llvm dev - Mar 2014 - [LLVMdev] leave uncodnitional fall-through branches in llc -O0

Currently SelectionDAGBuilder::visitBr() doesn't emit unconditional
fall-through branches (to next block) without reference of optimization level.
So the following LLVM IR

entry:
  br label %return, !dbg !26
return:                                           ; preds = %entry
  ret void, !dbg !27

has no generated DAG node for "br" instruction. If we are going to
generate debug line table for this branch in -O0 mode, we should leave it in
place. For example such a branch can be generated by a FE for some
"return;" statement at the end of void function or for one of
"break;" statement if switch-related exit block is next to BB of the
"break;".

This change emits all unconditional branches in -O0 mode:

--- lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -1584,8 +1584,8 @@ void SelectionDAGBuilder::visitBr(const BranchInst
     // Update machine-CFG edges.
     BrMBB->addSuccessor(Succ0MBB);
-    // If this is not a fall-through branch, emit the branch.
-    if (Succ0MBB != NextBlock)
+    // If this is not a fall-through branch or optimizations are off
+    if (Succ0MBB != NextBlock || CodeGenOpt::None == TM.getOptLevel())
       DAG.setRoot(DAG.getNode(ISD::BR, getCurSDLoc(),
                               MVT::Other, getControlRoot(),
                               DAG.getBasicBlock(Succ0MBB)));

Regards,
Daniil
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