Fix another case of live_in at irreducible loop entry.

GVN was implicitly extending the liveness of an instruction across
an irreducible loop.

Fix this problem by clearing the value set at loop entries that contain
an irreducible loop.

bug:28252896
Change-Id: I68823cb88dceb4c2b4545286ba54fd0c958a48b0

1 files changed, 7 insertions, 2 deletions

diff --git a/compiler/optimizing/gvn.cc b/compiler/optimizing/gvn.cc
index d0d52bf6cc..1e86b75075 100644
--- a/compiler/optimizing/gvn.cc
+++ b/compiler/optimizing/gvn.cc
@@ -454,11 +454,16 @@ void GlobalValueNumberer::VisitBasicBlock(HBasicBlock* block) {
 
     if (!set->IsEmpty()) {
       if (block->IsLoopHeader()) {
-        if (block->GetLoopInformation()->IsIrreducible()) {
+        if (block->GetLoopInformation()->ContainsIrreducibleLoop()) {
           // To satisfy our linear scan algorithm, no instruction should flow in an irreducible
-          // loop header.
+          // loop header. We clear the set at entry of irreducible loops and any loop containing
+          // an irreducible loop, as in both cases, GVN can extend the liveness of an instruction
+          // across the irreducible loop.
+          // Note that, if we're not compiling OSR, we could still do GVN and introduce
+          // phis at irreducible loop headers. We decided it was not worth the complexity.
           set->Clear();
         } else {
+          DCHECK(!block->GetLoopInformation()->IsIrreducible());
           DCHECK_EQ(block->GetDominator(), block->GetLoopInformation()->GetPreHeader());
           set->Kill(side_effects_.GetLoopEffects(block));
         }