optimizing: add block-scoped constructor fence merging pass

Introduce a new "Constructor Fence Redundancy Elimination" pass.
The pass currently performs local optimization only, i.e. within instructions
in the same basic block.

All constructor fences preceding a publish (e.g. store, invoke) get
merged into one instruction.

==============

OptStat#ConstructorFenceGeneratedNew:   43825
OptStat#ConstructorFenceGeneratedFinal: 17631  <+++
OptStat#ConstructorFenceRemovedLSE:     164
OptStat#ConstructorFenceRemovedPFRA:    9391
OptStat#ConstructorFenceRemovedCFRE:    16133  <---

Removes ~91.5% of the 'final' constructor fences in RitzBenchmark:

(We do not distinguish the exact reason that a fence was created, so
it's possible some "new" fences were also removed.)

==============

Test: art/test/run-test --host --optimizing 476-checker-ctor-fence-redun-elim
Bug: 36656456
Change-Id: I8020217b448ad96ce9b7640aa312ae784690ad99

1 files changed, 5 insertions, 0 deletions

diff --git a/compiler/optimizing/code_sinking.cc b/compiler/optimizing/code_sinking.cc
index 6c3a9fd6b5..b558eb17a7 100644
--- a/compiler/optimizing/code_sinking.cc
+++ b/compiler/optimizing/code_sinking.cc
@@ -64,6 +64,11 @@ static bool IsInterestingInstruction(HInstruction* instruction) {
     // A fence with "0" inputs is dead and should've been removed in a prior pass.
     DCHECK_NE(0u, ctor_fence->InputCount());
 
+    // TODO: this should be simplified to 'return true' since it's
+    // potentially pessimizing any code sinking for inlined constructors with final fields.
+    // TODO: double check that if the final field assignments are not moved,
+    // then the fence is not moved either.
+
     return ctor_fence->GetAssociatedAllocation() != nullptr;
   }