Fix a crash in block placement due to an inner loop that happened to be

reversed in the function's original ordering, and we happened to
encounter it while handling an outer unnatural CFG structure.

Thanks to the test case reduced from GCC's source by Benjamin Kramer.
This may also fix a crasher in gzip that Duncan reduced for me, but
I haven't yet gotten to testing that one.

llvm-svn: 145094

1 files changed, 4 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/MachineBlockPlacement.cpp b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
index 4cbc94e0b7d..0d15e8a726f 100644
--- a/llvm/lib/CodeGen/MachineBlockPlacement.cpp
+++ b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
@@ -488,7 +488,10 @@ MachineBasicBlock *MachineBlockPlacement::getFirstUnplacedBlock(
       continue;
     if (BlockToChain[I] != &PlacedChain) {
       PrevUnplacedBlockIt = I;
-      return I;
+      // Now select the head of the chain to which the unplaced block belongs
+      // as the block to place. This will force the entire chain to be placed,
+      // and satisfies the requirements of merging chains.
+      return *BlockToChain[I]->begin();
     }
   }
   return 0;