When processing frame index virtual registers, consider all available registers

(if there are any) and use the one which remains available for the longest
rather than just using the first one. This should help enable better re-use
of the loaded frame index values. rdar://7318760

llvm-svn: 107847

llvm/include/llvm/CodeGen/RegisterScavenging.h

@@ -98,10 +98,24 @@ public:
   /// getRegsUsed - return all registers currently in use in used.
   void getRegsUsed(BitVector &used, bool includeReserved);
 
+  /// getRegsAvailable - Return all available registers in the register class
+  /// in Mask.
+  void getRegsAvailable(const TargetRegisterClass *RC, BitVector &Mask);
+
   /// FindUnusedReg - Find a unused register of the specified register class.
   /// Return 0 if none is found.
   unsigned FindUnusedReg(const TargetRegisterClass *RegClass) const;
 
+  /// findSurvivorReg - Return the candidate register that is unused for the
+  /// longest after StartMI. UseMI is set to the instruction where the search
+  /// stopped.
+  ///
+  /// No more than InstrLimit instructions are inspected.
+  unsigned findSurvivorReg(MachineBasicBlock::iterator StartMI,
+                           BitVector &Candidates,
+                           unsigned InstrLimit,
+                           MachineBasicBlock::iterator &UseMI);
+
   /// setScavengingFrameIndex / getScavengingFrameIndex - accessor and setter of
   /// ScavengingFrameIndex.
   void setScavengingFrameIndex(int FI) { ScavengingFrameIndex = FI; }
@@ -147,11 +161,6 @@ private:
   /// Add Reg and its aliases to BV.
   void addRegWithAliases(BitVector &BV, unsigned Reg);
 
-  unsigned findSurvivorReg(MachineBasicBlock::iterator MI,
-                           BitVector &Candidates,
-                           unsigned InstrLimit,
-                           MachineBasicBlock::iterator &UseMI);
-
 };
 
 } // End llvm namespace

llvm/lib/CodeGen/PrologEpilogInserter.cpp

@@ -885,10 +885,20 @@ void PEI::scavengeFrameVirtualRegs(MachineFunction &Fn) {
             // Scavenge a new scratch register
             CurrentVirtReg = Reg;
             const TargetRegisterClass *RC = Fn.getRegInfo().getRegClass(Reg);
-            CurrentScratchReg = RS->FindUnusedReg(RC);
-            if (CurrentScratchReg == 0)
+            const TargetRegisterInfo *TRI = Fn.getTarget().getRegisterInfo();
+            BitVector Candidates(TRI->getNumRegs());
+            RS->getRegsAvailable(RC, Candidates);
+
+            // If there are any registers available, use the one that's
+            // unused for the longest after this instruction. That increases
+            // the ability to reuse the value.
+            if (Candidates.any()) {
+              MachineBasicBlock::iterator UMI;
+              CurrentScratchReg = RS->findSurvivorReg(I, Candidates, 25, UMI);
+            } else {
               // No register is "free". Scavenge a register.
               CurrentScratchReg = RS->scavengeRegister(RC, I, SPAdj);
+            }
 
             PrevValue = Value;
           }

llvm/lib/CodeGen/RegisterScavenging.cpp

@@ -242,8 +242,18 @@ unsigned RegScavenger::FindUnusedReg(const TargetRegisterClass *RC) const {
   return 0;
 }
 
+/// getRegsAvailable - Return all available registers in the register class
+/// in Mask.
+void RegScavenger::getRegsAvailable(const TargetRegisterClass *RC,
+                                    BitVector &Mask) {
+  for (TargetRegisterClass::iterator I = RC->begin(), E = RC->end();
+       I != E; ++I)
+    if (!isAliasUsed(*I))
+      Mask.set(*I);
+}
+
 /// findSurvivorReg - Return the candidate register that is unused for the
-/// longest after MBBI. UseMI is set to the instruction where the search
+/// longest after StargMII. UseMI is set to the instruction where the search
 /// stopped.
 ///
 /// No more than InstrLimit instructions are inspected.