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

[Zhang Kang via Phabricator via llvm-commits]

ZhangKang updated this revision to Diff 209762.
ZhangKang added a comment.

This new patch will do below optimization in the end of block-placement pass:

  bb.1:
    B %bb.11
  
  bb.2:
    renamable $x3 = LI8 1
    BLR8 implicit $lr8, implicit $rm, implicit killed $x3



  bb.1:
    renamable $x3 = LI8 1
    BLR8 implicit $lr8, implicit $rm, implicit killed $x3 

Above pattern will may be created in `block-placement` pass which is after the `tail duplication` pass.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D63972/new/

https://reviews.llvm.org/D63972

Files:
  llvm/lib/CodeGen/MachineBlockPlacement.cpp
  llvm/test/CodeGen/PowerPC/block-placement.mir


Index: llvm/test/CodeGen/PowerPC/block-placement.mir
===================================================================
--- llvm/test/CodeGen/PowerPC/block-placement.mir
+++ llvm/test/CodeGen/PowerPC/block-placement.mir
@@ -209,9 +209,9 @@
     BLR8 implicit $lr8, implicit $rm, implicit killed $x3
 
   ; CHECK:      bb.5.if.else.i:
-  ; CHECK:      B %bb.11
-
-  ; CHECK:      bb.11:
+  ; CHECK-NOT:    B %bb.11
   ; CHECK:        renamable $x3 = LI8 1
   ; CHECK-NEXT:   BLR8 implicit $lr8, implicit $rm, implicit killed $x3
+
+  ; CHECK-NOT:  bb.11:
 ...
Index: llvm/lib/CodeGen/MachineBlockPlacement.cpp
===================================================================
--- llvm/lib/CodeGen/MachineBlockPlacement.cpp
+++ llvm/lib/CodeGen/MachineBlockPlacement.cpp
@@ -2730,6 +2730,30 @@
         TII->removeBranch(*ChainBB);
         TII->insertBranch(*ChainBB, FBB, TBB, Cond, dl);
         ChainBB->updateTerminator();
+      } else if (Cond.empty() && TBB && ChainBB != TBB && 
+                 !TBB->canFallThrough()) {
+        // If ChainBB unconditional branch to the TBB, and TBB has no 
+        // fallthrough predecessor and fallthrough successor, or ChainBB has
+        // only one unconditonal branch and TBB has only one predecessor. We can
+        // do the tail dupliction for ChainBB and remove TBB.
+        MachineFunction::iterator I(TBB);
+        if (((TBB == &*F->begin()) || !std::prev(I)->isSuccessor(TBB)) &&
+             (TailDup.isSimpleBB(ChainBB) || (TBB->pred_size() == 1))) {
+          TII->removeBranch(*ChainBB);
+          ChainBB->removeSuccessor(TBB);
+
+          // Update the CFG.
+          for (MachineBasicBlock::pred_iterator PI = TBB->pred_begin(),
+               PE = TBB->pred_end(); PI != PE; PI++) 
+            (*PI)->ReplaceUsesOfBlockWith(TBB, ChainBB);
+
+          for (MachineBasicBlock *Succ : TBB->successors())
+            ChainBB->addSuccessor(Succ, MBPI->getEdgeProbability(TBB, Succ));
+
+          // Move all the instructions of TBB to ChainBB.
+          ChainBB->splice(ChainBB->end(), TBB, TBB->begin(), TBB->end());
+          F->remove(TBB);
+        }
       }
     }
   }


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