When adding blocks to the list of those which no longer have any CFG

conflicts, we should only be adding the first block of the chain to the
list, lest we try to merge into the middle of that chain. Most of the
places we were doing this we already happened to be looking at the first
block, but there is no reason to assume that, and in some cases it was
clearly wrong.

I've added a couple of tests here. One already worked, but I like having
an explicit test for it. The other is reduced from a test case Duncan
reduced for me and used to crash. Now it is handled correctly.

llvm-svn: 145119

1 files changed, 3 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/MachineBlockPlacement.cpp b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
index 45d5af2..870e248 100644
--- a/llvm/lib/CodeGen/MachineBlockPlacement.cpp
+++ b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
@@ -315,7 +315,7 @@ void MachineBlockPlacement::markChainSuccessors(
       // This is a cross-chain edge that is within the loop, so decrement the
       // loop predecessor count of the destination chain.
       if (SuccChain.LoopPredecessors > 0 && --SuccChain.LoopPredecessors == 0)
-        BlockWorkList.push_back(*SI);
+        BlockWorkList.push_back(*SuccChain.begin());
     }
   }
 }
@@ -594,7 +594,7 @@ void MachineBlockPlacement::buildLoopChains(MachineFunction &F,
     }
 
     if (Chain.LoopPredecessors == 0)
-      BlockWorkList.push_back(*BI);
+      BlockWorkList.push_back(*Chain.begin());
   }
 
   buildChain(*L.block_begin(), LoopChain, BlockWorkList, &LoopBlockSet);
@@ -692,7 +692,7 @@ void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
     }
 
     if (Chain.LoopPredecessors == 0)
-      BlockWorkList.push_back(BB);
+      BlockWorkList.push_back(*Chain.begin());
   }
 
   BlockChain &FunctionChain = *BlockToChain[&F.front()];