/// <summary>
/// Replace a copy along a chain of variables (e.g. `b = a; c = b`) with a direct copy (e.g. `b = a; c = a`)
/// </summary>
/// <param name="cfg"></param>
/// <param name="ssa"></param>
/// <returns></returns>
public static IControlFlowGraph FoldUnnecessaryCopies(this IControlFlowGraph cfg, ISingleStaticAssignmentTable ssa)
{
IControlFlowGraph InnerFold(IControlFlowGraph cfgi)
{
// Copy across vertices, modifying them in the process
return(cfgi.Modify((a, b) =>
{
// generate data flow graph for this block
var dfg = (IDataFlowGraph) new DataFlowGraph.DataFlowGraph(a, ssa);
// Count how many times each variable in the whole program is read
var readsInProgram = cfgi.FindReadCounts().ToDictionary(x => x.Item1, x => x.Item2);
// Find variables which are written in this block (we're in SSA, so everything is written exactly once)
var writesInBlock = a.FindWrites(ssa).ToHashSet();
// Find variables which are written in this block and read (just once) in this block
var copiesInBlock = (
from x in a.FindReadCounts()
where writesInBlock.Contains(x.Item1)
where x.Item2 == readsInProgram[x.Item1]
where x.Item2 == 1
where !x.Item1.IsExternal
select x.Item1
).ToArray();
// Select a single var to optimise
var work = (
from op in dfg.Outputs
let ass = op.ToStatement() as Assignment
where ass != null
where copiesInBlock.Contains(ass.Left)
where ass.Right is Variable
select(ass, op)
).FirstOrDefault();
// Early out if there is nothing to optimise
if (work == default)
{
a.CopyTo(b);
return;
}
// Select the variable to replace and the expression to replace it with
var varToReplace = work.ass.Left;
var expToSubstitute = work.ass.Right;
var modified = new SubstituteVariable(varToReplace, expToSubstitute).Visit(a);
foreach (var item in modified.Statements)
{
b.Add(item);
}
}));
}
// Keep applying folding until no more folds are done
return(cfg.Fixpoint(InnerFold));
}
/// <summary>
/// Replace expressions which result in a constant with the result
/// </summary>
/// <param name="cfg"></param>
/// <param name="ssa"></param>
/// <returns></returns>
[NotNull] public static IControlFlowGraph FoldConstants([NotNull] this IControlFlowGraph cfg, ISingleStaticAssignmentTable ssa)
{
// Keep finding and replacing constants until nothing is found
cfg = cfg.Fixpoint(c => {
// Find variables which are assigned a value which is not tainted by external reads
var constants = c.FindConstants(ssa);
// Replace reads of a constant variable with the expression assigned to that variable
c = c.VisitBlocks(() => new ReplaceConstantSubexpressions(constants));
return(c);
});
// Replace constant subexpressions with their value
cfg = cfg.VisitBlocks(() => new ConstantFoldingVisitor(true));
return(cfg);
}
