->getSubtarget<SISubtarget>();
-  const SIRegisterInfo *TRI = ST.getRegisterInfo();
-
-  MachineFrameInfo &FrameInfo = MF->getFrameInfo();
-  MachineRegisterInfo &MRI = MF->getRegInfo();
-  int64_t Offset = FrameInfo.getObjectOffset(FrameIndex);
-  Offset += SubIdx * 4;
-
-  unsigned LaneVGPRIdx = Offset / (64 * 4);
-  unsigned Lane = (Offset / 4) % 64;
-
-  struct SpilledReg Spill;
-  Spill.Lane = Lane;
-
-  if (!LaneVGPRs.count(LaneVGPRIdx)) {
-    unsigned LaneVGPR = TRI->findUnusedRegister(MRI, &AMDGPU::VGPR_32RegClass,
-                                                *MF);
+/// Reserve a slice of a VGPR to support spilling for FrameIndex \p FI.
+bool SIMachineFunctionInfo::allocateSGPRSpillToVGPR(MachineFunction &MF,
+                                                    int FI) {
+  std::vector<SpilledReg> &SpillLanes = SGPRToVGPRSpills[FI];
 
-    if (LaneVGPR == AMDGPU::NoRegister)
-      // We have no VGPRs left for spilling SGPRs.
-      return Spill;
+  // This has already been allocated.
+  if (!SpillLanes.empty())
+    return true;
 
-    LaneVGPRs[LaneVGPRIdx] = LaneVGPR;
-
-    // Add this register as live-in to all blocks to avoid machine verifer
-    // complaining about use of an undefined physical register.
-    for (MachineFunction::iterator BI = MF->begin(), BE = MF->end();
-         BI != BE; ++BI) {
-      BI->addLiveIn(LaneVGPR);
+  const SISubtarget &ST = MF.getSubtarget<SISubtarget>();
+  const SIRegisterInfo *TRI = ST.getRegisterInfo();
+  MachineFrameInfo &FrameInfo = MF.getFrameInfo();
+  MachineRegisterInfo &MRI = MF.getRegInfo();
+  unsigned WaveSize = ST.getWavefrontSize();
+
+  unsigned Size = FrameInfo.getObjectSize(FI);
+  assert(Size >= 4 && Size <= 64 && "invalid sgpr spill size");
+  assert(TRI->spillSGPRToVGPR() && "not spilling SGPRs to VGPRs");
+
+  int NumLanes = Size / 4;
+
+  // Make sure to handle the case where a wide SGPR spill may span between two
+  // VGPRs.
+  for (int I = 0; I < NumLanes; ++I, ++NumVGPRSpillLanes) {
+    unsigned LaneVGPR;
+    unsigned VGPRIndex = (NumVGPRSpillLanes % WaveSize);
+
+    if (VGPRIndex == 0) {
+      LaneVGPR = TRI->findUnusedRegister(MRI, &AMDGPU::VGPR_32RegClass, MF);
+      if (LaneVGPR == AMDGPU::NoRegister) {
+        // We have no VGPRs left for spilling SGPRs. Reset because we won't
+        // partially spill the SGPR to VGPRs.
+        SGPRToVGPRSpills.erase(FI);
+        NumVGPRSpillLanes -= I;
+        return false;
+      }
+
+      SpillVGPRs.push_back(LaneVGPR);
+
+      // Add this register as live-in to all blocks to avoid machine verifer
+      // complaining about use of an undefined physical register.
+      for (MachineBasicBlock &BB : MF)
+        BB.addLiveIn(LaneVGPR);
+    } else {
+      LaneVGPR = SpillVGPRs.back();
     }
+
+    SpillLanes.push_back(SpilledReg(LaneVGPR, VGPRIndex));
   }
 
-  Spill.VGPR = LaneVGPRs[LaneVGPRIdx];
-  return Spill;
+  return true;
+}
+
+void SIMachineFunctionInfo::removeSGPRToVGPRFrameIndices(MachineFrameInfo &MFI) {
+  for (auto &R : SGPRToVGPRSpills)
+    MFI.RemoveStackObject(R.first);
 }