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
MachineBlockPlacement::findBestLoopTop(MachineFunction &F,
MachineLoop &L,
const BlockFilterSet &LoopBlockSet) {
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;
BlockFrequency BestExitEdgeFreq;
MachineBasicBlock *ExitingBB = 0;
MachineBasicBlock *LoopingBB = 0;