pkg/compiler/lib/src/ssa/codegen_helpers.dart

// Copyright (c) 2012, the Dart 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.

import '../constants/values.dart';
import '../elements/entities.dart';
import '../js_backend/js_backend.dart';
import '../js_backend/interceptor_data.dart';
import '../options.dart';
import '../types/abstract_value_domain.dart';
import '../universe/selector.dart' show Selector;
import '../world.dart' show JClosedWorld;
import 'nodes.dart';

/**
 * Replaces some instructions with specialized versions to make codegen easier.
 * Caches codegen information on nodes.
 */
class SsaInstructionSelection extends HBaseVisitor {
  final JClosedWorld _closedWorld;
  final InterceptorData _interceptorData;
  HGraph graph;

  SsaInstructionSelection(this._closedWorld, this._interceptorData);

  AbstractValueDomain get _abstractValueDomain =>
      _closedWorld.abstractValueDomain;

  void visitGraph(HGraph graph) {
    this.graph = graph;
    visitDominatorTree(graph);
  }

  visitBasicBlock(HBasicBlock block) {
    HInstruction instruction = block.first;
    while (instruction != null) {
      HInstruction next = instruction.next;
      HInstruction replacement = instruction.accept(this);
      if (replacement != instruction && replacement != null) {
        block.rewrite(instruction, replacement);

        // If the replacement instruction does not know its source element, use
        // the source element of the instruction.
        if (replacement.sourceElement == null) {
          replacement.sourceElement = instruction.sourceElement;
        }
        if (replacement.sourceInformation == null) {
          replacement.sourceInformation = instruction.sourceInformation;
        }
        if (!replacement.isInBasicBlock()) {
          // The constant folding can return an instruction that is already
          // part of the graph (like an input), so we only add the replacement
          // if necessary.
          block.addAfter(instruction, replacement);
          // Visit the replacement as the next instruction in case it can also
          // be constant folded away.
          next = replacement;
        }
        block.remove(instruction);
      }
      instruction = next;
    }
  }

  HInstruction visitInstruction(HInstruction node) {
    return node;
  }

  HInstruction visitIs(HIs node) {
    if (node.kind == HIs.RAW_CHECK) {
      HInstruction interceptor = node.interceptor;
      if (interceptor != null) {
        return new HIsViaInterceptor(
            node.typeExpression, interceptor, _abstractValueDomain.boolType);
      }
    }
    return node;
  }

  HInstruction visitIdentity(HIdentity node) {
    node.singleComparisonOp = simpleOp(node.left, node.right);
    return node;
  }

  /// Returns the single JavaScript comparison (`==` or `===`) if that
  /// implements `identical(left, right)`, or returns `null` if a more complex
  /// expression is needed.
  String simpleOp(HInstruction left, HInstruction right) {
    AbstractValue leftType = left.instructionType;
    AbstractValue rightType = right.instructionType;
    if (_abstractValueDomain.canBeNull(leftType) &&
        _abstractValueDomain.canBeNull(rightType)) {
      // Can't use `===` on Dart `null` since it is implemented by JavaScript
      // `null` and `undefined`.
      if (left.isConstantNull() || right.isConstantNull()) {
        return '==';
      }
      if (_abstractValueDomain.isNumberOrNull(leftType) &&
          _abstractValueDomain.isNumberOrNull(rightType)) {
        return '==';
      }
      if (_abstractValueDomain.isStringOrNull(leftType) &&
          _abstractValueDomain.isStringOrNull(rightType)) {
        return '==';
      }
      if (_abstractValueDomain.isBooleanOrNull(leftType) &&
          _abstractValueDomain.isBooleanOrNull(rightType)) {
        return '==';
      }

      // TODO(34439): There are more cases that can compile to `==` without
      // triggering a conversion in the JavaScript evaluation. `==` will work
      // for most Dart objects, but we have to ensure neither side can be a
      // JavaScript Number, String, Symbol or Boolean.
      return null;
    }
    return '===';
  }

  HInstruction visitInvokeDynamic(HInvokeDynamic node) {
    if (node.isInterceptedCall) {
      tryReplaceInterceptorWithDummy(node, node.selector, node.mask);
    }
    return node;
  }

