private static void MoveOutward(GotoStatement stmt, int gLevel, int lLevel)
{
AstBlock origin = ParentBlock(stmt.Goto);
AstBlock block = origin;
//Check if a loop is enclosing the goto, and the block that is
//directly related to the label is above the loop block.
//In that case, we need to introduce a break to get out of the loop.
AstBlock loopBlock = origin;
int loopLevel = gLevel;
while (loopLevel > lLevel)
{
AstBlock child = loopBlock;
loopBlock = loopBlock.Parent;
loopLevel--;
if (child.Type == AstBlockType.DoWhile)
{
EncloseSingleInst(stmt, Instruction.LoopBreak);
block.Remove(stmt.Goto);
loopBlock.AddAfter(child, stmt.Goto);
block = loopBlock;
gLevel = loopLevel;
}
}
//Insert ifs to skip the parts that shouldn't be executed due to the goto.
bool tryInsertElse = stmt.IsUnconditional && origin.Type == AstBlockType.If;
while (gLevel > lLevel)
{
Enclose(block, AstBlockType.If, stmt.Condition, Next(stmt.Goto));
block.Remove(stmt.Goto);
AstBlock child = block;
//We can't move the goto in the middle of a if and a else block, in
//this case we need to move it after the else.
//IsLoop may need to be updated if the label is inside the else, as
//introducing a loop is the only way to ensure the else will be executed.
if (Next(child) is AstBlock elseBlock && elseBlock.Type == AstBlockType.Else)
{
child = elseBlock;
}
block = block.Parent;
block.AddAfter(child, stmt.Goto);
gLevel--;
if (tryInsertElse && child == origin)
{
AstBlock lBlock = ParentBlock(stmt.Label);
IAstNode last = block == lBlock && !stmt.IsLoop ? stmt.Label : null;
AstBlock newBlock = Enclose(block, AstBlockType.Else, null, Next(stmt.Goto), last);
if (newBlock != null)
{
block.Remove(stmt.Goto);
block.AddAfter(newBlock, stmt.Goto);
}
}
}
}
//This is a modified version of the algorithm presented on the paper
//"Taming Control Flow: A Structured Approach to Eliminating Goto Statements".
public static void Eliminate(GotoStatement[] gotos)
{
for (int index = gotos.Length - 1; index >= 0; index--)
{
GotoStatement stmt = gotos[index];
AstBlock gBlock = ParentBlock(stmt.Goto);
AstBlock lBlock = ParentBlock(stmt.Label);
int gLevel = Level(gBlock);
int lLevel = Level(lBlock);
if (IndirectlyRelated(gBlock, lBlock, gLevel, lLevel))
{
AstBlock drBlock = gBlock;
int drLevel = gLevel;
do
{
drBlock = drBlock.Parent;
drLevel--;
}while (!DirectlyRelated(drBlock, lBlock, drLevel, lLevel));
MoveOutward(stmt, gLevel, drLevel);
gBlock = drBlock;
gLevel = drLevel;
if (Previous(stmt.Goto) is AstBlock elseBlock && elseBlock.Type == AstBlockType.Else)
{
//It's possible that the label was enclosed inside an else block,
//in this case we need to update the block and level.
//We also need to set the IsLoop for the case when the label is
//now before the goto, due to the newly introduced else block.
lBlock = ParentBlock(stmt.Label);
lLevel = Level(lBlock);
if (!IndirectlyRelated(elseBlock, lBlock, gLevel + 1, lLevel))
{
stmt.IsLoop = true;
}
}
}
if (DirectlyRelated(gBlock, lBlock, gLevel, lLevel))
{
if (gLevel > lLevel)
{
MoveOutward(stmt, gLevel, lLevel);
}
else
{
if (stmt.IsLoop)
{
Lift(stmt);
}
MoveInward(stmt);
}
}
gBlock = ParentBlock(stmt.Goto);
if (stmt.IsLoop)
{
EncloseDoWhile(stmt, gBlock, stmt.Label);
}
else
{
Enclose(gBlock, AstBlockType.If, stmt.Condition, Next(stmt.Goto), stmt.Label);
}
gBlock.Remove(stmt.Goto);
}
}
