No SuspendChecks when branching to return block

The return basic block is usually placed at the beginning of the DEX
file, making the branches to it back edges where the Optimizing's
graph builder places SuspendCheck instructions, only to be removed
later by the instruction_simplifier pass. Since huge auto-generated
methods tend to contain hundreds/thousands of these, this patch
recognizes the pattern and prevents builder from generating the
redundant check in the first place.

Change-Id: I065a3c2f71964b8fc2e53dc20730ba42938b78a1

2 files changed, 32 insertions, 22 deletions

diff --git a/compiler/optimizing/builder.cc b/compiler/optimizing/builder.cc
index 4b97a621a..eed56146e 100644
--- a/compiler/optimizing/builder.cc
+++ b/compiler/optimizing/builder.cc
@@ -190,37 +190,37 @@ void HGraphBuilder::InitializeParameters(uint16_t number_of_parameters) {
 template<typename T>
 void HGraphBuilder::If_22t(const Instruction& instruction, uint32_t dex_pc) {
   int32_t target_offset = instruction.GetTargetOffset();
-  PotentiallyAddSuspendCheck(target_offset, dex_pc);
+  HBasicBlock* branch_target = FindBlockStartingAt(dex_pc + target_offset);
+  HBasicBlock* fallthrough_target = FindBlockStartingAt(dex_pc + instruction.SizeInCodeUnits());
+  DCHECK(branch_target != nullptr);
+  DCHECK(fallthrough_target != nullptr);
+  PotentiallyAddSuspendCheck(branch_target, dex_pc);
   HInstruction* first = LoadLocal(instruction.VRegA(), Primitive::kPrimInt);
   HInstruction* second = LoadLocal(instruction.VRegB(), Primitive::kPrimInt);
   T* comparison = new (arena_) T(first, second);
   current_block_->AddInstruction(comparison);
   HInstruction* ifinst = new (arena_) HIf(comparison);
   current_block_->AddInstruction(ifinst);
-  HBasicBlock* target = FindBlockStartingAt(dex_pc + target_offset);
-  DCHECK(target != nullptr);
-  current_block_->AddSuccessor(target);
-  target = FindBlockStartingAt(dex_pc + instruction.SizeInCodeUnits());
-  DCHECK(target != nullptr);
-  current_block_->AddSuccessor(target);
+  current_block_->AddSuccessor(branch_target);
+  current_block_->AddSuccessor(fallthrough_target);
   current_block_ = nullptr;
 }
 
 template<typename T>
 void HGraphBuilder::If_21t(const Instruction& instruction, uint32_t dex_pc) {
   int32_t target_offset = instruction.GetTargetOffset();
-  PotentiallyAddSuspendCheck(target_offset, dex_pc);
+  HBasicBlock* branch_target = FindBlockStartingAt(dex_pc + target_offset);
+  HBasicBlock* fallthrough_target = FindBlockStartingAt(dex_pc + instruction.SizeInCodeUnits());
+  DCHECK(branch_target != nullptr);
+  DCHECK(fallthrough_target != nullptr);
+  PotentiallyAddSuspendCheck(branch_target, dex_pc);
   HInstruction* value = LoadLocal(instruction.VRegA(), Primitive::kPrimInt);
   T* comparison = new (arena_) T(value, GetIntConstant(0));
   current_block_->AddInstruction(comparison);
   HInstruction* ifinst = new (arena_) HIf(comparison);
   current_block_->AddInstruction(ifinst);
-  HBasicBlock* target = FindBlockStartingAt(dex_pc + target_offset);
-  DCHECK(target != nullptr);
-  current_block_->AddSuccessor(target);
-  target = FindBlockStartingAt(dex_pc + instruction.SizeInCodeUnits());
-  DCHECK(target != nullptr);
-  current_block_->AddSuccessor(target);
+  current_block_->AddSuccessor(branch_target);
+  current_block_->AddSuccessor(fallthrough_target);
   current_block_ = nullptr;
 }
 