  HInstruction visitInvokeSuper(HInvokeSuper node) {
    if (node.isInterceptedCall) {
      AbstractValue mask = node.getDartReceiver(_closedWorld).instructionType;
      tryReplaceInterceptorWithDummy(node, node.selector, mask);
    }
    return node;
  }

  void tryReplaceInterceptorWithDummy(
      HInvoke node, Selector selector, AbstractValue mask) {
    // Calls of the form
    //
    //     a.foo$1(a, x)
    //
    // where the interceptor calling convention is used come from recognizing
    // that 'a' is a 'self-interceptor'.  If the selector matches only methods
    // that ignore the explicit receiver parameter, replace occurences of the
    // receiver argument with a dummy receiver '0':
    //
    //     a.foo$1(a, x)   --->   a.foo$1(0, x)
    //
    // This often reduces the number of references to 'a' to one, allowing 'a'
    // to be generated at use to avoid a temporary, e.g.
    //
    //     t1 = b.get$thing();
    //     t1.foo$1(t1, x)
    // --->
    //     b.get$thing().foo$1(0, x)
    //

    // TODO(15933): Make automatically generated property extraction closures
    // work with the dummy receiver optimization.
    if (selector.isGetter) return;

    // This assignment of inputs is uniform for HInvokeDynamic and HInvokeSuper.
    HInstruction interceptor = node.inputs[0];
    HInstruction receiverArgument = node.inputs[1];

    if (interceptor.nonCheck() == receiverArgument.nonCheck()) {
      if (_interceptorData.isInterceptedSelector(selector) &&
          !_interceptorData.isInterceptedMixinSelector(
              selector, mask, _closedWorld)) {
        ConstantValue constant = new SyntheticConstantValue(
            SyntheticConstantKind.DUMMY_INTERCEPTOR,
            receiverArgument.instructionType);
        HConstant dummy = graph.addConstant(constant, _closedWorld);
        receiverArgument.usedBy.remove(node);
        node.inputs[1] = dummy;
        dummy.usedBy.add(node);
      }
    }
  }

  HInstruction visitFieldSet(HFieldSet setter) {
    // Pattern match
    //     t1 = x.f; t2 = t1 + 1; x.f = t2; use(t2)   -->  ++x.f
    //     t1 = x.f; t2 = t1 op y; x.f = t2; use(t2)  -->  x.f op= y
    //     t1 = x.f; t2 = t1 + 1; x.f = t2; use(t1)   -->  x.f++
    HBasicBlock block = setter.block;
    HInstruction op = setter.value;
    HInstruction receiver = setter.receiver;

    bool isMatchingRead(HInstruction candidate) {
      if (candidate is HFieldGet) {
        if (candidate.element != setter.element) return false;
        if (candidate.receiver != setter.receiver) return false;
        // Recognize only three instructions in sequence in the same block. This
        // could be broadened to allow non-interfering interleaved instructions.
        if (op.block != block) return false;
        if (candidate.block != block) return false;
        if (setter.previous != op) return false;
        if (op.previous != candidate) return false;
        return true;
      }
      return false;
    }

    HInstruction noMatchingRead() {
      // If we have other HFieldSet optimizations, they go here.
      return null;
    }

    HInstruction replaceOp(HInstruction replacement, HInstruction getter) {
      block.addBefore(setter, replacement);
      block.remove(setter);
      block.rewrite(op, replacement);
      block.remove(op);
      block.remove(getter);
      return null;
    }

    HInstruction plusOrMinus(String assignOp, String incrementOp) {
      HInvokeBinary binary = op;
      HInstruction left = binary.left;
      HInstruction right = binary.right;
      if (isMatchingRead(left)) {
        if (left.usedBy.length == 1) {
          if (right is HConstant && right.constant.isOne) {
            HInstruction rmw = new HReadModifyWrite.preOp(
                setter.element, incrementOp, receiver, op.instructionType);
            return replaceOp(rmw, left);
          } else {
            HInstruction rmw = new HReadModifyWrite.assignOp(
                setter.element, assignOp, receiver, right, op.instructionType);
            return replaceOp(rmw, left);
          }
        } else if (op.usedBy.length == 1 &&
            right is HConstant &&
            right.constant.isOne) {
          HInstruction rmw = new HReadModifyWrite.postOp(
              setter.element, incrementOp, receiver, op.instructionType);
          block.addAfter(left, rmw);
          block.remove(setter);
          block.remove(op);
          block.rewrite(left, rmw);
          block.remove(left);
          return null;
        }
      }
      return noMatchingRead();
    }

