/// <summary>
/// Determines the type of the loop and the condition of the loop. Adds additional nodes into the loop body.
/// </summary>
/// <param name="loopBody"></param>
/// <param name="header"></param>
/// <param name="latchingNodes"></param>
/// <param name="interval"></param>
/// <param name="loopCondition"></param>
/// <returns></returns>
private LoopType DetermineLoopType(HashSet<ILogicalConstruct> loopBody, HashSet<ILogicalConstruct> latchingNodes,
IntervalConstruct interval, DominatorTree dominatorTree, out ConditionLogicalConstruct loopCondition)
{
ILogicalConstruct header = interval.Entry as ILogicalConstruct;
HashSet<ILogicalConstruct> legalExits = new HashSet<ILogicalConstruct>(latchingNodes);
legalExits.Add(header);
ILogicalConstruct parentConstruct = header.Parent as ILogicalConstruct;
DFSTree dfsTree = DFSTBuilder.BuildTree(parentConstruct);
//B - nodes in the loop body (= loopBody)
//I - nodes in the interval (= interval.Children)
//U - union of all of the dominance frontiers of the nodes in B
//exitDominanceFrontier = (U n I) \ B
//If a node is in the exitDominanceFrontier, then it is dominated by the header and is a successor (not necessarily direct) of more than one
//node in the loop body.
HashSet<ILogicalConstruct> exitDominanceFrontier = new HashSet<ILogicalConstruct>();
foreach (ILogicalConstruct loopNode in loopBody)
{
foreach (ILogicalConstruct frontierNode in dominatorTree.GetDominanceFrontier(loopNode))
{
if (interval.Children.Contains(frontierNode) && !loopBody.Contains(frontierNode))
{
exitDominanceFrontier.Add(frontierNode);
}
}
}
//This is leftover heuristic, that was used for determining a suitable successor that is going to be follow node of the loop.
//Changing it now will break a good number of the tests. Since the produced output is acceptable, until a better heuristic is found
//there is no need to change it.
if (exitDominanceFrontier.Count == 0)
{
//If the exit dominance frontier is empty then we look for the node, with minimum post order index, that is a successor of a condition loop exit.
//The desired exit should be a condition in order to reduce the number of infinite loops (heuristic).
foreach (DFSTNode dfsNode in dfsTree.ReversePostOrder)
{
ILogicalConstruct construct = dfsNode.Construct as ILogicalConstruct;
if (loopBody.Contains(construct))
{
continue;
}
loopCondition = GetLoopConditionWithMaxIndex(dfsTree, loopBody, legalExits, construct);
//By taking the successor with the minimum post order index and the loop exit with the maximum post order index, we ensure that
//the produced construct will always be the same, since the post order in our case is a total order.
//There are other various ways of finding the exit-successor pair that can bring consistent output, but none of them is found to yield
//better results than the rest.
if (loopCondition != null)
{
//We expand the loop body only when we've found a condition successor of the loop.
//This is done in order to avoid adding all of the dominated nodes of an infinite loop to the body. (Better readability of the final code.)
//E.g.: An infinite loop on the top level of the logical tree (i.e. child of the method block construct). If it dominates all of its
//succeeding nodes then they will be in its interval, which means that they will be added to the loop. As a result there will
//be an infinite loop at the end of the method, that encloses a cood part of the code, for no apparent reason.
ExpandLoopBody(interval, loopBody, construct);
if (loopCondition == header)
{
return LoopType.PreTestedLoop;
}
else
{
return LoopType.PostTestedLoop;
}
}
}
if (CanBeLoopCondition(header, loopBody))
{
loopCondition = header as ConditionLogicalConstruct;
return LoopType.PreTestedLoop;
}
else
{
loopCondition = null;
return LoopType.InfiniteLoop;
}
}
else
{
//If there are nodes in the exitDominanceFrontier, then we choose the one with the minimum postorder index for successor of the loop.