@@ -1034,7 +1034,9 @@ void HGraphBuilder::BuildSwitchCaseHelper(const Instruction& instruction, size_t
                                           bool is_last_case, const SwitchTable& table,
                                           HInstruction* value, int32_t case_value_int,
                                           int32_t target_offset, uint32_t dex_pc) {
-  PotentiallyAddSuspendCheck(target_offset, dex_pc);
+  HBasicBlock* case_target = FindBlockStartingAt(dex_pc + target_offset);
+  DCHECK(case_target != nullptr);
+  PotentiallyAddSuspendCheck(case_target, dex_pc);
 
   // The current case's value.
   HInstruction* this_case_value = GetIntConstant(case_value_int);
@@ -1046,8 +1048,6 @@ void HGraphBuilder::BuildSwitchCaseHelper(const Instruction& instruction, size_t
   current_block_->AddInstruction(ifinst);
 
   // Case hit: use the target offset to determine where to go.
-  HBasicBlock* case_target = FindBlockStartingAt(dex_pc + target_offset);
-  DCHECK(case_target != nullptr);
   current_block_->AddSuccessor(case_target);
 
   // Case miss: go to the next case (or default fall-through).
@@ -1072,10 +1072,20 @@ void HGraphBuilder::BuildSwitchCaseHelper(const Instruction& instruction, size_t
   }
 }
 
-void HGraphBuilder::PotentiallyAddSuspendCheck(int32_t target_offset, uint32_t dex_pc) {
+void HGraphBuilder::PotentiallyAddSuspendCheck(HBasicBlock* target, uint32_t dex_pc) {
+  int32_t target_offset = target->GetDexPc() - dex_pc;
   if (target_offset <= 0) {
-    // Unconditionnally add a suspend check to backward branches. We can remove
-    // them after we recognize loops in the graph.
+    // DX generates back edges to the first encountered return. We can save
+    // time of later passes by not adding redundant suspend checks.
+    if (target_offset != 0) {
+      DCHECK(target->GetLastInstruction() != nullptr);
+      if (target->GetLastInstruction()->IsReturn()) {
+        return;
+      }
+    }
+
+    // Add a suspend check to backward branches which may potentially loop. We
+    // can remove them after we recognize loops in the graph.
     current_block_->AddInstruction(new (arena_) HSuspendCheck(dex_pc));
   }
 }
@@ -1197,9 +1207,9 @@ bool HGraphBuilder::AnalyzeDexInstruction(const Instruction& instruction, uint32
     case Instruction::GOTO_16:
     case Instruction::GOTO_32: {
       int32_t offset = instruction.GetTargetOffset();
-      PotentiallyAddSuspendCheck(offset, dex_pc);
       HBasicBlock* target = FindBlockStartingAt(offset + dex_pc);
       DCHECK(target != nullptr);
+      PotentiallyAddSuspendCheck(target, dex_pc);
       current_block_->AddInstruction(new (arena_) HGoto());
       current_block_->AddSuccessor(target);
       current_block_ = nullptr;
diff --git a/compiler/optimizing/builder.h b/compiler/optimizing/builder.h
index 592db2348..8ee27a1b3 100644
--- a/compiler/optimizing/builder.h
+++ b/compiler/optimizing/builder.h
@@ -106,7 +106,7 @@ class HGraphBuilder : public ValueObject {
   HLocal* GetLocalAt(int register_index) const;
   void UpdateLocal(int register_index, HInstruction* instruction) const;
   HInstruction* LoadLocal(int register_index, Primitive::Type type) const;
-  void PotentiallyAddSuspendCheck(int32_t target_offset, uint32_t dex_pc);
+  void PotentiallyAddSuspendCheck(HBasicBlock* target, uint32_t dex_pc);
   void InitializeParameters(uint16_t number_of_parameters);
   bool NeedsAccessCheck(uint32_t type_index) const;