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

7 files changed, 56 insertions, 34 deletions

diff --git a/compiler/dex/global_value_numbering.cc b/compiler/dex/global_value_numbering.cc
index e2b99871c8..94ba4fad2a 100644
--- a/compiler/dex/global_value_numbering.cc
+++ b/compiler/dex/global_value_numbering.cc
@@ -160,20 +160,10 @@ uint16_t GlobalValueNumbering::GetArrayLocation(uint16_t base, uint16_t index) {
   return location;
 }
 
-bool GlobalValueNumbering::HasNullCheckLastInsn(const BasicBlock* pred_bb,
-                                                BasicBlockId succ_id) {
-  if (pred_bb->block_type != kDalvikByteCode || pred_bb->last_mir_insn == nullptr) {
-    return false;
-  }
-  Instruction::Code last_opcode = pred_bb->last_mir_insn->dalvikInsn.opcode;
-  return ((last_opcode == Instruction::IF_EQZ && pred_bb->fall_through == succ_id) ||
-      (last_opcode == Instruction::IF_NEZ && pred_bb->taken == succ_id));
-}
-
 bool GlobalValueNumbering::NullCheckedInAllPredecessors(
     const ScopedArenaVector<uint16_t>& merge_names) const {
   // Implicit parameters:
-  //   - *work_lvn: the LVN for which we're checking predecessors.
+  //   - *work_lvn_: the LVN for which we're checking predecessors.
   //   - merge_lvns_: the predecessor LVNs.
   DCHECK_EQ(merge_lvns_.size(), merge_names.size());
   for (size_t i = 0, size = merge_lvns_.size(); i != size; ++i) {
@@ -198,7 +188,7 @@ bool GlobalValueNumbering::NullCheckedInAllPredecessors(
 bool GlobalValueNumbering::DivZeroCheckedInAllPredecessors(
     const ScopedArenaVector<uint16_t>& merge_names) const {
   // Implicit parameters:
-  //   - *work_lvn: the LVN for which we're checking predecessors.
+  //   - *work_lvn_: the LVN for which we're checking predecessors.
   //   - merge_lvns_: the predecessor LVNs.
   DCHECK_EQ(merge_lvns_.size(), merge_names.size());
   for (size_t i = 0, size = merge_lvns_.size(); i != size; ++i) {
@@ -217,15 +207,11 @@ bool GlobalValueNumbering::IsBlockEnteredOnTrue(uint16_t cond, BasicBlockId bb_i
   if (bb->predecessors.size() == 1u) {
     BasicBlockId pred_id = bb->predecessors[0];
     BasicBlock* pred_bb = mir_graph_->GetBasicBlock(pred_id);
-    if (pred_bb->last_mir_insn != nullptr) {
-      Instruction::Code opcode = pred_bb->last_mir_insn->dalvikInsn.opcode;
-      if ((opcode == Instruction::IF_NEZ && pred_bb->taken == bb_id) ||
-          (opcode == Instruction::IF_EQZ && pred_bb->fall_through == bb_id)) {
-        DCHECK(lvns_[pred_id] != nullptr);
-        uint16_t operand = lvns_[pred_id]->GetSregValue(pred_bb->last_mir_insn->ssa_rep->uses[0]);
-        if (operand == cond) {
-          return true;
-        }
+    if (pred_bb->BranchesToSuccessorOnlyIfNotZero(bb_id)) {
+      DCHECK(lvns_[pred_id] != nullptr);
+      uint16_t operand = lvns_[pred_id]->GetSregValue(pred_bb->last_mir_insn->ssa_rep->uses[0]);
+      if (operand == cond) {
+        return true;
       }
     }
   }
diff --git a/compiler/dex/global_value_numbering.h b/compiler/dex/global_value_numbering.h
index bd2f187d17..c514f75dcc 100644
--- a/compiler/dex/global_value_numbering.h
+++ b/compiler/dex/global_value_numbering.h
@@ -194,7 +194,9 @@ class GlobalValueNumbering : public DeletableArenaObject<kArenaAllocMisc> {
     return mir_graph_->GetBasicBlock(bb_id);
   }
 
