/*
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* Copyright (C) 2018 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#include "loop_analysis.h"
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#include "base/bit_vector-inl.h"
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#include "induction_var_range.h"
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namespace art {
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void LoopAnalysis::CalculateLoopBasicProperties(HLoopInformation* loop_info,
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LoopAnalysisInfo* analysis_results,
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int64_t trip_count) {
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analysis_results->trip_count_ = trip_count;
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for (HBlocksInLoopIterator block_it(*loop_info);
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!block_it.Done();
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block_it.Advance()) {
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HBasicBlock* block = block_it.Current();
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// Check whether one of the successor is loop exit.
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for (HBasicBlock* successor : block->GetSuccessors()) {
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if (!loop_info->Contains(*successor)) {
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analysis_results->exits_num_++;
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// We track number of invariant loop exits which correspond to HIf instruction and
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// can be eliminated by loop peeling; other control flow instruction are ignored and will
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// not cause loop peeling to happen as they either cannot be inside a loop, or by
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// definition cannot be loop exits (unconditional instructions), or are not beneficial for
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// the optimization.
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HIf* hif = block->GetLastInstruction()->AsIf();
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if (hif != nullptr && !loop_info->Contains(*hif->InputAt(0)->GetBlock())) {
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analysis_results->invariant_exits_num_++;
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}
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}
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}
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for (HInstructionIterator it(block->GetInstructions()); !it.Done(); it.Advance()) {
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HInstruction* instruction = it.Current();
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if (it.Current()->GetType() == DataType::Type::kInt64) {
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analysis_results->has_long_type_instructions_ = true;
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}
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if (MakesScalarPeelingUnrollingNonBeneficial(instruction)) {
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analysis_results->has_instructions_preventing_scalar_peeling_ = true;
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analysis_results->has_instructions_preventing_scalar_unrolling_ = true;
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}
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analysis_results->instr_num_++;
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}
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analysis_results->bb_num_++;
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}
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}
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int64_t LoopAnalysis::GetLoopTripCount(HLoopInformation* loop_info,
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const InductionVarRange* induction_range) {
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int64_t trip_count;
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if (!induction_range->HasKnownTripCount(loop_info, &trip_count)) {
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trip_count = LoopAnalysisInfo::kUnknownTripCount;
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}
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return trip_count;
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}
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// Default implementation of loop helper; used for all targets unless a custom implementation
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// is provided. Enables scalar loop peeling and unrolling with the most conservative heuristics.
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class ArchDefaultLoopHelper : public ArchNoOptsLoopHelper {
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public:
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// Scalar loop unrolling parameters and heuristics.
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//
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// Maximum possible unrolling factor.
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static constexpr uint32_t kScalarMaxUnrollFactor = 2;
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// Loop's maximum instruction count. Loops with higher count will not be peeled/unrolled.
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static constexpr uint32_t kScalarHeuristicMaxBodySizeInstr = 17;
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// Loop's maximum basic block count. Loops with higher count will not be peeled/unrolled.
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static constexpr uint32_t kScalarHeuristicMaxBodySizeBlocks = 6;
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// Maximum number of instructions to be created as a result of full unrolling.
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static constexpr uint32_t kScalarHeuristicFullyUnrolledMaxInstrThreshold = 35;
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bool IsLoopNonBeneficialForScalarOpts(LoopAnalysisInfo* analysis_info) const override {
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return analysis_info->HasLongTypeInstructions() ||
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IsLoopTooBig(analysis_info,
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kScalarHeuristicMaxBodySizeInstr,
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kScalarHeuristicMaxBodySizeBlocks);
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}
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uint32_t GetScalarUnrollingFactor(const LoopAnalysisInfo* analysis_info) const override {
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int64_t trip_count = analysis_info->GetTripCount();
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// Unroll only loops with known trip count.
