Revert r368509 "[CodeGen] Do the Simple Early Return in block-placement pass to optimize the blocks"

> In `block-placement` pass, it will create some patterns for unconditional we can do the simple early retrun.
> But the `early-ret` pass is before `block-placement`, we don't want to run it again.
> This patch is to do the simple early return to optimize the blocks at the last of `block-placement`.
>
> Reviewed By: efriedma
>
> Differential Revision: https://reviews.llvm.org/D63972

This also revertes follow-ups r368514 and r368532.

llvm-svn: 368560

llvm/lib/CodeGen/MachineBlockPlacement.cpp

@@ -2755,7 +2755,6 @@ void MachineBlockPlacement::optimizeBranches() {
   // cannot because all branches may not be analyzable.
   // E.g., the target may be able to remove an unconditional branch to
   // a fallthrough when it occurs after predicated terminators.
-  SmallVector<MachineBasicBlock*, 4> EmptyBB;
   for (MachineBasicBlock *ChainBB : FunctionChain) {
     Cond.clear();
     MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
@@ -2775,42 +2774,9 @@ void MachineBlockPlacement::optimizeBranches() {
         TII->removeBranch(*ChainBB);
         TII->insertBranch(*ChainBB, FBB, TBB, Cond, dl);
         ChainBB->updateTerminator();
-      } else if (Cond.empty() && TBB && ChainBB != TBB &&
-                 !TBB->canFallThrough()) {
-        // When ChainBB is unconditional branch to the TBB, and TBB has no
-        // fallthrough predecessor and fallthrough successor, try to merge
-        // ChainBB and TBB. This is legal under the one of following conditions:
-        // 1. ChainBB is empty except for an unconditional branch.
-        // 2. TBB has only one predecessor.
-        MachineFunction::iterator I(TBB);
-        if (((TBB == &*F->begin()) || !std::prev(I)->canFallThrough()) &&
-             (TailDup.isSimpleBB(ChainBB) || (TBB->pred_size() == 1))) {
-          TII->removeBranch(*ChainBB);
-          ChainBB->removeSuccessor(TBB);
-
-          // Update the CFG.
-          while (!TBB->pred_empty()) {
-            MachineBasicBlock *Pred = *(TBB->pred_end()-1);
-            Pred->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());
-          EmptyBB.push_back(TBB);
-        }
       }
     }
   }
-
-  for (auto BB: EmptyBB) {
-    MLI->removeBlock(BB);
-    FunctionChain.remove(BB);
-    BlockToChain.erase(BB);
-    F->erase(BB);
-  }
 }
 
 void MachineBlockPlacement::alignBlocks() {
@@ -3130,9 +3096,6 @@ bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &MF) {
     }
   }
 
-  // optimizeBranches() may change the blocks, but we haven't updated the
-  // post-dominator tree. Because the post-dominator tree won't be used after
-  // this function and this pass don't preserve the post-dominator tree.
   optimizeBranches();
   alignBlocks();
 

llvm/test/CodeGen/PowerPC/block-placement-1.mir

@@ -299,8 +299,14 @@ body:             |
   bb.11.unreachable:
 
   ; CHECK:      bb.4.catch4:
-  ; CHECK:        successors: %bb.6(0x00000800)
+  ; CHECK:        successors: %bb.11(0x7ffff800), %bb.6(0x00000800)
+  ; CHECK:        B %bb.11
 
   ; CHECK:      bb.1.for.body (align 4):
-  ; CHECK:        successors: %bb.3(0x00000800)
+  ; CHECK:        successors: %bb.2(0x7ffff800), %bb.3(0x00000800)
+  ; CHECK:        B %bb.2
+
+  ; CHECK:      bb.2..noexc:
+
+  ; CHECK:      bb.11.unreachable: 
 ...

llvm/test/CodeGen/PowerPC/block-placement.mir

@@ -209,10 +209,14 @@ body:             |
     BLR8 implicit $lr8, implicit $rm, implicit killed $x3
 
   ; CHECK:      bb.5.if.else.i:
-  ; CHECK-NEXT:   renamable $x3 = LI8 1
+  ; CHECK:        successors: %bb.11(0x80000000)
-  ; CHECK-NEXT:   BLR8 implicit $lr8, implicit $rm, implicit killed $x3
+  ; CHECK:        B %bb.11
 
   ; CHECK:      bb.8.while.body.i (align 4):
-  ; CHECK:        successors: %bb.5(0x04000000), %bb.9(0x7c000000)
+  ; CHECK:        successors: %bb.11(0x04000000), %bb.9(0x7c000000)
-  ; CHECK:        BCC 76, killed renamable $cr0, %bb.5
+  ; CHECK:        BCC 76, killed renamable $cr0, %bb.11
+
+  ; CHECK:      bb.11:
+  ; CHECK:        renamable $x3 = LI8 1
+  ; CHECK-NEXT:   BLR8 implicit $lr8, implicit $rm, implicit killed $x3
 ...