    HInstruction simple(
        String assignOp, HInstruction left, HInstruction right) {
      if (isMatchingRead(left)) {
        if (left.usedBy.length == 1) {
          HInstruction rmw = new HReadModifyWrite.assignOp(
              setter.element, assignOp, receiver, right, op.instructionType);
          return replaceOp(rmw, left);
        }
      }
      return noMatchingRead();
    }

    HInstruction simpleBinary(String assignOp) {
      HInvokeBinary binary = op;
      return simple(assignOp, binary.left, binary.right);
    }

    HInstruction bitop(String assignOp) {
      // HBitAnd, HBitOr etc. are more difficult because HBitAnd(a.x, y)
      // sometimes needs to be forced to unsigned: a.x = (a.x & y) >>> 0.
      if (op.isUInt31(_abstractValueDomain)) return simpleBinary(assignOp);
      return noMatchingRead();
    }

    if (op is HAdd) return plusOrMinus('+', '++');
    if (op is HSubtract) return plusOrMinus('-', '--');

    if (op is HStringConcat) return simple('+', op.left, op.right);

    if (op is HMultiply) return simpleBinary('*');
    if (op is HDivide) return simpleBinary('/');

    if (op is HBitAnd) return bitop('&');
    if (op is HBitOr) return bitop('|');
    if (op is HBitXor) return bitop('^');

    return noMatchingRead();
  }
}

/**
 * Remove [HTypeKnown] instructions from the graph, to make codegen
 * analysis easier.
 */
class SsaTypeKnownRemover extends HBaseVisitor {
  void visitGraph(HGraph graph) {
    visitDominatorTree(graph);
  }

  void visitBasicBlock(HBasicBlock block) {
    HInstruction instruction = block.first;
    while (instruction != null) {
      HInstruction next = instruction.next;
      instruction.accept(this);
      instruction = next;
    }
  }

  void visitTypeKnown(HTypeKnown instruction) {
    for (HInstruction user in instruction.usedBy) {
      if (user is HTypeConversion) {
        user.inputType = instruction.instructionType;
      }
    }
    instruction.block.rewrite(instruction, instruction.checkedInput);
    instruction.block.remove(instruction);
  }
}

/**
 * Remove [HTypeConversion] instructions from the graph in '--trust-primitives'
 * mode.
 */
class SsaTrustedCheckRemover extends HBaseVisitor {
  final CompilerOptions _options;

  SsaTrustedCheckRemover(this._options);

  void visitGraph(HGraph graph) {
    if (!_options.trustPrimitives) return;
    visitDominatorTree(graph);
  }

  void visitBasicBlock(HBasicBlock block) {
    HInstruction instruction = block.first;
    while (instruction != null) {
      HInstruction next = instruction.next;
      instruction.accept(this);
      instruction = next;
    }
  }

  void visitTypeConversion(HTypeConversion instruction) {
    if (instruction.isReceiverTypeCheck || instruction.isArgumentTypeCheck) {
      instruction.block.rewrite(instruction, instruction.checkedInput);
      instruction.block.remove(instruction);
    }
  }
}

/**
 * Instead of emitting each SSA instruction with a temporary variable
 * mark instructions that can be emitted at their use-site.
 * For example, in:
 *   t0 = 4;
 *   t1 = 3;
 *   t2 = add(t0, t1);
 * t0 and t1 would be marked and the resulting code would then be:
 *   t2 = add(4, 3);
 */
class SsaInstructionMerger extends HBaseVisitor {
  final AbstractValueDomain _abstractValueDomain;
  final SuperMemberData _superMemberData;
  /**
   * List of [HInstruction] that the instruction merger expects in
   * order when visiting the inputs of an instruction.
   */
  List<HInstruction> expectedInputs;
  /**
   * Set of pure [HInstruction] that the instruction merger expects to
   * find. The order of pure instructions do not matter, as they will
   * not be affected by side effects.
   */
  Set<HInstruction> pureInputs;
  Set<HInstruction> generateAtUseSite;

  void markAsGenerateAtUseSite(HInstruction instruction) {
    assert(!instruction.isJsStatement());
    generateAtUseSite.add(instruction);
  }

