[PATCH] MachineCSE: Add a target query for the LookAheadLimit heurisitic

[Tom Stellard]

Hi hfinkel, ributzka, ab, MatzeB, bkramer,

This heurisitc is used to determine whether or not to CSE physical register defs.  My understanding is that this pass will only CSE a physical register def that is within 'LookAheadLimit' instructions of a common expression.

I'm adding a target query for this because the R600 backend needs this LookAheadLimit to be MAX_UINT.

I'm not sure what the original intent of this heuristic was and why the value is 5, so another possibility would be to remove it entirely.

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D9472

Files:
  include/llvm/Target/TargetInstrInfo.h
  lib/CodeGen/MachineCSE.cpp
  lib/Target/R600/SIInstrInfo.h

Index: include/llvm/Target/TargetInstrInfo.h
===================================================================
--- include/llvm/Target/TargetInstrInfo.h
+++ include/llvm/Target/TargetInstrInfo.h
@@ -1235,6 +1235,14 @@
     return false;
   }
 
+  /// \brief Return the value to use for the MachineCSE's LookAheadLimit,
+  /// which is a heuristic used for CSE'ing phys reg defs.
+  virtual unsigned getMachineCSELookAheadLimit () const {
+    // 5 is the value for this heuristic that used to be hard-coded into
+    // the MachineCSE pass.  I have no idea why 5 was choosen.
+    return 5;
+  }
+
 private:
   int CallFrameSetupOpcode, CallFrameDestroyOpcode;
 };
Index: lib/CodeGen/MachineCSE.cpp
===================================================================
--- lib/CodeGen/MachineCSE.cpp
+++ lib/CodeGen/MachineCSE.cpp
@@ -48,7 +48,7 @@
     MachineRegisterInfo *MRI;
   public:
     static char ID; // Pass identification
-    MachineCSE() : MachineFunctionPass(ID), LookAheadLimit(5), CurrVN(0) {
+    MachineCSE() : MachineFunctionPass(ID), LookAheadLimit(0), CurrVN(0) {
       initializeMachineCSEPass(*PassRegistry::getPassRegistry());
     }
 
@@ -69,7 +69,7 @@
     }
 
   private:
-    const unsigned LookAheadLimit;
+    unsigned LookAheadLimit;
     typedef RecyclingAllocator<BumpPtrAllocator,
         ScopedHashTableVal<MachineInstr*, unsigned> > AllocatorTy;
     typedef ScopedHashTable<MachineInstr*, unsigned,
@@ -716,5 +716,6 @@
   MRI = &MF.getRegInfo();
   AA = &getAnalysis<AliasAnalysis>();
   DT = &getAnalysis<MachineDominatorTree>();
+  LookAheadLimit = TII->getMachineCSELookAheadLimit();
   return PerformCSE(DT->getRootNode());
 }
Index: lib/Target/R600/SIInstrInfo.h
===================================================================
--- lib/Target/R600/SIInstrInfo.h
+++ lib/Target/R600/SIInstrInfo.h
@@ -142,6 +142,8 @@
   bool FoldImmediate(MachineInstr *UseMI, MachineInstr *DefMI,
                      unsigned Reg, MachineRegisterInfo *MRI) const final;
 
+  unsigned getMachineCSELookAheadLimit() const override { return UINT_MAX; }
+
   bool isSALU(uint16_t Opcode) const {
     return get(Opcode).TSFlags & SIInstrFlags::SALU;
   }

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