//Then we try to find a condition exit of the loop, with maximum post order index, that is predecessor of the successor node.
int minOrderIndexOfSuccessor = dfsTree.ReversePostOrder.Count;
foreach (ILogicalConstruct successor in exitDominanceFrontier)
{
int currentOrderIndex = dfsTree.ConstructToNodeMap[successor].ReversePostOrderIndex;
if(currentOrderIndex < minOrderIndexOfSuccessor)
{
minOrderIndexOfSuccessor = currentOrderIndex;
}
}
ILogicalConstruct loopSuccessor = dfsTree.ReversePostOrder[minOrderIndexOfSuccessor].Construct as ILogicalConstruct;
loopCondition = GetLoopConditionWithMaxIndex(dfsTree, loopBody, legalExits, loopSuccessor);
ExpandLoopBody(interval, loopBody, loopSuccessor);
if (loopCondition != null)
{
if (loopCondition == header)
{
return LoopType.PreTestedLoop;
}
else
{
return LoopType.PostTestedLoop;
}
}
else
{
return LoopType.InfiniteLoop;
}
}
}
/// <summary>
/// Tries to build an if construct with condition - the specified condition.
/// </summary>
/// <remarks>
/// The idea is to get the dominated nodes of the true successor to create the then block and the dominated nodes of the false successor
/// to create the else block.
/// If both the then and else blocks have successors, then they must have a common successor to create the if construct.
/// </remarks>
/// <param name="condition"></param>
/// <returns>True on success.</returns>
private bool TryBuildIfConstruct(ConditionLogicalConstruct condition, DominatorTree dominatorTree, DFSTree dfsTree)
{
//Store the true and false successors for optimization.
ILogicalConstruct falseSuccessor = condition.FalseSuccessor;
ILogicalConstruct trueSuccessor = condition.TrueSuccessor;
HashSet<ISingleEntrySubGraph> falseSuccessorFrontier = dominatorTree.GetDominanceFrontier(falseSuccessor);
HashSet<ISingleEntrySubGraph> trueSuccessorFrontier = dominatorTree.GetDominanceFrontier(trueSuccessor);
ILogicalConstruct exitSuccessor = CheckSuccessor(condition, trueSuccessor, falseSuccessorFrontier, dfsTree) ??
CheckSuccessor(condition, falseSuccessor, trueSuccessorFrontier, dfsTree);
HashSet<ISingleEntrySubGraph> frontierIntersection = new HashSet<ISingleEntrySubGraph>(trueSuccessorFrontier);
frontierIntersection.IntersectWith(falseSuccessorFrontier);
if (exitSuccessor == null && falseSuccessorFrontier.Count > 0 && trueSuccessorFrontier.Count > 0 && frontierIntersection.Count == 0)
{
//If none of the successors can be a proper exit and the false and true successor frontiers are not empty but have no common node,
//then we do not make the if since it will not have a common exit.
return false;
}
HashSet<ILogicalConstruct> thenBody = GetBlockBody(dominatorTree, trueSuccessor, condition);
HashSet<ILogicalConstruct> elseBody = GetBlockBody(dominatorTree, falseSuccessor, condition);
if (thenBody == null && elseBody == null)
{
return false;
}
else if(thenBody == null)
{
condition.Negate(typeSystem);
ILogicalConstruct swapHelper = trueSuccessor;
trueSuccessor = falseSuccessor;
falseSuccessor = swapHelper;
thenBody = elseBody;
elseBody = null;
}
//If the else body is null but the false successor is not a successor of the then body then we do not make the if.
if(elseBody == null && !CheckSuccessors(thenBody, falseSuccessor))
{
return false;
}
if (ShouldInvertIfAndRemoveElse(thenBody, trueSuccessor, elseBody, falseSuccessor))
{
///This is performed for cosmetic reasons.
condition.Negate(typeSystem);
ILogicalConstruct successorSwapHelper = trueSuccessor;
trueSuccessor = falseSuccessor;
falseSuccessor = successorSwapHelper;
HashSet<ILogicalConstruct> swapHelper = thenBody;
thenBody = elseBody;
elseBody = swapHelper;
elseBody = null;
}
if (elseBody != null && !HasSuccessors(thenBody) &&
SubtreeEndsInInstructionCode(trueSuccessor.FirstBlock.TheBlock,new Code[]{Code.Ret, Code.Throw})) // check if all ends are throw and/or return -> allow mixed ends as well
{
// we don't need the else
elseBody = null;
}
BlockLogicalConstruct theThenBlock = new BlockLogicalConstruct(trueSuccessor, thenBody);
BlockLogicalConstruct theElseBlock = elseBody != null ? new BlockLogicalConstruct(falseSuccessor, elseBody) : null;
IfLogicalConstruct theIfConstruct = IfLogicalConstruct.GroupInIfConstruct(condition, theThenBlock, theElseBlock);
UpdateDominatorTree(dominatorTree, theIfConstruct);
return true;
}
