ART: Implement loop full unrolling.

Performs whole loop unrolling for small loops with small
trip count to eliminate the loop check overhead, to have
more opportunities for inter-iteration optimizations.

caffeinemark/FloatAtom: 1.2x performance on arm64 Cortex-A57.

Test: 530-checker-peel-unroll.
Test: test-art-host, test-art-target.
Change-Id: Idf3fe3cb611376935d176c60db8c49907222e28a

1 files changed, 10 insertions, 0 deletions

diff --git a/compiler/optimizing/loop_analysis.cc b/compiler/optimizing/loop_analysis.cc
index efb23e7d3e..d355cedb35 100644
--- a/compiler/optimizing/loop_analysis.cc
+++ b/compiler/optimizing/loop_analysis.cc
@@ -84,6 +84,8 @@ class ArchDefaultLoopHelper : public ArchNoOptsLoopHelper {
   static constexpr uint32_t kScalarHeuristicMaxBodySizeInstr = 17;
   // Loop's maximum basic block count. Loops with higher count will not be peeled/unrolled.
   static constexpr uint32_t kScalarHeuristicMaxBodySizeBlocks = 6;
+  // Maximum number of instructions to be created as a result of full unrolling.
+  static constexpr uint32_t kScalarHeuristicFullyUnrolledMaxInstrThreshold = 35;
 
   bool IsLoopNonBeneficialForScalarOpts(LoopAnalysisInfo* analysis_info) const OVERRIDE {
     return analysis_info->HasLongTypeInstructions() ||
@@ -108,6 +110,14 @@ class ArchDefaultLoopHelper : public ArchNoOptsLoopHelper {
 
   bool IsLoopPeelingEnabled() const OVERRIDE { return true; }
 
+  bool IsFullUnrollingBeneficial(LoopAnalysisInfo* analysis_info) const OVERRIDE {
+    int64_t trip_count = analysis_info->GetTripCount();
+    // We assume that trip count is known.
+    DCHECK_NE(trip_count, LoopAnalysisInfo::kUnknownTripCount);
+    size_t instr_num = analysis_info->GetNumberOfInstructions();
+    return (trip_count * instr_num < kScalarHeuristicFullyUnrolledMaxInstrThreshold);
+  }
+
  protected:
   bool IsLoopTooBig(LoopAnalysisInfo* loop_analysis_info,
                     size_t instr_threshold,