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if (trip_count == LoopAnalysisInfo::kUnknownTripCount) {
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return LoopAnalysisInfo::kNoUnrollingFactor;
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}
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uint32_t desired_unrolling_factor = kScalarMaxUnrollFactor;
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if (trip_count < desired_unrolling_factor || trip_count % desired_unrolling_factor != 0) {
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return LoopAnalysisInfo::kNoUnrollingFactor;
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}
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return desired_unrolling_factor;
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}
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bool IsLoopPeelingEnabled() const override { return true; }
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bool IsFullUnrollingBeneficial(LoopAnalysisInfo* analysis_info) const override {
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int64_t trip_count = analysis_info->GetTripCount();
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// We assume that trip count is known.
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DCHECK_NE(trip_count, LoopAnalysisInfo::kUnknownTripCount);
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size_t instr_num = analysis_info->GetNumberOfInstructions();
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return (trip_count * instr_num < kScalarHeuristicFullyUnrolledMaxInstrThreshold);
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}
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protected:
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bool IsLoopTooBig(LoopAnalysisInfo* loop_analysis_info,
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size_t instr_threshold,
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size_t bb_threshold) const {
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size_t instr_num = loop_analysis_info->GetNumberOfInstructions();
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size_t bb_num = loop_analysis_info->GetNumberOfBasicBlocks();
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return (instr_num >= instr_threshold || bb_num >= bb_threshold);
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}
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};
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// Custom implementation of loop helper for arm64 target. Enables heuristics for scalar loop
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// peeling and unrolling and supports SIMD loop unrolling.
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class Arm64LoopHelper : public ArchDefaultLoopHelper {
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public:
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// SIMD loop unrolling parameters and heuristics.
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//
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// Maximum possible unrolling factor.
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static constexpr uint32_t kArm64SimdMaxUnrollFactor = 8;
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// Loop's maximum instruction count. Loops with higher count will not be unrolled.
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static constexpr uint32_t kArm64SimdHeuristicMaxBodySizeInstr = 50;
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// Loop's maximum instruction count. Loops with higher count will not be peeled/unrolled.
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static constexpr uint32_t kArm64ScalarHeuristicMaxBodySizeInstr = 40;
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// Loop's maximum basic block count. Loops with higher count will not be peeled/unrolled.
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static constexpr uint32_t kArm64ScalarHeuristicMaxBodySizeBlocks = 8;
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bool IsLoopNonBeneficialForScalarOpts(LoopAnalysisInfo* loop_analysis_info) const override {
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return IsLoopTooBig(loop_analysis_info,
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kArm64ScalarHeuristicMaxBodySizeInstr,
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kArm64ScalarHeuristicMaxBodySizeBlocks);
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}
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uint32_t GetSIMDUnrollingFactor(HBasicBlock* block,
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int64_t trip_count,
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uint32_t max_peel,
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uint32_t vector_length) const override {
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// Don't unroll with insufficient iterations.
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// TODO: Unroll loops with unknown trip count.
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DCHECK_NE(vector_length, 0u);
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if (trip_count < (2 * vector_length + max_peel)) {
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return LoopAnalysisInfo::kNoUnrollingFactor;
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}
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// Don't unroll for large loop body size.
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uint32_t instruction_count = block->GetInstructions().CountSize();
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if (instruction_count >= kArm64SimdHeuristicMaxBodySizeInstr) {
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return LoopAnalysisInfo::kNoUnrollingFactor;
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}
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// Find a beneficial unroll factor with the following restrictions:
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// - At least one iteration of the transformed loop should be executed.
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// - The loop body shouldn't be "too big" (heuristic).
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uint32_t uf1 = kArm64SimdHeuristicMaxBodySizeInstr / instruction_count;
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uint32_t uf2 = (trip_count - max_peel) / vector_length;
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uint32_t unroll_factor =
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TruncToPowerOfTwo(std::min({uf1, uf2, kArm64SimdMaxUnrollFactor}));
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DCHECK_GE(unroll_factor, 1u);
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return unroll_factor;
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}
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};
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ArchNoOptsLoopHelper* ArchNoOptsLoopHelper::Create(InstructionSet isa,
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ArenaAllocator* allocator) {
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switch (isa) {
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case InstructionSet::kArm64: {
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return new (allocator) Arm64LoopHelper;
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}
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default: {
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return new (allocator) ArchDefaultLoopHelper;
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}
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}
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}
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} // namespace art
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