  SsaInstructionMerger(
      this._abstractValueDomain, this.generateAtUseSite, this._superMemberData);

  void visitGraph(HGraph graph) {
    visitDominatorTree(graph);
  }

  void analyzeInputs(HInstruction user, int start) {
    List<HInstruction> inputs = user.inputs;
    for (int i = start; i < inputs.length; i++) {
      HInstruction input = inputs[i];
      if (!generateAtUseSite.contains(input) &&
          !input.isCodeMotionInvariant() &&
          input.usedBy.length == 1 &&
          input is! HPhi &&
          input is! HLocalValue &&
          !input.isJsStatement()) {
        if (isEffectivelyPure(input)) {
          // Only consider a pure input if it is in the same loop.
          // Otherwise, we might move GVN'ed instruction back into the
          // loop.
          if (user.hasSameLoopHeaderAs(input)) {
            // Move it closer to [user], so that instructions in
            // between do not prevent making it generate at use site.
            input.moveBefore(user);
            pureInputs.add(input);
            // Previous computations done on [input] are now invalid
            // because we moved [input] to another place. So all
            // non code motion invariant instructions need
            // to be removed from the [generateAtUseSite] set.
            input.inputs.forEach((instruction) {
              if (!instruction.isCodeMotionInvariant()) {
                generateAtUseSite.remove(instruction);
              }
            });
            // Visit the pure input now so that the expected inputs
            // are after the expected inputs of [user].
            input.accept(this);
          }
        } else {
          expectedInputs.add(input);
        }
      }
    }
  }

  // Some non-pure instructions may be treated as pure. HLocalGet depends on
  // assignments, but we can ignore the initializing assignment since it will by
  // construction always precede a use.
  bool isEffectivelyPure(HInstruction instruction) {
    if (instruction is HLocalGet) return !isAssignedLocal(instruction.local);
    return instruction.isPure(_abstractValueDomain);
  }

  bool isAssignedLocal(HLocalValue local) {
    // [HLocalValue]s have an initializing assignment which is guaranteed to
    // precede the use, except for [HParameterValue]s which are 'assigned' at
    // entry.
    int initializingAssignmentCount = (local is HParameterValue) ? 0 : 1;
    return local.usedBy
        .where((user) => user is HLocalSet)
        .skip(initializingAssignmentCount)
        .isNotEmpty;
  }

  void visitInstruction(HInstruction instruction) {
    // A code motion invariant instruction is dealt before visiting it.
    assert(!instruction.isCodeMotionInvariant());
    analyzeInputs(instruction, 0);
  }

  void visitInvokeSuper(HInvokeSuper instruction) {
    MemberEntity superMethod = instruction.element;
    Selector selector = instruction.selector;
    // If aliased super members cannot be used, we will generate code like
    //
    //     C.prototype.method.call(instance)
    //
    // where instance is the [this] object for the method. In such a case, the
    // get of prototype might be evaluated before instance is created if we
    // generate instance at use site, which in turn might update the prototype
    // after first access if we use lazy initialization.
    // In this case, we therefore don't allow the receiver (the first argument)
    // to be generated at use site, and only analyze all other arguments.
    if (!_superMemberData.canUseAliasedSuperMember(superMethod, selector)) {
      analyzeInputs(instruction, 1);
    } else {
      super.visitInvokeSuper(instruction);
    }
  }

  void visitIs(HIs instruction) {
    // In the general case the input might be used multple multiple times, so it
    // must not be set generate at use site.

