[llvm-commits] [llvm] r65120 - /llvm/trunk/lib/CodeGen/StackSlotColoring.cpp

[Owen Anderson]

Why?  The scan is stupidly fast and can improve code quality in rare  
cases even when no color is done.

--Owen

On Feb 20, 2009, at 7:15 PM, Evan Cheng wrote:

> It probably should only do DCE when any coloring was done, right?
>
> Evan
>
> On Feb 20, 2009, at 1:11 AM, Owen Anderson wrote:
>
>> Author: resistor
>> Date: Fri Feb 20 03:11:36 2009
>> New Revision: 65120
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=65120&view=rev
>> Log:
>> 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.
>>
>> Modified:
>>   llvm/trunk/lib/CodeGen/StackSlotColoring.cpp
>>
>> Modified: llvm/trunk/lib/CodeGen/StackSlotColoring.cpp
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/StackSlotColoring.cpp?rev=65120&r1=65119&r2=65120&view=diff
>>
>> =
>> =
>> =
>> =
>> =
>> =
>> =
>> =
>> = 
>> =====================================================================
>> --- llvm/trunk/lib/CodeGen/StackSlotColoring.cpp (original)
>> +++ llvm/trunk/lib/CodeGen/StackSlotColoring.cpp Fri Feb 20 03:11:36
>> 2009
>> @@ -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 @@
>>             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 @@
>>
>>    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
>>      AU.addRequired<LiveStacks>();
>> +
>>      AU.addPreservedID(MachineLoopInfoID);
>>      AU.addPreservedID(MachineDominatorsID);
>>      MachineFunctionPass::getAnalysisUsage(AU);
>> @@ -82,6 +93,7 @@
>>    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 @@
>>  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 @@
>>    Assignments[i].clear();
>>  Assignments.clear();
>>
>> +  if (EnableDCE) {
>> +    for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I !
>> = E; ++I)
>> +      Changed |= removeDeadStores(I);
>> +  }
>> +
>>  return Changed;
>> }
>>
>>
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