// Copyright (c) 2017, the R8 project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
package com.android.tools.r8.ir.optimize;

import static com.android.tools.r8.ir.code.Opcodes.CONST_CLASS;
import static com.android.tools.r8.ir.code.Opcodes.CONST_NUMBER;
import static com.android.tools.r8.ir.code.Opcodes.CONST_STRING;
import static com.android.tools.r8.ir.code.Opcodes.DEX_ITEM_BASED_CONST_STRING;
import static com.android.tools.r8.ir.code.Opcodes.STATIC_GET;

import com.android.tools.r8.errors.Unreachable;
import com.android.tools.r8.graph.AppView;
import com.android.tools.r8.graph.DexClass;
import com.android.tools.r8.graph.DexEncodedField;
import com.android.tools.r8.graph.DexType;
import com.android.tools.r8.graph.ProgramMethod;
import com.android.tools.r8.ir.analysis.value.AbstractValue;
import com.android.tools.r8.ir.analysis.value.SingleFieldValue;
import com.android.tools.r8.ir.code.BasicBlock;
import com.android.tools.r8.ir.code.ConstClass;
import com.android.tools.r8.ir.code.ConstNumber;
import com.android.tools.r8.ir.code.ConstString;
import com.android.tools.r8.ir.code.DexItemBasedConstString;
import com.android.tools.r8.ir.code.IRCode;
import com.android.tools.r8.ir.code.Instruction;
import com.android.tools.r8.ir.code.InstructionListIterator;
import com.android.tools.r8.ir.code.StaticGet;
import com.android.tools.r8.ir.code.Value;
import it.unimi.dsi.fastutil.Hash.Strategy;
import it.unimi.dsi.fastutil.objects.Object2ObjectLinkedOpenCustomHashMap;
import it.unimi.dsi.fastutil.objects.Object2ObjectMap;
import it.unimi.dsi.fastutil.objects.Object2ObjectSortedMap.FastSortedEntrySet;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

/**
 * Canonicalize constants.
 */
public class ConstantCanonicalizer {
  // Threshold to limit the number of constant canonicalization.
  private static final int MAX_CANONICALIZED_CONSTANT = 22;

  private final CodeRewriter codeRewriter;

  public ConstantCanonicalizer(CodeRewriter codeRewriter) {
    this.codeRewriter = codeRewriter;
  }

  public void canonicalize(AppView<?> appView, IRCode code) {
    ProgramMethod context = code.context();
    Object2ObjectLinkedOpenCustomHashMap<Instruction, List<Value>> valuesDefinedByConstant =
        new Object2ObjectLinkedOpenCustomHashMap<>(
            new Strategy<Instruction>() {

              @Override
              public int hashCode(Instruction candidate) {
                assert candidate.instructionTypeCanBeCanonicalized();
                switch (candidate.opcode()) {
                  case CONST_CLASS:
                    return candidate.asConstClass().getValue().hashCode();
                  case CONST_NUMBER:
                    return Long.hashCode(candidate.asConstNumber().getRawValue())
                        + 13 * candidate.outType().hashCode();
                  case CONST_STRING:
                    return candidate.asConstString().getValue().hashCode();
                  case DEX_ITEM_BASED_CONST_STRING:
                    return candidate.asDexItemBasedConstString().getItem().hashCode();
                  case STATIC_GET:
                    return candidate.asStaticGet().getField().hashCode();
                  default:
                    throw new Unreachable();
                }
              }

              @Override
              public boolean equals(Instruction a, Instruction b) {
                // Constants with local info must not be canonicalized and must be filtered.
                assert a == null || !a.outValue().hasLocalInfo();
                assert b == null || !b.outValue().hasLocalInfo();
                return a == b || (a != null && b != null && a.identicalNonValueNonPositionParts(b));
              }
            });

