[MachineOutliner][NFC] Don't add MBBs with a size < 2 to the search space

The suffix tree won't ever consider sequences with a length less than 2.

Therefore, we really ought to not even consider them in the first place.

Also add a FIXME explaining that this should be defined in terms of the size
in B of an outlined call versus the size in B of the MBB.

llvm-svn: 342688

diff --git a/llvm/lib/CodeGen/MachineOutliner.cpp b/llvm/lib/CodeGen/MachineOutliner.cpp
index 5cefaf4..4b65d97 100644 (file)
--- a/llvm/lib/CodeGen/MachineOutliner.cpp
+++ b/llvm/lib/CodeGen/MachineOutliner.cpp
@@ -1386,7 +1386,11 @@ void MachineOutliner::populateMapper(InstructionMapper &Mapper, Module &M,
     for (MachineBasicBlock &MBB : *MF) {
       // If there isn't anything in MBB, then there's no point in outlining from
       // it.
-      if (MBB.empty())
+      // If there are fewer than 2 instructions in the MBB, then it can't ever
+      // contain something worth outlining.
+      // FIXME: This should be based off of the maximum size in B of an outlined
+      // call versus the size in B of the MBB.
+      if (MBB.empty() || MBB.size() < 2)
         continue;
 
       // Check if MBB could be the target of an indirect branch. If it is, then