[GlobalISel] Don't translate br to layout successor.

MI can represent fallthrough to layout successor blocks, and our
post-isel representation uses that extensively.

We might as well use it too, to avoid translating and carrying along
unnecessary branches.

llvm-svn: 298459

1 files changed, 5 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp b/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
index b6e6fbd03d0..b3f131b5eeb 100644
--- a/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
@@ -223,10 +223,13 @@ bool IRTranslator::translateBr(const User &U, MachineIRBuilder &MIRBuilder) {
 
   const BasicBlock &BrTgt = *cast<BasicBlock>(BrInst.getSuccessor(Succ));
   MachineBasicBlock &TgtBB = getMBB(BrTgt);
-  MIRBuilder.buildBr(TgtBB);
+  MachineBasicBlock &CurBB = MIRBuilder.getMBB();
+
+  // If the unconditional target is the layout successor, fallthrough.
+  if (!CurBB.isLayoutSuccessor(&TgtBB))
+    MIRBuilder.buildBr(TgtBB);
 
   // Link successors.
-  MachineBasicBlock &CurBB = MIRBuilder.getMBB();
   for (const BasicBlock *Succ : BrInst.successors())
     CurBB.addSuccessor(&getMBB(*Succ));
   return true;