Make a somewhat subtle change in the logic of block placement. Sometimes

the loop header has a non-loop predecessor which has been pre-fused into
its chain due to unanalyzable branches. In this case, rotating the
header into the body of the loop in order to place a loop exit at the
bottom of the loop is a Very Bad Idea as it makes the loop
non-contiguous.

I'm working on a good test case for this, but it's a bit annoynig to
craft. I should get one shortly, but I'm submitting this now so I can
begin the (lengthy) performance analysis process. An initial run of LNT
looks really, really good, but there is too much noise there for me to
trust it much.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154395 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/MachineBlockPlacement.cpp

@@ -547,6 +547,18 @@ MachineBasicBlock *
 MachineBlockPlacement::findBestLoopTop(MachineFunction &F,
                                        MachineLoop &L,
                                        const BlockFilterSet &LoopBlockSet) {
+  // We don't want to layout the loop linearly in all cases. If the loop header
+  // is just a normal basic block in the loop, we want to look for what block
+  // within the loop is the best one to layout at the top. However, if the loop
+  // header has be pre-merged into a chain due to predecessors not having
+  // analyzable branches, *and* the predecessor it is merged with is *not* part
+  // of the loop, rotating the header into the middle of the loop will create
+  // a non-contiguous range of blocks which is Very Bad. So start with the
+  // header and only rotate if safe.
+  BlockChain &HeaderChain = *BlockToChain[L.getHeader()];
+  if (!LoopBlockSet.count(*HeaderChain.begin()))
+    return L.getHeader();
+
   BlockFrequency BestExitEdgeFreq;
   MachineBasicBlock *ExitingBB = 0;
   MachineBasicBlock *LoopingBB = 0;