[ADT] Notify ilist traits about in-list transfers

Summary:
Previously no client of ilist traits has needed to know about transfers
of nodes within the same list, so as an optimization, ilist doesn't call
transferNodesFromList in that case. However, now there are clients that
want to use ilist traits to cache instruction ordering information to
optimize dominance queries of instructions in the same basic block.
This change updates the existing ilist traits users to detect in-list
transfers and do nothing in that case.

After this change, we can start caching instruction ordering information
in LLVM IR data structures. There are two main ways to do that:
- by putting an order integer into the Instruction class
- by maintaining order integers in a hash table on BasicBlock

I plan to implement and measure both, but I wanted to commit this change
first to enable other out of tree ilist clients to implement this
optimization as well.

Reviewers: lattner, hfinkel, chandlerc

Subscribers: hiraditya, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D57120

llvm-svn: 351992

1 files changed, 2 insertions, 1 deletions

diff --git a/llvm/lib/IR/SymbolTableListTraitsImpl.h b/llvm/lib/IR/SymbolTableListTraitsImpl.h
index a74ac8a960f..f399c823d6f 100644
--- a/llvm/lib/IR/SymbolTableListTraitsImpl.h
+++ b/llvm/lib/IR/SymbolTableListTraitsImpl.h
@@ -83,7 +83,8 @@ void SymbolTableListTraits<ValueSubClass>::transferNodesFromList(
     SymbolTableListTraits &L2, iterator first, iterator last) {
   // We only have to do work here if transferring instructions between BBs
   ItemParentClass *NewIP = getListOwner(), *OldIP = L2.getListOwner();
-  assert(NewIP != OldIP && "Expected different list owners");
+  if (NewIP == OldIP)
+    return;
 
   // We only have to update symbol table entries if we are transferring the
   // instructions to a different symtab object...