[PATCH] D63972: [PowerPC] Do the Early Return for the li and unconditional branch

[Zhang Kang via Phabricator via llvm-commits]

ZhangKang marked an inline comment as done.
ZhangKang added inline comments.


================
Comment at: llvm/lib/CodeGen/MachineBlockPlacement.cpp:2741
+        if (((TBB == &*F->begin()) || !std::prev(I)->isSuccessor(TBB)) &&
+             (TailDup.isSimpleBB(ChainBB) || (TBB->pred_size() == 1))) {
+          TII->removeBranch(*ChainBB);
----------------
efriedma wrote:
> If you have a block that ends in an unconditional branch, and the successor block has exactly one predecessor and no fallthrough successors, you can merge the two blocks; that's straightforward.  But I don't see how the isSimpleBB check is related to that, and the isSuccessor() check seems redundant.
Here, I will move all the instructions of TBB to ChainBB and remove TBB, if `TBB->pred_size() > 1`, I must ensure that ChainBB has only an unconditonal branch.
For below example:
```
BB:
   XOR 3, 3, 4
   B TBB
...
ChainBB:
   LI 3, 1
   B TBB
...
TBB:
   ADD 3, 3, 4
   BLR
```
TBB has two predecessors, I cannot move all the instructions of TBB to ChainBB and remove TBB.  Of course, if I reserve TBB, I can also do the tail-duplication, but it will increase the size of program or has other side-effect, for those complex pattern, it need to be done by the `tail-duplicaton` pass. And here, I only optimize the pattern may be created by the `block-placement` pass.

But if ChainBB has only uncontional branch, I can do it.
```
BB:
   XOR 3, 3, 4
   B TBB
...
ChainBB:
   B TBB
...
TBB:
   ADD 3, 3, 4
   BLR
```
Can be optimized to:
```
BB:
   XOR 3, 3, 4
   B ChainBB
...
ChainBB:
   ADD 3, 3, 4
   BLR
```

Also `TBB->pred_size() == 1`, the only one predcessor is ChainBB, I will not care whether ChainBB has only one branch.


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  https://reviews.llvm.org/D63972/new/

https://reviews.llvm.org/D63972