Merge rust-lang#29

29: In SelectionDAG lowering, always emit a branch. r=ptersilie a=vext01



Co-authored-by: Edd Barrett <vext01@gmail.com>

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -135,6 +135,12 @@ static cl::opt<unsigned> SwitchPeelThreshold(
              "switch statement. A value greater than 100 will void this "
              "optimization"));
 
+static cl::opt<bool> YkNoFallThrough(
+    "yk-no-fallthrough", cl::Hidden, cl::init(false),
+    cl::desc("Always emit a branch even if fallthrough is possible. This "
+             "is required for the yk JIT, so that the machine IR has the "
+             "same block structure as the high-level IR"));
+
 // Limit the width of DAG chains. This is important in general to prevent
 // DAG-based analysis from blowing up. For example, alias analysis and
 // load clustering may not complete in reasonable time. It is difficult to
@@ -2348,7 +2354,8 @@ void SelectionDAGBuilder::visitBr(const BranchInst &I) {
 
     // If this is not a fall-through branch or optimizations are switched off,
     // emit the branch.
-    if (Succ0MBB != NextBlock(BrMBB) || TM.getOptLevel() == CodeGenOpt::None)
+    if ((YkNoFallThrough) || (Succ0MBB != NextBlock(BrMBB) ||
+        TM.getOptLevel() == CodeGenOpt::None))
       DAG.setRoot(DAG.getNode(ISD::BR, getCurSDLoc(),
                               MVT::Other, getControlRoot(),
                               DAG.getBasicBlock(Succ0MBB)));