Be more clever about calculating live variables through new basic blocks.

When splitting a critical edge, the registers live through the edge are:

- Used in a PHI instruction, or
- Live out from the predecessor, and
- Live in to the successor.

This allows the coalescer to eliminate even more phi joins.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@89530 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/LiveVariables.cpp

@@ -656,35 +656,45 @@ void LiveVariables::analyzePHINodes(const MachineFunction& Fn) {
           .push_back(BBI->getOperand(i).getReg());
 }
 
+bool LiveVariables::VarInfo::isLiveIn(const MachineBasicBlock &MBB,
+                                      unsigned Reg,
+                                      MachineRegisterInfo &MRI) {
+  unsigned Num = MBB.getNumber();
+
+  // Reg is live-through.
+  if (AliveBlocks.test(Num))
+    return true;
+
+  // Registers defined in MBB cannot be live in.
+  const MachineInstr *Def = MRI.getVRegDef(Reg);
+  if (Def && Def->getParent() == &MBB)
+    return false;
+
+ // Reg was not defined in MBB, was it killed here?
+  return findKill(&MBB);
+}
+
 /// addNewBlock - Add a new basic block BB as an empty succcessor to DomBB. All
 /// variables that are live out of DomBB will be marked as passing live through
 /// BB.
 void LiveVariables::addNewBlock(MachineBasicBlock *BB,
-                                MachineBasicBlock *DomBB) {
+                                MachineBasicBlock *DomBB,
+                                MachineBasicBlock *SuccBB) {
   const unsigned NumNew = BB->getNumber();
-  const unsigned NumDom = DomBB->getNumber();
+
+  // All registers used by PHI nodes in SuccBB must be live through BB.
+  for (MachineBasicBlock::const_iterator BBI = SuccBB->begin(),
+         BBE = SuccBB->end();
+       BBI != BBE && BBI->getOpcode() == TargetInstrInfo::PHI; ++BBI)
+    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);
 
   // Update info for all live variables
   for (unsigned Reg = TargetRegisterInfo::FirstVirtualRegister,
          E = MRI->getLastVirtReg()+1; Reg != E; ++Reg) {
     VarInfo &VI = getVarInfo(Reg);
-
-    // Anything live through DomBB is also live through BB.
-    if (VI.AliveBlocks.test(NumDom)) {
+    if (!VI.AliveBlocks.test(NumNew) && VI.isLiveIn(*SuccBB, Reg, *MRI))
       VI.AliveBlocks.set(NumNew);
-      continue;
-    }
-
-    // Variables not defined in DomBB cannot be live out.
-    const MachineInstr *Def = MRI->getVRegDef(Reg);
-    if (!Def || Def->getParent() != DomBB)
-      continue;
-
-    // Killed by DomBB?
-    if (VI.findKill(DomBB))
-      continue;
-
-    // This register is defined in DomBB and live out
-    VI.AliveBlocks.set(NumNew);
   }
 }