diff --git a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
--- a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
@@ -1398,6 +1398,11 @@
 
 // A basic block can only return if it terminates with a ReturnInst and does not
 // contain calls to noreturn functions.
+// If we could assume that something like SimplifyCFG would always run before
+// FunctionAttrs and replace code after instructions that don't return with
+// unreachable, we would only need to look at the terminator. However, that's
+// not always true; for example the new PM's CGSCC pipeline first runs
+// FunctionAttrs, then function simplification passes.
 static bool basicBlockCanReturn(BasicBlock &BB) {
   if (!isa<ReturnInst>(BB.getTerminator()))
     return false;