ART: NullCheckElimination should converge with MIR_IGNORE_NULL_CHECK

If the MIRGraph::EliminateNullChecksAndInferTypes() function managed
to prove that some regs are non-null then it sets the flag
MIR_IGNORE_NULL_CHECK and resets this flag for all the other regs.
If some previous optimizations have already set MIR_IGNORE_NULL_CHECK
then it can be reset by EliminateNullChecksAndInferTypes. This way
NullCheckElimination discards some optimization efforts.
Optimization passes should not reset MIR_IGNORE_NULL_CHECK unless
they 100% sure NullCheck is needed.

This patch makes the NCE_TypeInference pass merge its own
calculated MIR_IGNORE_NULL_CHECK with the one came from previous
optimizations. Technically NCE_TypeInference calculates the flag
in a temporary MIR_MARK-th bit by preserving MIR_IGNORE_NULL_CHECK.
Then at the end of NCE pass MIR_MARK is or-ed with
MIR_IGNORE_NULL_CHECK.

Change-Id: Ib26997c70ecf2c158f61496dee9b1fe45c812096
Signed-off-by: Yevgeny Rouban <yevgeny.y.rouban@intel.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/compiler/dex/compiler_enums.h b/compiler/dex/compiler_enums.h
index e4003bf088..78da420339 100644
--- a/compiler/dex/compiler_enums.h
+++ b/compiler/dex/compiler_enums.h
@@ -311,7 +311,8 @@ enum MIROptimizationFlagPositions {
   kMIRCallee,                         // Instruction is inlined from callee.
   kMIRIgnoreSuspendCheck,
   kMIRDup,
-  kMIRMark,                           // Temporary node mark.
+  kMIRMark,                           // Temporary node mark can be used by
+                                      // opt passes for their private needs.
   kMIRStoreNonTemporal,
   kMIRLastMIRFlag,
 };