[SystemZ] Remove no-op MVCs

The stack coloring pass has code to delete stores and loads that become
trivially dead after coloring.  Extend it to cope with single instructions
that copy from one frame index to another.

The testcase happens to show an example of this kicking in at the moment.
It did occur in Real Code too though.

llvm-svn: 185705

3 files changed, 37 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/StackSlotColoring.cpp b/llvm/lib/CodeGen/StackSlotColoring.cpp
index e1b63680e02..d0c9e1d677b 100644
--- a/llvm/lib/CodeGen/StackSlotColoring.cpp
+++ b/llvm/lib/CodeGen/StackSlotColoring.cpp
@@ -377,10 +377,19 @@ bool StackSlotColoring::RemoveDeadStores(MachineBasicBlock* MBB) {
     if (DCELimit != -1 && (int)NumDead >= DCELimit)
       break;
 
+    int FirstSS, SecondSS;
+    if (TII->isStackSlotCopy(I, FirstSS, SecondSS) &&
+        FirstSS == SecondSS &&
+        FirstSS != -1) {
+      ++NumDead;
+      changed = true;
+      toErase.push_back(I);
+      continue;
+    }
+        
     MachineBasicBlock::iterator NextMI = llvm::next(I);
     if (NextMI == MBB->end()) continue;
 
-    int FirstSS, SecondSS;
     unsigned LoadReg = 0;
     unsigned StoreReg = 0;
     if (!(LoadReg = TII->isLoadFromStackSlot(I, FirstSS))) continue;
diff --git a/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp b/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp
index 81d72e05740..cfd270ae434 100644
--- a/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp
+++ b/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp
@@ -104,6 +104,31 @@ unsigned SystemZInstrInfo::isStoreToStackSlot(const MachineInstr *MI,
   return isSimpleMove(MI, FrameIndex, SystemZII::SimpleBDXStore);
 }
 
+bool SystemZInstrInfo::isStackSlotCopy(const MachineInstr *MI,
+                                       int &DestFrameIndex,
+                                       int &SrcFrameIndex) const {
+  // Check for MVC 0(Length,FI1),0(FI2)
+  const MachineFrameInfo *MFI = MI->getParent()->getParent()->getFrameInfo();
+  if (MI->getOpcode() != SystemZ::MVC ||
+      !MI->getOperand(0).isFI() ||
+      MI->getOperand(1).getImm() != 0 ||
+      !MI->getOperand(3).isFI() ||
+      MI->getOperand(4).getImm() != 0)
+    return false;
+
+  // Check that Length covers the full slots.
+  int64_t Length = MI->getOperand(2).getImm();
+  unsigned FI1 = MI->getOperand(0).getIndex();
+  unsigned FI2 = MI->getOperand(3).getIndex();
+  if (MFI->getObjectSize(FI1) != Length ||
+      MFI->getObjectSize(FI2) != Length)
+    return false;
+
+  DestFrameIndex = FI1;
+  SrcFrameIndex = FI2;
+  return true;
+}
+
 bool SystemZInstrInfo::AnalyzeBranch(MachineBasicBlock &MBB,
                                      MachineBasicBlock *&TBB,
                                      MachineBasicBlock *&FBB,
diff --git a/llvm/lib/Target/SystemZ/SystemZInstrInfo.h b/llvm/lib/Target/SystemZ/SystemZInstrInfo.h
index 11d486cafba..3fe71d88547 100644
--- a/llvm/lib/Target/SystemZ/SystemZInstrInfo.h
+++ b/llvm/lib/Target/SystemZ/SystemZInstrInfo.h
@@ -91,6 +91,8 @@ public:
                                        int &FrameIndex) const LLVM_OVERRIDE;
   virtual unsigned isStoreToStackSlot(const MachineInstr *MI,
                                       int &FrameIndex) const LLVM_OVERRIDE;
+  virtual bool isStackSlotCopy(const MachineInstr *MI, int &DestFrameIndex,
+                               int &SrcFrameIndex) const LLVM_OVERRIDE;
   virtual bool AnalyzeBranch(MachineBasicBlock &MBB,
                              MachineBasicBlock *&TBB,
                              MachineBasicBlock *&FBB,