Local spiller optimization:

Turn this:
movswl  %ax, %eax
movl    %eax, -36(%ebp)
xorl    %edi, -36(%ebp)
into
movswl  %ax, %eax
xorl    %edi, %eax
movl    %eax, -36(%ebp)
by unfolding the load / store xorl into an xorl and a store when we know the
value in the spill slot is available in a register. This doesn't change the
number of instructions but reduce the number of times memory is accessed.

Also unfold some load folding instructions and reuse the value when similar
situation presents itself.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@42947 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 54 insertions, 4 deletions

diff --git a/lib/CodeGen/VirtRegMap.cpp b/lib/CodeGen/VirtRegMap.cpp
index ebdbb86..d6dc92a 100644
--- a/lib/CodeGen/VirtRegMap.cpp
+++ b/lib/CodeGen/VirtRegMap.cpp
@@ -139,6 +139,11 @@ void VirtRegMap::virtFolded(unsigned VirtReg, MachineInstr *OldMI,
   MI2VirtMap.insert(IP, std::make_pair(NewMI, std::make_pair(VirtReg, MRInfo)));
 }
 
+void VirtRegMap::virtFolded(unsigned VirtReg, MachineInstr *MI, ModRef MRInfo) {
+  MI2VirtMapTy::iterator IP = MI2VirtMap.lower_bound(MI);
+  MI2VirtMap.insert(IP, std::make_pair(MI, std::make_pair(VirtReg, MRInfo)));
+}
+
 void VirtRegMap::print(std::ostream &OS) const {
   const MRegisterInfo* MRI = MF.getTarget().getRegisterInfo();
 
@@ -1059,6 +1064,19 @@ void LocalSpiller::RewriteMBB(MachineBasicBlock &MBB, VirtRegMap &VRM) {
             Erased = true;
             goto ProcessNextInst;
           }
+        } else {
+          unsigned PhysReg = Spills.getSpillSlotOrReMatPhysReg(SS);
+          SmallVector<MachineInstr*, 4> NewMIs;
+          if (PhysReg &&
+              MRI->unfoldMemoryOperand(MF, &MI, PhysReg, false, false, NewMIs)) {
+            MBB.insert(MII, NewMIs[0]);
+            VRM.RemoveFromFoldedVirtMap(&MI);
+            MBB.erase(&MI);
+            Erased = true;
+            --NextMII;  // backtrack to the unfolded instruction.
+            BackTracked = true;
+            goto ProcessNextInst;
+          }
         }
       }
 
@@ -1066,16 +1084,44 @@ void LocalSpiller::RewriteMBB(MachineBasicBlock &MBB, VirtRegMap &VRM) {
       // Otherwise, the store to this stack slot is not dead anymore.
       MachineInstr* DeadStore = MaybeDeadStores[SS];
       if (DeadStore) {
-        if (!(MR & VirtRegMap::isRef)) {  // Previous store is dead.
+        bool isDead = true;
+        MachineInstr *NewStore = NULL;
+        if (MR & VirtRegMap::isRef) {
+          unsigned PhysReg = Spills.getSpillSlotOrReMatPhysReg(SS);
+          SmallVector<MachineInstr*, 4> NewMIs;
+          if (PhysReg &&
+              DeadStore->findRegisterUseOperandIdx(PhysReg, true) != -1 &&
+              MRI->unfoldMemoryOperand(MF, &MI, PhysReg, false, true, NewMIs)) {
+            MBB.insert(MII, NewMIs[0]);
+            NewStore = NewMIs[1];
+            MBB.insert(MII, NewStore);
+            VRM.RemoveFromFoldedVirtMap(&MI);
+            MBB.erase(&MI);
+            Erased = true;
+            --NextMII;
+            --NextMII;  // backtrack to the unfolded instruction.
+            BackTracked = true;
+          } else
+            isDead = false;
+        }
+
+        if (isDead) {  // Previous store is dead.
           // If we get here, the store is dead, nuke it now.
-          assert(VirtRegMap::isMod && "Can't be modref!");
           DOUT << "Removed dead store:\t" << *DeadStore;
           InvalidateKills(*DeadStore, RegKills, KillOps);
-          MBB.erase(DeadStore);
           VRM.RemoveFromFoldedVirtMap(DeadStore);
-          ++NumDSE;
+          MBB.erase(DeadStore);
+          if (!NewStore)
+            ++NumDSE;
         }
+
         MaybeDeadStores[SS] = NULL;
+        if (NewStore) {
+          // Treat this store as a spill merged into a copy. That makes the
+          // stack slot value available.
+          VRM.virtFolded(VirtReg, NewStore, VirtRegMap::isMod);
+          goto ProcessNextInst;
+        }
       }
 
       // If the spill slot value is available, and this is a new definition of
diff --git a/lib/CodeGen/VirtRegMap.h b/lib/CodeGen/VirtRegMap.h
index 8385f3d..f6d305e 100644
--- a/lib/CodeGen/VirtRegMap.h
+++ b/lib/CodeGen/VirtRegMap.h
@@ -181,6 +181,10 @@ namespace llvm {
     void virtFolded(unsigned VirtReg, MachineInstr *OldMI, unsigned OpNum,
                     MachineInstr *NewMI);
 
+    /// @brief Updates information about the specified virtual register's value
+    /// folded into the specified machine instruction.
+    void virtFolded(unsigned VirtReg, MachineInstr *MI, ModRef MRInfo);
+
     /// @brief returns the virtual registers' values folded in memory
     /// operands of this instruction
     std::pair<MI2VirtMapTy::const_iterator, MI2VirtMapTy::const_iterator>