[PowerPC] eliminate unconditional branch to the next instruction

This patch makes analyzeBranch eliminate unconditional branch to the next instruction.
After basic blocks are re-organized by optimizers, such as machine block placement, a BB may end with an unconditional branch to the next (fallthrough) BB. This patch removes such redundant branch instruction.

Differential Revision: https://reviews.llvm.org/D37730

llvm-svn: 314297

1 files changed, 14 insertions, 0 deletions

diff --git a/llvm/lib/Target/PowerPC/PPCInstrInfo.cpp b/llvm/lib/Target/PowerPC/PPCInstrInfo.cpp
index 80530ec19be..1e4cc4a3c80 100644
--- a/llvm/lib/Target/PowerPC/PPCInstrInfo.cpp
+++ b/llvm/lib/Target/PowerPC/PPCInstrInfo.cpp
@@ -494,6 +494,20 @@ bool PPCInstrInfo::analyzeBranch(MachineBasicBlock &MBB,
   if (!isUnpredicatedTerminator(*I))
     return false;
 
+  if (AllowModify) {
+    // If the BB ends with an unconditional branch to the fallthrough BB,
+    // we eliminate the branch instruction.
+    if (I->getOpcode() == PPC::B &&
+        MBB.isLayoutSuccessor(I->getOperand(0).getMBB())) {
+      I->eraseFromParent();
+
+      // We update iterator after deleting the last branch.
+      I = MBB.getLastNonDebugInstr();
+      if (I == MBB.end() || !isUnpredicatedTerminator(*I))
+        return false;
+    }
+  }
+
   // Get the last instruction in the block.
   MachineInstr &LastInst = *I;