Fix a silly use-after-free issue. A much earlier version of this code

need lots of fanciness around retaining a reference to a Chain's slot in
the BlockToChain map, but that's all gone now. We can just go directly
to allocating the new chain (which will update the mapping for us) and
using it.

Somewhat gross mechanically generated test case replicates the issue
Duncan spotted when actually testing this out.

llvm-svn: 145120

1 files changed, 2 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/MachineBlockPlacement.cpp b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
index 870e24884af..55d804b31e6 100644
--- a/llvm/lib/CodeGen/MachineBlockPlacement.cpp
+++ b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
@@ -640,8 +640,8 @@ void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
   SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
   for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
     MachineBasicBlock *BB = FI;
-    BlockChain *&Chain = BlockToChain[BB];
-    Chain = new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB);
+    BlockChain *Chain
+      = new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB);
     // Also, merge any blocks which we cannot reason about and must preserve
     // the exact fallthrough behavior for.
     for (;;) {