[MachineCombiner] Fix initialisation of LastUpdate for incremental update.

Summary:
Fixes a bogus iterator resulting from the removal of a block's first instruction at the point that incremental update is enabled.

Patch by Paul Walker.

Reviewers: fhahn, Gerolf, efriedma, MatzeB

Reviewed By: fhahn

Subscribers: aemerson, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D38734

llvm-svn: 315502

1 files changed, 4 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/MachineCombiner.cpp b/llvm/lib/CodeGen/MachineCombiner.cpp
index d563370dd4f..3ffef682334 100644
--- a/llvm/lib/CodeGen/MachineCombiner.cpp
+++ b/llvm/lib/CodeGen/MachineCombiner.cpp
@@ -415,7 +415,7 @@ bool MachineCombiner::combineInstructions(MachineBasicBlock *MBB) {
 
   bool IncrementalUpdate = false;
   auto BlockIter = MBB->begin();
-  auto LastUpdate = BlockIter;
+  decltype(BlockIter) LastUpdate;
   // Check if the block is in a loop.
   const MachineLoop *ML = MLI->getLoopFor(MBB);
   if (!MinInstr)
@@ -503,9 +503,11 @@ bool MachineCombiner::combineInstructions(MachineBasicBlock *MBB) {
                                     InstrIdxForVirtReg, P,
                                     !IncrementalUpdate) &&
             preservesResourceLen(MBB, BlockTrace, InsInstrs, DelInstrs)) {
-          if (MBB->size() > inc_threshold)
+          if (MBB->size() > inc_threshold) {
             // Use incremental depth updates for basic blocks above treshold
             IncrementalUpdate = true;
+            LastUpdate = BlockIter;
+          }
 
           insertDeleteInstructions(MBB, MI, InsInstrs, DelInstrs, MinInstr,
                                    RegUnits, IncrementalUpdate);