[PATCH] D36415: Insert IMPLICIT_DEFS for undef uses in tail merging

[Matthias Braun via Phabricator via llvm-commits]

MatzeB added inline comments.


================
Comment at: lib/CodeGen/BranchFolding.cpp:385-398
+      CalcLiveIns.init(*TRI);
+      CalcLiveIns.addLiveOutsNoPristines(*Pred);
+      for (const MachineInstr &MI : llvm::reverse(*Pred))
+        CalcLiveIns.stepBackward(MI);
+
+      LivePhysRegs LiveIns(*TRI);
+      LiveIns.addLiveIns(*Pred);
----------------
I'm confused: Don't you need to compare the live-outs of `Pred` with the live-ins of `NewDest`? Adding the liveouts of Pred and stepping backwards over the instructions gives you the live-ins of Pred, but I don't see what they are good for.

I would have expected this to look like this:
```
PredLiveOuts.init(*TRI);
PrevLiveOuts.addLiveOutsNoPristines(*Pred);
// Add implicit defs for NewDest live-in values not available at end of Pred.
for (const MachineBasicBlock::RegisterMaskPair &LI : NewDest->liveins()) {
  // computeLiveIn() currently doesn't add partial live-ins so we are fine with an assert.
  assert(LI.LaneMask == LaneBitmask::getAll() && "found partial live-in");
  if (PredLiveOuts.available(*MRI, LI.PhysReg)) {
    BuildMI ...
  }
}
```


Repository:
  rL LLVM

https://reviews.llvm.org/D36415