    // If the code will generate 'instanceof' then we can generate at use site.
    if (instruction.useInstanceOf) {
      analyzeInputs(instruction, 0);
    }

    // Compound and variable checks use a separate instruction to compute the
    // result.
    if (instruction.isCompoundCheck || instruction.isVariableCheck) {
      analyzeInputs(instruction, 0);
    }
  }

  // A bounds check method must not have its first input generated at use site,
  // because it's using it twice.
  void visitBoundsCheck(HBoundsCheck instruction) {
    analyzeInputs(instruction, 1);
  }

  // An identity operation must only have its inputs generated at use site if
  // does not require an expression with multiple uses (because of null /
  // undefined).
  void visitIdentity(HIdentity instruction) {
    if (instruction.singleComparisonOp != null) {
      super.visitIdentity(instruction);
    }
    // Do nothing.
  }

  void visitTypeConversion(HTypeConversion instruction) {
    if (!instruction.isArgumentTypeCheck && !instruction.isReceiverTypeCheck) {
      assert(instruction.isCheckedModeCheck || instruction.isCastTypeCheck);
      // Checked mode checks and cast checks compile to code that
      // only use their input once, so we can safely visit them
      // and try to merge the input.
      visitInstruction(instruction);
    }
  }

  void visitTypeKnown(HTypeKnown instruction) {
    // [HTypeKnown] instructions are removed before code generation.
    assert(false);
  }

  void tryGenerateAtUseSite(HInstruction instruction) {
    if (instruction.isControlFlow()) return;
    markAsGenerateAtUseSite(instruction);
  }

  bool isBlockSinglePredecessor(HBasicBlock block) {
    return block.successors.length == 1 &&
        block.successors[0].predecessors.length == 1;
  }

  void visitBasicBlock(HBasicBlock block) {
    // Compensate from not merging blocks: if the block is the
    // single predecessor of its single successor, let the successor
    // visit it.
    if (isBlockSinglePredecessor(block)) return;

    tryMergingExpressions(block);
  }

  void tryMergingExpressions(HBasicBlock block) {
    // Visit each instruction of the basic block in last-to-first order.
    // Keep a list of expected inputs of the current "expression" being
    // merged. If instructions occur in the expected order, they are
    // included in the expression.

    // The expectedInputs list holds non-trivial instructions that may
    // be generated at their use site, if they occur in the correct order.
    if (expectedInputs == null) expectedInputs = new List<HInstruction>();
    if (pureInputs == null) pureInputs = new Set<HInstruction>();

    // Pop instructions from expectedInputs until instruction is found.
    // Return true if it is found, or false if not.
    bool findInInputsAndPopNonMatching(HInstruction instruction) {
      assert(!isEffectivelyPure(instruction));
      while (!expectedInputs.isEmpty) {
        HInstruction nextInput = expectedInputs.removeLast();
        assert(!generateAtUseSite.contains(nextInput));
        assert(nextInput.usedBy.length == 1);
        if (identical(nextInput, instruction)) {
          return true;
        }
      }
      return false;
    }

