Quick: Fix optimizations for empty if blocks.

If a block ending with if-eqz or if-nez has the same "taken"
and "fallthrough", we cannot assume that the value has been
checked against zero in one of the succesors. This affects
the null check elimination pass as well as GVN. Refactor all
those checks to a single function in BasicBlock and check
that the "taken" and "falthrough" are different when needed.

Bug: 21614284
Change-Id: I8c6ac23e96cdaf5984786a555ebbd28110f095cb

1 files changed, 6 insertions, 12 deletions

diff --git a/compiler/dex/mir_optimization.cc b/compiler/dex/mir_optimization.cc
index 645511ed9f..727d0fd759 100644
--- a/compiler/dex/mir_optimization.cc
+++ b/compiler/dex/mir_optimization.cc
@@ -978,18 +978,12 @@ bool MIRGraph::EliminateNullChecks(BasicBlock* bb) {
       BasicBlock* pred_bb = GetBasicBlock(pred_id);
       DCHECK(pred_bb != nullptr);
       MIR* null_check_insn = nullptr;
-      if (pred_bb->block_type == kDalvikByteCode) {
-        // Check to see if predecessor had an explicit null-check.
-        MIR* last_insn = pred_bb->last_mir_insn;
-        if (last_insn != nullptr) {
-          Instruction::Code last_opcode = last_insn->dalvikInsn.opcode;
-          if ((last_opcode == Instruction::IF_EQZ && pred_bb->fall_through == bb->id) ||
-              (last_opcode == Instruction::IF_NEZ && pred_bb->taken == bb->id)) {
-            // Remember the null check insn if there's no other predecessor requiring null check.
-            if (!copied_first || !vregs_to_check->IsBitSet(last_insn->dalvikInsn.vA)) {
-              null_check_insn = last_insn;
-            }
-          }
+      // Check to see if predecessor had an explicit null-check.
+      if (pred_bb->BranchesToSuccessorOnlyIfNotZero(bb->id)) {
+        // Remember the null check insn if there's no other predecessor requiring null check.
+        if (!copied_first || !vregs_to_check->IsBitSet(pred_bb->last_mir_insn->dalvikInsn.vA)) {
+          null_check_insn = pred_bb->last_mir_insn;
+          DCHECK(null_check_insn != nullptr);
         }
       }
       if (!copied_first) {