LiveVariables: Compute a set of defs and kills to speed up updating LV during critical edge splitting.

Previously we checked if the register is def'd in a block via the def/use list a
nd walked the list of kills to check if the register is killed in a block. Both
of these checks can be made much cheaper by walking the block first and
recording all defs and kills.

This reduces the compile time of the test case from PR13651 from 40s to 15s at
-O2. The compile time is still dominated by LV updating but now the main culprit
is SparseBitVector's slowness.

llvm-svn: 163478

1 files changed, 30 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/LiveVariables.cpp b/llvm/lib/CodeGen/LiveVariables.cpp
index 6c94690e771..f294124228d 100644
--- a/llvm/lib/CodeGen/LiveVariables.cpp
+++ b/llvm/lib/CodeGen/LiveVariables.cpp
@@ -808,18 +808,44 @@ void LiveVariables::addNewBlock(MachineBasicBlock *BB,
                                 MachineBasicBlock *SuccBB) {
   const unsigned NumNew = BB->getNumber();
 
-  // All registers used by PHI nodes in SuccBB must be live through BB.
-  for (MachineBasicBlock::iterator BBI = SuccBB->begin(),
-         BBE = SuccBB->end(); BBI != BBE && BBI->isPHI(); ++BBI)
+  SmallSet<unsigned, 16> Defs, Kills;
+
+  MachineBasicBlock::iterator BBI = SuccBB->begin(), BBE = SuccBB->end();
+  for (; BBI != BBE && BBI->isPHI(); ++BBI) {
+    // Record the def of the PHI node.
+    Defs.insert(BBI->getOperand(0).getReg());
+
+    // All registers used by PHI nodes in SuccBB must be live through BB.
     for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2)
       if (BBI->getOperand(i+1).getMBB() == BB)
         getVarInfo(BBI->getOperand(i).getReg()).AliveBlocks.set(NumNew);
+  }
+
+  // Record all vreg defs and kills of all instructions in SuccBB.
+  for (; BBI != BBE; ++BBI) {
+    for (MachineInstr::mop_iterator I = BBI->operands_begin(),
+         E = BBI->operands_end(); I != E; ++I) {
+      if (I->isReg() && TargetRegisterInfo::isVirtualRegister(I->getReg())) {
+        if (I->isDef())
+          Defs.insert(I->getReg());
+        else if (I->isKill())
+          Kills.insert(I->getReg());
+      }
+    }
+  }
 
   // Update info for all live variables
   for (unsigned i = 0, e = MRI->getNumVirtRegs(); i != e; ++i) {
     unsigned Reg = TargetRegisterInfo::index2VirtReg(i);
+
+    // If the Defs is defined in the successor it can't be live in BB.
+    if (Defs.count(Reg))
+      continue;
+
+    // If the register is either killed in or live through SuccBB it's also live
+    // through BB.
     VarInfo &VI = getVarInfo(Reg);
-    if (!VI.AliveBlocks.test(NumNew) && VI.isLiveIn(*SuccBB, Reg, *MRI))
+    if (Kills.count(Reg) || VI.AliveBlocks.test(SuccBB->getNumber()))
       VI.AliveBlocks.set(NumNew);
   }
 }