Add a comment about a missed opportunity.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@85635 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp
index 859b87566667921705f6fc9336b73aa7a9600bfe..8e1fb98b704c6c70ca2d9bde6a28286899b1b2d2 100644 (file)
--- a/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -1753,8 +1753,13 @@ static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI) {
 /// nodes in this block. This doesn't try to be clever about PHI nodes
 /// which differ only in the order of the incoming values, but instcombine
 /// orders them so it usually won't matter.
+///
 static bool EliminateDuplicatePHINodes(BasicBlock *BB) {
   bool Changed = false;
+  
+  // This implementation doesn't currently consider undef operands
+  // specially. Theroetically, two phis which are identical except for
+  // one having an undef where the other doesn't could be collapsed.
 
   // Map from PHI hash values to PHI nodes. If multiple PHIs have
   // the same hash value, the element is the first PHI in the