5 files changed, 83 insertions, 12 deletions

diff --git a/compiler/optimizing/code_generator.cc b/compiler/optimizing/code_generator.cc
index a164b15a04..a353d0758e 100644
--- a/compiler/optimizing/code_generator.cc
+++ b/compiler/optimizing/code_generator.cc
@@ -1114,7 +1114,7 @@ static void CheckLoopEntriesCanBeUsedForOsr(const HGraph& graph,
   for (HBasicBlock* block : graph.GetReversePostOrder()) {
     if (block->IsLoopHeader()) {
       HSuspendCheck* suspend_check = block->GetLoopInformation()->GetSuspendCheck();
-      if (!suspend_check->GetEnvironment()->IsFromInlinedInvoke()) {
+      if (suspend_check != nullptr && !suspend_check->GetEnvironment()->IsFromInlinedInvoke()) {
         loop_headers.push_back(suspend_check);
       }
     }
diff --git a/compiler/optimizing/graph_checker.cc b/compiler/optimizing/graph_checker.cc
index d1769cea0d..55a505c5ea 100644
--- a/compiler/optimizing/graph_checker.cc
+++ b/compiler/optimizing/graph_checker.cc
@@ -674,13 +674,14 @@ void GraphChecker::HandleLoop(HBasicBlock* loop_header) {
         loop_information->GetPreHeader()->GetSuccessors().size()));
   }
 
-  if (loop_information->GetSuspendCheck() == nullptr) {
-    AddError(StringPrintf(
-        "Loop with header %d does not have a suspend check.",
-        loop_header->GetBlockId()));
+  if (!GetGraph()->SuspendChecksAreAllowedToBeRemoved() &&
+      loop_information->GetSuspendCheck() == nullptr) {
+    AddError(StringPrintf("Loop with header %d does not have a suspend check.",
+                          loop_header->GetBlockId()));
   }
 
-  if (loop_information->GetSuspendCheck() != loop_header->GetFirstInstructionDisregardMoves()) {
+  if (!GetGraph()->SuspendChecksAreAllowedToBeRemoved() &&
+      loop_information->GetSuspendCheck() != loop_header->GetFirstInstructionDisregardMoves()) {
     AddError(StringPrintf(
         "Loop header %d does not have the loop suspend check as the first instruction.",
         loop_header->GetBlockId()));
diff --git a/compiler/optimizing/loop_optimization.cc b/compiler/optimizing/loop_optimization.cc
index 02ee4ec057..cd054822cd 100644
--- a/compiler/optimizing/loop_optimization.cc
+++ b/compiler/optimizing/loop_optimization.cc
@@ -636,6 +636,47 @@ bool HLoopOptimization::TraverseLoopsInnerToOuter(LoopNode* node) {
 }
 
 //
+// This optimization applies to loops with plain simple operations
+// (I.e. no calls to java code or runtime) with a known small trip_count * instr_count
+// value.
+//
+bool HLoopOptimization::TryToRemoveSuspendCheckFromLoopHeader(LoopAnalysisInfo* analysis_info,
+                                                              bool generate_code) {
+  if (!graph_->SuspendChecksAreAllowedToBeRemoved()) {
+    return false;
+  }
+
+  int64_t trip_count = analysis_info->GetTripCount();
+
+  if (trip_count == LoopAnalysisInfo::kUnknownTripCount) {
+    return false;
+  }
+
+  int64_t instruction_count = analysis_info->GetNumberOfInstructions();
+  int64_t total_instruction_count = trip_count * instruction_count;
+
+  // The inclusion of the HasInstructionsPreventingScalarOpts() prevents this
+  // optimization from being applied to loops that have calls.
+  bool can_optimize =
+      total_instruction_count <= HLoopOptimization::kMaxTotalInstRemoveSuspendCheck &&
+      !analysis_info->HasInstructionsPreventingScalarOpts();
+
+  if (!can_optimize) {
+    return false;
+  }
+
+  if (generate_code) {
+    HLoopInformation* loop_info = analysis_info->GetLoopInfo();
+    HBasicBlock* header = loop_info->GetHeader();
+    HInstruction* instruction = header->GetLoopInformation()->GetSuspendCheck();
+    header->RemoveInstruction(instruction);
+    loop_info->SetSuspendCheck(nullptr);
+  }
+
+  return true;
+}
+
+//
 // Optimization.
 //
 
@@ -779,7 +820,7 @@ bool HLoopOptimization::TryOptimizeInnerLoopFinite(LoopNode* node) {
 }
 
 bool HLoopOptimization::OptimizeInnerLoop(LoopNode* node) {
-  return TryOptimizeInnerLoopFinite(node) || TryPeelingAndUnrolling(node);
+  return TryOptimizeInnerLoopFinite(node) || TryLoopScalarOpts(node);
 }
 
 
@@ -885,7 +926,7 @@ bool HLoopOptimization::TryFullUnrolling(LoopAnalysisInfo* analysis_info, bool g
   return true;
 }
 