    // Collect usages of constants that can be canonicalized.
    for (Instruction current : code.instructions()) {
      // Interested only in instructions types that can be canonicalized, i.e., ConstClass,
      // ConstNumber, (DexItemBased)?ConstString, and StaticGet.
      if (!current.instructionTypeCanBeCanonicalized()) {
        continue;
      }
      // Do not canonicalize ConstClass that may have side effects. Its original instructions
      // will not be removed by dead code remover due to the side effects.
      if (current.isConstClass() && current.instructionMayHaveSideEffects(appView, context)) {
        continue;
      }
      // Do not canonicalize ConstString instructions if there are monitor operations in the code.
      // That could lead to unbalanced locking and could lead to situations where OOM exceptions
      // could leave a synchronized method without unlocking the monitor.
      if ((current.isConstString() || current.isDexItemBasedConstString())
          && code.metadata().mayHaveMonitorInstruction()) {
        continue;
      }
      // Canonicalize effectively final fields that are guaranteed to be written before they are
      // read. This is only OK if the instruction cannot have side effects.
      if (current.isStaticGet()) {
        AbstractValue abstractValue = current.outValue().getAbstractValue(appView, context);
        if (!abstractValue.isSingleFieldValue()) {
          continue;
        }
        SingleFieldValue singleFieldValue = abstractValue.asSingleFieldValue();
        DexType fieldHolderType = singleFieldValue.getField().getHolderType();
        if (context.getDefinition().isClassInitializer()
            && context.getHolderType() == fieldHolderType) {
          // Avoid that canonicalization inserts a read before the unique write in the class
          // initializer, as that would change the program behavior.
          continue;
        }
        DexClass fieldHolder = appView.definitionFor(fieldHolderType);
        DexEncodedField field = singleFieldValue.getField().lookupOnClass(fieldHolder);
        if (field == null
            || !field.isEnum()
            || current.instructionMayHaveSideEffects(appView, context)) {
          // Only allow canonicalization of enums.
          continue;
        }
      }
      // Constants with local info must not be canonicalized and must be filtered.
      if (current.outValue().hasLocalInfo()) {
        continue;
      }
      // Constants that are used by invoke range are not canonicalized to be compliant with the
      // optimization splitRangeInvokeConstant that gives the register allocator more freedom in
      // assigning register to ranged invokes which can greatly reduce the number of register
      // needed (and thereby code size as well). Thus no need to do a transformation that should
      // be removed later by another optimization.
      if (constantUsedByInvokeRange(current)) {
        continue;
      }
      List<Value> oldValuesDefinedByConstant =
          valuesDefinedByConstant.computeIfAbsent(current, k -> new ArrayList<>());
      oldValuesDefinedByConstant.add(current.outValue());
    }

    if (valuesDefinedByConstant.isEmpty()) {
      return;
    }

    // Double-check the entry block does not have catch handlers.
    // Otherwise, we need to split it before moving canonicalized const-string, which may throw.
    assert !code.entryBlock().hasCatchHandlers();
    FastSortedEntrySet<Instruction, List<Value>> entries =
        valuesDefinedByConstant.object2ObjectEntrySet();
    // Sort the most frequently used constant first and exclude constant use only one time, such
    // as the {@code MAX_CANONICALIZED_CONSTANT} will be canonicalized into the entry block.
    Iterator<Object2ObjectMap.Entry<Instruction, List<Value>>> iterator =
        entries.stream()
            .filter(a -> a.getValue().size() > 1)
            .sorted((a, b) -> Integer.compare(b.getValue().size(), a.getValue().size()))
            .limit(MAX_CANONICALIZED_CONSTANT)
            .iterator();

    if (!iterator.hasNext()) {
      return;
    }

    boolean shouldSimplifyIfs = false;
    do {
      Object2ObjectMap.Entry<Instruction, List<Value>> entry = iterator.next();
      Instruction canonicalizedConstant = entry.getKey();
      assert canonicalizedConstant.instructionTypeCanBeCanonicalized();
      Instruction newConst;
      if (canonicalizedConstant.getBlock().isEntry()) {
        newConst = canonicalizedConstant;
      } else {
        switch (canonicalizedConstant.opcode()) {
          case CONST_CLASS:
            newConst = ConstClass.copyOf(code, canonicalizedConstant.asConstClass());
            break;
          case CONST_NUMBER:
            newConst = ConstNumber.copyOf(code, canonicalizedConstant.asConstNumber());
            break;
          case CONST_STRING:
            newConst = ConstString.copyOf(code, canonicalizedConstant.asConstString());
            break;
          case DEX_ITEM_BASED_CONST_STRING:
            newConst =
                DexItemBasedConstString.copyOf(
                    code, canonicalizedConstant.asDexItemBasedConstString());
            break;
          case STATIC_GET:
            newConst = StaticGet.copyOf(code, canonicalizedConstant.asStaticGet());
            break;
          default:
            throw new Unreachable();
        }
        insertCanonicalizedConstant(code, newConst);
      }
      for (Value outValue : entry.getValue()) {
        outValue.replaceUsers(newConst.outValue());
      }
      shouldSimplifyIfs |= newConst.outValue().hasUserThatMatches(Instruction::isIf);
    } while (iterator.hasNext());

    shouldSimplifyIfs |= code.removeAllDeadAndTrivialPhis();

    if (shouldSimplifyIfs) {
      codeRewriter.simplifyIf(code);
    }

    assert code.isConsistentSSA(appView);
  }

  private static void insertCanonicalizedConstant(IRCode code, Instruction canonicalizedConstant) {
    BasicBlock entryBlock = code.entryBlock();
    // Insert the constant instruction at the start of the block right after the argument
    // instructions. It is important that the const instruction is put before any instruction
    // that can throw exceptions (since the value could be used on the exceptional edge).
    InstructionListIterator it = entryBlock.listIterator(code);
    while (it.hasNext()) {
      Instruction next = it.next();
      if (!next.isArgument()) {
        canonicalizedConstant.setPosition(code.getEntryPosition());
        it.previous();
        break;
      }
    }
    it.add(canonicalizedConstant);
  }

  private static boolean constantUsedByInvokeRange(Instruction constant) {
    for (Instruction user : constant.outValue().uniqueUsers()) {
      if (user.isInvoke() && user.asInvoke().requiredArgumentRegisters() > 5) {
        return true;
      }
    }
    return false;
  }
}