Add a quick pass to the stack slot colorer to eliminate some trivially redundant spills after coloring.

Ideally these would never get created in the first place, but until we enhance the spiller to have a more
global picture of what's happening, this is necessary for code quality in some circumstances.

llvm-svn: 65120

llvm/lib/CodeGen/StackSlotColoring.cpp

@@ -19,6 +19,8 @@
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/Debug.h"
+#include "llvm/Target/TargetInstrInfo.h"
+#include "llvm/Target/TargetMachine.h"
 #include "llvm/ADT/BitVector.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
@@ -30,12 +32,20 @@ DisableSharing("no-stack-slot-sharing",
              cl::init(false), cl::Hidden,
              cl::desc("Surpress slot sharing during stack coloring"));
 
+static cl::opt<bool>
+EnableDCE("enable-ssc-dce",
+               cl::init(false), cl::Hidden,
+               cl::desc("Enable slot coloring DCE"));
+
 STATISTIC(NumEliminated,   "Number of stack slots eliminated due to coloring");
+STATISTIC(NumDeadAccesses,
+                          "Number of trivially dead stack accesses eliminated");
 
 namespace {
   class VISIBILITY_HIDDEN StackSlotColoring : public MachineFunctionPass {
     LiveStacks* LS;
     MachineFrameInfo *MFI;
+    const TargetInstrInfo  *TII;
 
     // SSIntervals - Spill slot intervals.
     std::vector<LiveInterval*> SSIntervals;
@@ -67,6 +77,7 @@ namespace {
     
     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
       AU.addRequired<LiveStacks>();
+      
       AU.addPreservedID(MachineLoopInfoID);
       AU.addPreservedID(MachineDominatorsID);
       MachineFunctionPass::getAnalysisUsage(AU);
@@ -82,6 +93,7 @@ namespace {
     bool OverlapWithAssignments(LiveInterval *li, int Color) const;
     int ColorSlot(LiveInterval *li);
     bool ColorSlots(MachineFunction &MF);
+    bool removeDeadStores(MachineBasicBlock* MBB);
   };
 } // end anonymous namespace
 
@@ -260,10 +272,55 @@ bool StackSlotColoring::ColorSlots(MachineFunction &MF) {
   return true;
 }
 
+/// removeDeadStores - Scan through a basic block and look for loads followed
+/// by stores.  If they're both using the same stack slot, then the store is
+/// definitely dead.  This could obviously be much more aggressive (consider
+/// pairs with instructions between them), but such extensions might have a
+/// considerable compile time impact.
+bool StackSlotColoring::removeDeadStores(MachineBasicBlock* MBB) {
+  // FIXME: This could be much more aggressive, but we need to investigate
+  // the compile time impact of doing so.
+  bool changed = false;
+
+  SmallVector<MachineInstr*, 4> toErase;
+
+  for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
+       I != E; ++I) {
+    MachineBasicBlock::iterator NextMI = next(I);
+    if (NextMI == MBB->end()) continue;
+    
+    int FirstSS, SecondSS;
+    unsigned LoadReg = 0;
+    unsigned StoreReg = 0;
+    if (!(LoadReg = TII->isLoadFromStackSlot(I, FirstSS))) continue;
+    if (!(StoreReg = TII->isStoreToStackSlot(NextMI, SecondSS))) continue;
+    if (FirstSS != SecondSS || LoadReg != StoreReg || FirstSS == -1) continue;
+    
+    ++NumDeadAccesses;
+    changed = true;
+    
+    if (NextMI->findRegisterUseOperandIdx(LoadReg, true, 0) != -1) {
+      ++NumDeadAccesses;
+      toErase.push_back(I);
+    }
+    
+    toErase.push_back(NextMI);
+    ++I;
+  }
+  
+  for (SmallVector<MachineInstr*, 4>::iterator I = toErase.begin(),
+       E = toErase.end(); I != E; ++I)
+    (*I)->eraseFromParent();
+  
+  return changed;
+}
+
+
 bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
   DOUT << "********** Stack Slot Coloring **********\n";
 
   MFI = MF.getFrameInfo();
+  TII = MF.getTarget().getInstrInfo();
   LS = &getAnalysis<LiveStacks>();
 
   bool Changed = false;
@@ -280,5 +337,10 @@ bool StackSlotColoring::runOnMachineFunction(MachineFunction &MF) {
     Assignments[i].clear();
   Assignments.clear();
 
+  if (EnableDCE) {
+    for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
+      Changed |= removeDeadStores(I);
+  }
+
   return Changed;
 }