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

[Krzysztof Parzyszek via Phabricator via llvm-commits]

kparzysz 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);
----------------
MatzeB wrote:
> 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 ...
>   }
> }
> ```
You're right, this won't work.  Still, we need to traverse the block, and it will have to use stepForward.  Consider this case:

```
MBB:
  liveins: R0
  R0<def,dead> = ... R0<kill>    ; stays in MBB
  ... = R0<undef>                ; goes into common tail, but no longer <undef>
```

We need to add R0<def> = IMPLICIT_DEF here.

We can't rely on addLiveOuts from LivePhysRegs, because NewDest->liveins() will (by definition) be a subset of Pred->addLiveOuts.


Repository:
  rL LLVM

https://reviews.llvm.org/D36415