Reuse operand location when updating PHI instructions.

Calling RemoveOperand is very expensive on huge PHI instructions. This makes
early tail duplication run twice as fast on the Firefox JavaScript
interpreter.

llvm-svn: 95832

1 files changed, 26 insertions, 7 deletions

diff --git a/llvm/lib/CodeGen/TailDuplication.cpp b/llvm/lib/CodeGen/TailDuplication.cpp
index 9ab4058b19b..3223e53d5da 100644
--- a/llvm/lib/CodeGen/TailDuplication.cpp
+++ b/llvm/lib/CodeGen/TailDuplication.cpp
@@ -403,26 +403,45 @@ TailDuplicatePass::UpdateSuccessorsPHIs(MachineBasicBlock *FromBB, bool isDead,
             II->RemoveOperand(i);
           }
         }
-        II->RemoveOperand(Idx+1);
-        II->RemoveOperand(Idx);
-      }
+      } else
+        Idx = 0;
+
+      // If Idx is set, the operands at Idx and Idx+1 must be removed.
+      // We reuse the location to avoid expensive RemoveOperand calls.
+
       DenseMap<unsigned,AvailableValsTy>::iterator LI=SSAUpdateVals.find(Reg);
       if (LI != SSAUpdateVals.end()) {
         // This register is defined in the tail block.
         for (unsigned j = 0, ee = LI->second.size(); j != ee; ++j) {
           MachineBasicBlock *SrcBB = LI->second[j].first;
           unsigned SrcReg = LI->second[j].second;
-          II->addOperand(MachineOperand::CreateReg(SrcReg, false));
-          II->addOperand(MachineOperand::CreateMBB(SrcBB));
+          if (Idx != 0) {
+            II->getOperand(Idx).setReg(SrcReg);
+            II->getOperand(Idx+1).setMBB(SrcBB);
+            Idx = 0;
+          } else {
+            II->addOperand(MachineOperand::CreateReg(SrcReg, false));
+            II->addOperand(MachineOperand::CreateMBB(SrcBB));
+          }
         }
       } else {
         // Live in tail block, must also be live in predecessors.
         for (unsigned j = 0, ee = TDBBs.size(); j != ee; ++j) {
           MachineBasicBlock *SrcBB = TDBBs[j];
-          II->addOperand(MachineOperand::CreateReg(Reg, false));
-          II->addOperand(MachineOperand::CreateMBB(SrcBB));
+          if (Idx != 0) {
+            II->getOperand(Idx).setReg(Reg);
+            II->getOperand(Idx+1).setMBB(SrcBB);
+            Idx = 0;
+          } else {
+            II->addOperand(MachineOperand::CreateReg(Reg, false));
+            II->addOperand(MachineOperand::CreateMBB(SrcBB));
+          }
         }
       }
+      if (Idx != 0) {
+        II->RemoveOperand(Idx+1);
+        II->RemoveOperand(Idx);
+      }
     }
   }
 }