-  static bool HasNullCheckLastInsn(const BasicBlock* pred_bb, BasicBlockId succ_id);
+  static bool HasNullCheckLastInsn(const BasicBlock* pred_bb, BasicBlockId succ_id) {
+    return pred_bb->BranchesToSuccessorOnlyIfNotZero(succ_id);
+  }
 
   bool NullCheckedInAllPredecessors(const ScopedArenaVector<uint16_t>& merge_names) const;
 
diff --git a/compiler/dex/mir_graph.h b/compiler/dex/mir_graph.h
index f038397e1e..dbe906280f 100644
--- a/compiler/dex/mir_graph.h
+++ b/compiler/dex/mir_graph.h
@@ -452,6 +452,21 @@ class BasicBlock : public DeletableArenaObject<kArenaAllocBB> {
   MIR* GetFirstNonPhiInsn();
 
   /**
+   * @brief Checks whether the block ends with if-nez or if-eqz that branches to
+   *        the given successor only if the register in not zero.
+   */
+  bool BranchesToSuccessorOnlyIfNotZero(BasicBlockId succ_id) const {
+    if (last_mir_insn == nullptr) {
+      return false;
+    }
+    Instruction::Code last_opcode = last_mir_insn->dalvikInsn.opcode;
+    return ((last_opcode == Instruction::IF_EQZ && fall_through == succ_id) ||
+        (last_opcode == Instruction::IF_NEZ && taken == succ_id)) &&
+        // Make sure the other successor isn't the same (empty if), b/21614284.
+        (fall_through != taken);
+  }
+
+  /**
    * @brief Used to obtain the next MIR that follows unconditionally.
    * @details The implementation does not guarantee that a MIR does not
    * follow even if this method returns nullptr.
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) {
diff --git a/test/800-smali/expected.txt b/test/800-smali/expected.txt
index 7be78b7160..6c00c82983 100644
--- a/test/800-smali/expected.txt
+++ b/test/800-smali/expected.txt
@@ -19,4 +19,5 @@ b/20224106
 b/17410612
 b/21863767
 b/21873167
+b/21614284
 Done!
diff --git a/test/800-smali/smali/b_21614284.smali b/test/800-smali/smali/b_21614284.smali
new file mode 100644
index 0000000000..3cb1bd0ce2
--- /dev/null
+++ b/test/800-smali/smali/b_21614284.smali
@@ -0,0 +1,22 @@
+.class public LB21614284;
+.super Ljava/lang/Object;
+
+.field private a:I
+
+.method public constructor <init>()V
+    .registers 2
+    invoke-direct {p0}, Ljava/lang/Object;-><init>()V
+    const v0, 42
+    iput v0, p0, LB21614284;->a:I
+    return-void
+.end method
+
+.method public static test(LB21614284;)I
+    .registers 2
+    # Empty if, testing p0.
+    if-nez p0, :label
+    :label
+    # p0 still needs a null check.
+    iget v0, p0, LB21614284;->a:I
+    return v0
+.end method
diff --git a/test/800-smali/src/Main.java b/test/800-smali/src/Main.java
index 2180186ac5..ab4457e919 100644
--- a/test/800-smali/src/Main.java
+++ b/test/800-smali/src/Main.java
@@ -86,6 +86,8 @@ public class Main {
         testCases.add(new TestCase("b/21863767", "B21863767", "run", null, null,
                 null));
         testCases.add(new TestCase("b/21873167", "B21873167", "test", null, null, null));
+        testCases.add(new TestCase("b/21614284", "B21614284", "test", new Object[] { null },
+            new NullPointerException(), null));
     }
 
     public void runTests() {