/// <summary>
/// Removes unused declared variables from the specified block.
/// </summary>
/// <param name="block">The block to remove unused declared variables from.</param>
/// <returns>The result of removing unused variables.</returns>
/// <example>
/// <code>{ int x, int y; x + 1; }</code>
/// becomes
/// <code>{ int x; x + 1; }</code>
/// </example>
private static BlockExpression RemoveUnusedVariables(BlockExpression block)
{
var variables = block.Variables;
var n = variables.Count;
if (n > 0)
{
// REVIEW: This does a lot of repeated scanning of the same child nodes.
// We could consider storing these results for later reuse.
var finder = new FreeVariableFinder();
finder.Visit(block.Expressions);
var freeVariables = finder.FreeVariables;
var remainingVariables = default(List <ParameterExpression>);
for (var i = 0; i < n; i++)
{
var variable = variables[i];
if (!freeVariables.Contains(variable))
{
if (remainingVariables == null)
{
remainingVariables = new List <ParameterExpression>(n);
for (var j = 0; j < i; j++)
{
remainingVariables.Add(variables[j]);
}
}
}
else
{
remainingVariables?.Add(variable);
}
}
if (remainingVariables != null)
{
return(block.Update(remainingVariables, block.Expressions));
}
}
return(block);
}
/// <summary>
/// Tries to beta-reduce the specified <paramref name="expression"/> if it's semantically sound to do so. This
/// includes preserving the order and plurality of side-effects of the arguments passed to the lambda expression.
/// </summary>
/// <param name="expression">The expression to beta-reduce.</param>
/// <param name="result">The result of beta-reducing the specified <paramref name="expression"/>, if semantically sound to do so.</param>
/// <returns><c>true</c> if a beta reduction was performed; otherwise, <c>false</c>.</returns>
public bool TryReduce(InvocationExpression expression, out Expression result)
{
var lambda = (LambdaExpression)expression.Expression;
var parameters = lambda.Parameters;
var arguments = expression.Arguments;
var n = arguments.Count;
if (n == 0)
{
result = lambda.Body;
return(true);
}
var findRestrictions = new FindInliningRestrictions();
var bindings = new Dictionary <ParameterExpression, Binding>(n);
var bindingOrder = new List <ParameterExpression>(n);
for (var i = 0; i < n; i++)
{
var parameter = parameters[i];
var argument = arguments[i];
findRestrictions.Visit(argument);
if (findRestrictions.Unsafe)
{
break;
}
var fvs = new FreeVariableFinder();
fvs.Visit(argument);
var binding = new Binding
{
Argument = argument,
FreeVariables = fvs.FreeVariables,
IsConstant = _provider.HasConstantValue(argument),
IsPure = _provider.IsPure(argument),
CanRepeat = CanInlineMany(argument),
};
bindings.Add(parameter, binding);
if (!binding.IsPure)
{
bindingOrder.Add(parameter);
}
}
//
// If we found any lval in an expression we'd be inlining, we can't proceed because we risk reordering
// reads and writes. For example:
//
// (x => a + x)(a = 1)
//
// would evaluate as:
//
// a = 1;
// (x => a + x)(a);
//
// and produces 2. However, if we inline the side-effect of assignment, we end up with:
//
// a + (a = 1)
//
// which will read the original value of a for the left operand.
//
// CONSIDER: The analysis could be made more precise to pick up on the storage locations being written
// to and the reads being performed by the lambda body.
//
if (!findRestrictions.Unsafe)
{
var impl = new Impl(this, bindings, bindingOrder);
var tmp = impl.Visit(lambda.Body);
impl.EnsureAllBound();
if (impl.Status == Status.AllBound)
{
result = tmp;
return(true);
}
}
result = null;
return(false);
}