    block.last.accept(this);
    for (HInstruction instruction = block.last.previous;
        instruction != null;
        instruction = instruction.previous) {
      if (generateAtUseSite.contains(instruction)) {
        continue;
      }
      if (instruction.isCodeMotionInvariant()) {
        markAsGenerateAtUseSite(instruction);
        continue;
      }
      if (isEffectivelyPure(instruction)) {
        if (pureInputs.contains(instruction)) {
          tryGenerateAtUseSite(instruction);
        } else {
          // If the input is not in the [pureInputs] set, it has not
          // been visited or should not be generated at use-site. The most
          // likely reason for the latter, is that the instruction is used
          // in more than one location.
          // We must either clear the expectedInputs, or move the pure
          // instruction's inputs in front of the existing ones.
          // Example:
          //   t1 = foo();  // side-effect.
          //   t2 = bar();  // side-effect.
          //   t3 = pure(t2);    // used more than once.
          //   f(t1, t3);   // expected inputs of 'f': t1.
          //   use(t3);
          //
          // If we don't clear the expected inputs we end up in a situation
          // where pure pushes "t2" on top of "t1" leading to:
          //   t3 = pure(bar());
          //   f(foo(), t3);
          //   use(t3);
          //
          // If we clear the expected-inputs list we have the correct
          // output:
          //   t1 = foo();
          //   t3 = pure(bar());
          //   f(t1, t3);
          //   use(t3);
          //
          // Clearing is, however, not optimal.
          // Example:
          //   t1 = foo();  // t1 is now used by `pure`.
          //   t2 = bar();  // t2 is now used by `f`.
          //   t3 = pure(t1);
          //   f(t2, t3);
          //   use(t3);
          //
          // If we clear the expected-inputs we can't generate-at-use any of
          // the instructions.
          //
          // The optimal solution is to move the inputs of 'pure' in
          // front of the expectedInputs list. This makes sense, since we
          // push expected-inputs from left-to right, and the `pure` function
          // invocation is "more left" (i.e. before) the first argument of `f`.
          // With that approach we end up with:
          //   t3 = pure(foo());
          //   f(bar(), t3);
          //   use(t3);
          //
          int oldLength = expectedInputs.length;
          instruction.accept(this);
          if (oldLength != 0 && oldLength != expectedInputs.length) {
            // Move the pure instruction's inputs to the front.
            List<HInstruction> newInputs = expectedInputs.sublist(oldLength);
            int newCount = newInputs.length;
            expectedInputs.setRange(
                newCount, newCount + oldLength, expectedInputs);
            expectedInputs.setRange(0, newCount, newInputs);
          }
        }
      } else {
        if (findInInputsAndPopNonMatching(instruction)) {
          // The current instruction is the next non-trivial
          // expected input.
          tryGenerateAtUseSite(instruction);
        } else {
          assert(expectedInputs.isEmpty);
        }
        instruction.accept(this);
      }
    }

    if (block.predecessors.length == 1 &&
        isBlockSinglePredecessor(block.predecessors[0])) {
      assert(block.phis.isEmpty);
      tryMergingExpressions(block.predecessors[0]);
    } else {
      expectedInputs = null;
      pureInputs = null;
    }
  }
}

/**
 *  Detect control flow arising from short-circuit logical and
 *  conditional operators, and prepare the program to be generated
 *  using these operators instead of nested ifs and boolean variables.
 */
class SsaConditionMerger extends HGraphVisitor {
  Set<HInstruction> generateAtUseSite;
  Set<HInstruction> controlFlowOperators;

  void markAsGenerateAtUseSite(HInstruction instruction) {
    assert(!instruction.isJsStatement());
    generateAtUseSite.add(instruction);
  }

  SsaConditionMerger(this.generateAtUseSite, this.controlFlowOperators);

  void visitGraph(HGraph graph) {
    visitPostDominatorTree(graph);
  }

  /**
   * Check if a block has at least one statement other than
   * [instruction].
   */
  bool hasAnyStatement(HBasicBlock block, HInstruction instruction) {
    // If [instruction] is not in [block], then if the block is not
    // empty, we know there will be a statement to emit.
    if (!identical(instruction.block, block)) {
      return !identical(block.last, block.first);
    }

    // If [instruction] is not the last instruction of the block
    // before the control flow instruction, or the last instruction,
    // then we will have to emit a statement for that last instruction.
    if (instruction != block.last &&
        !identical(instruction, block.last.previous)) return true;

    // If one of the instructions in the block until [instruction] is
    // not generated at use site, then we will have to emit a
    // statement for it.
    // TODO(ngeoffray): we could generate a comma separated
    // list of expressions.
    for (HInstruction temp = block.first;
        !identical(temp, instruction);
        temp = temp.next) {
      if (!generateAtUseSite.contains(temp)) return true;
    }

    return false;
  }

  bool isSafeToGenerateAtUseSite(HInstruction user, HInstruction input) {
    // HCreate evaluates arguments in order and passes them to a constructor.
    if (user is HCreate) return true;
    // A [HForeign] instruction uses operators and if we generate [input] at use
    // site, the precedence or evaluation order might be wrong.
    if (user is HForeign) return false;
    // A [HCheck] instruction with control flow uses its input
    // multiple times, so we avoid generating it at use site.
    if (user is HCheck && user.isControlFlow()) return false;
    // A [HIs] instruction uses its input multiple times, so we
    // avoid generating it at use site.
    if (user is HIs) return false;
    // Avoid code motion into a loop.
    return user.hasSameLoopHeaderAs(input);
  }

  void visitBasicBlock(HBasicBlock block) {
    if (block.last is! HIf) return;
    HIf startIf = block.last;
    HBasicBlock end = startIf.joinBlock;

