[BlockPlacement] Add option to align all non-fall-through blocks.

[gberry]

Summary

This option is being added for testing purposes.

[mcrosier]

LGTM. Thanks for working on this, Geoff.

lib/CodeGen/MachineBlockPlacement.cpp

 STATISTIC(UncondBranchTakenFreq,
           "Potential frequency of taking unconditional branches");
 
 static cl::opt<unsigned> AlignAllBlock("align-all-blocks",
                                        cl::desc("Force the alignment of all "
                                                 "blocks in the function."),
                                        cl::init(0), cl::Hidden);
 
+static cl::opt<unsigned> AlignAllNonFallThruBlocks(
+    "align-all-nofallthru-blocks",
+    cl::desc("Force the alignment of all "
+             "blocks that have no fall-through predecessors (i.e. don't add "
+             "nops that are executed)."),
+    cl::init(0), cl::Hidden);
+
 static cl::opt<unsigned>
     AlignAllLoops("align-all-loops",
                   cl::desc("Force the alignment of all loops in the function."),
                   cl::init(0), cl::Hidden);
 
 // FIXME: Find a good default for this flag and remove the flag.
 static cl::opt<unsigned> ExitBlockBias(
     "block-placement-exit-block-bias",
bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
 
   BlockToChain.clear();
   ChainAllocator.DestroyAll();
 
   if (AlignAllBlock)
     // Align all of the blocks in the function to a specific alignment.
     for (MachineBasicBlock &MBB : F)
       MBB.setAlignment(AlignAllBlock);
+  else if (AlignAllNonFallThruBlocks) {
+    // Align all of the blocks that have no fall-through predecessors to a
+    // specific alignment.
+    for (auto MBI = std::next(F.begin()), MBE = F.end(); MBI != MBE; ++MBI) {
+      auto LayoutPred = std::prev(MBI);
+      if (!LayoutPred->isSuccessor(&*MBI))
+        MBI->setAlignment(AlignAllNonFallThruBlocks);
+    }
+  }
 
   // We always return true as we have no way to track whether the final order
   // differs from the original order.
   return true;
 }
 
 namespace {
 /// \brief A pass to compute block placement statistics.