-bool HLoopOptimization::TryPeelingAndUnrolling(LoopNode* node) {
+bool HLoopOptimization::TryLoopScalarOpts(LoopNode* node) {
   HLoopInformation* loop_info = node->loop_info;
   int64_t trip_count = LoopAnalysis::GetLoopTripCount(loop_info, &induction_range_);
   LoopAnalysisInfo analysis_info(loop_info);
@@ -898,10 +939,16 @@ bool HLoopOptimization::TryPeelingAndUnrolling(LoopNode* node) {
 
   if (!TryFullUnrolling(&analysis_info, /*generate_code*/ false) &&
       !TryPeelingForLoopInvariantExitsElimination(&analysis_info, /*generate_code*/ false) &&
-      !TryUnrollingForBranchPenaltyReduction(&analysis_info, /*generate_code*/ false)) {
+      !TryUnrollingForBranchPenaltyReduction(&analysis_info, /*generate_code*/ false) &&
+      !TryToRemoveSuspendCheckFromLoopHeader(&analysis_info, /*generate_code*/ false)) {
     return false;
   }
 
+  // Try the suspend check removal even for non-clonable loops. Also this
+  // optimization doesn't interfere with other scalar loop optimizations so it can
+  // be done prior to them.
+  bool removed_suspend_check = TryToRemoveSuspendCheckFromLoopHeader(&analysis_info);
+
   // Run 'IsLoopClonable' the last as it might be time-consuming.
   if (!LoopClonerHelper::IsLoopClonable(loop_info)) {
     return false;
@@ -909,7 +956,7 @@ bool HLoopOptimization::TryPeelingAndUnrolling(LoopNode* node) {
 
   return TryFullUnrolling(&analysis_info) ||
          TryPeelingForLoopInvariantExitsElimination(&analysis_info) ||
-         TryUnrollingForBranchPenaltyReduction(&analysis_info);
+         TryUnrollingForBranchPenaltyReduction(&analysis_info) || removed_suspend_check;
 }
 
 //
diff --git a/compiler/optimizing/loop_optimization.h b/compiler/optimizing/loop_optimization.h
index d3583ed8a6..0ca16b543d 100644
--- a/compiler/optimizing/loop_optimization.h
+++ b/compiler/optimizing/loop_optimization.h
@@ -47,6 +47,11 @@ class HLoopOptimization : public HOptimization {
 
   static constexpr const char* kLoopOptimizationPassName = "loop_optimization";
 
+  // The maximum number of total instructions (trip_count * instruction_count),
+  // where the optimization of removing SuspendChecks from the loop header could
+  // be performed.
+  static constexpr int64_t kMaxTotalInstRemoveSuspendCheck = 128;
+
  private:
   /**
    * A single loop inside the loop hierarchy representation.
@@ -163,8 +168,19 @@ class HLoopOptimization : public HOptimization {
   // should be actually applied.
   bool TryFullUnrolling(LoopAnalysisInfo* analysis_info, bool generate_code = true);
 
-  // Tries to apply scalar loop peeling and unrolling.
-  bool TryPeelingAndUnrolling(LoopNode* node);
+  // Tries to remove SuspendCheck for plain loops with a low trip count. The
+  // SuspendCheck in the codegen makes sure that the thread can be interrupted
+  // during execution for GC. Not being able to do so might decrease the
+  // responsiveness of GC when a very long loop or a long recursion is being
+  // executed. However, for plain loops with a small trip count, the removal of
+  // SuspendCheck should not affect the GC's responsiveness by a large margin.
+  // Consequently, since the thread won't be interrupted for plain loops, it is
+  // assumed that the performance might increase by removing SuspendCheck.
+  bool TryToRemoveSuspendCheckFromLoopHeader(LoopAnalysisInfo* analysis_info,
+                                             bool generate_code = true);
+
+  // Tries to apply scalar loop optimizations.
+  bool TryLoopScalarOpts(LoopNode* node);
 
   //
   // Vectorization analysis and synthesis.
diff --git a/compiler/optimizing/nodes.h b/compiler/optimizing/nodes.h
index 69ca520f5a..d6b3726fe1 100644
--- a/compiler/optimizing/nodes.h
+++ b/compiler/optimizing/nodes.h
@@ -675,6 +675,13 @@ class HGraph : public ArenaObject<kArenaAllocGraph> {
     return cha_single_implementation_list_;
   }
 
+  // In case of OSR we intend to use SuspendChecks as an entry point to the
+  // function; for debuggable graphs we might deoptimize to interpreter from
+  // SuspendChecks. In these cases we shouldn't remove them.
+  bool SuspendChecksAreAllowedToBeRemoved() const {
+    return !IsDebuggable() && !IsCompilingOsr();
+  }
+
   void AddCHASingleImplementationDependency(ArtMethod* method) {
     cha_single_implementation_list_.insert(method);
   }