    // We check that the structure is the following:
    //         If
    //       /    \
    //      /      \
    //   1 expr    goto
    //    goto     /
    //      \     /
    //       \   /
    // phi(expr, true|false)
    //
    // and the same for nested nodes:
    //
    //            If
    //          /    \
    //         /      \
    //      1 expr1    \
    //       If         \
    //      /  \         \
    //     /    \         goto
    //  1 expr2            |
    //    goto    goto     |
    //      \     /        |
    //       \   /         |
    //   phi1(expr2, true|false)
    //          \          |
    //           \         |
    //             phi(phi1, true|false)

    if (end == null) return;
    if (end.phis.isEmpty) return;
    if (!identical(end.phis.first, end.phis.last)) return;
    HBasicBlock elseBlock = startIf.elseBlock;

    if (!identical(end.predecessors[1], elseBlock)) return;
    HPhi phi = end.phis.first;
    HInstruction thenInput = phi.inputs[0];
    HInstruction elseInput = phi.inputs[1];
    if (thenInput.isJsStatement() || elseInput.isJsStatement()) return;

    if (hasAnyStatement(elseBlock, elseInput)) return;
    assert(elseBlock.successors.length == 1);
    assert(end.predecessors.length == 2);

    HBasicBlock thenBlock = startIf.thenBlock;
    // Skip trivial goto blocks.
    while (thenBlock.successors[0] != end && thenBlock.first is HGoto) {
      thenBlock = thenBlock.successors[0];
    }

    // If the [thenBlock] is already a control flow operation, and does not
    // have any statement and its join block is [end], we can emit a
    // sequence of control flow operation.
    if (controlFlowOperators.contains(thenBlock.last)) {
      HIf otherIf = thenBlock.last;
      if (!identical(otherIf.joinBlock, end)) {
        // This could be a join block that just feeds into our join block.
        HBasicBlock otherJoin = otherIf.joinBlock;
        if (otherJoin.first != otherJoin.last) return;
        if (otherJoin.successors.length != 1) return;
        if (otherJoin.successors[0] != end) return;
        if (otherJoin.phis.isEmpty) return;
        if (!identical(otherJoin.phis.first, otherJoin.phis.last)) return;
        HPhi otherPhi = otherJoin.phis.first;
        if (thenInput != otherPhi) return;
        if (elseInput != otherPhi.inputs[1]) return;
      }
      if (hasAnyStatement(thenBlock, otherIf)) return;
    } else {
      if (!identical(end.predecessors[0], thenBlock)) return;
      if (hasAnyStatement(thenBlock, thenInput)) return;
      assert(thenBlock.successors.length == 1);
    }

    // From now on, we have recognized a control flow operation built from
    // the builder. Mark the if instruction as such.
    controlFlowOperators.add(startIf);

    // Find the next non-HGoto instruction following the phi.
    HInstruction nextInstruction = phi.block.first;
    while (nextInstruction is HGoto) {
      nextInstruction = nextInstruction.block.successors[0].first;
    }

    // If the operation is only used by the first instruction
    // of its block and is safe to be generated at use site, mark it
    // so.
    if (phi.usedBy.length == 1 &&
        phi.usedBy[0] == nextInstruction &&
        isSafeToGenerateAtUseSite(phi.usedBy[0], phi)) {
      markAsGenerateAtUseSite(phi);
    }

    if (identical(elseInput.block, elseBlock)) {
      assert(elseInput.usedBy.length == 1);
      markAsGenerateAtUseSite(elseInput);
    }

    // If [thenInput] is defined in the first predecessor, then it is only used
    // by [phi] and can be generated at use site.
    if (identical(thenInput.block, end.predecessors[0])) {
      assert(thenInput.usedBy.length == 1);
      markAsGenerateAtUseSite(thenInput);
    }
  }
}