// For switch statements, we have an option of completely rewriting the switch header
// and switch sections into simpler constructs, i.e. we can rewrite the switch header
// using bound conditional goto statements and the rewrite the switch sections into
// bound labeled statements.
//
// However, all the logic for emitting the switch jump tables is language agnostic
// and includes IL optimizations. Hence we delay the switch jump table generation
// till the emit phase. This way we also get additional benefit of sharing this code
// between both VB and C# compilers.
//
// For string switch statements, we need to determine if we are generating a hash
// table based jump table or a non hash jump table, i.e. linear string comparisons
// with each case label. We use the Dev10 Heuristic to determine this
// (see SwitchStringJumpTableEmitter.ShouldGenerateHashTableSwitch() for details).
// If we are generating a hash table based jump table, we use a simple
// hash function to hash the string constants corresponding to the case labels.
// See SwitchStringJumpTableEmitter.ComputeStringHash().
// We need to emit this same function to compute the hash value into the compiler generated
// <PrivateImplementationDetails> class.
// If we have at least one string switch statement in a module that needs a
// hash table based jump table, we generate a single public string hash synthesized method
// that is shared across the module.
private void LowerBasicSwitch(DecisionTree.ByValue byValue)
{
var switchSections = ArrayBuilder <BoundSwitchSection> .GetInstance();
var noValueMatches = _factory.GenerateLabel("noValueMatches");
var underlyingSwitchType = byValue.Type.IsEnumType() ? byValue.Type.GetEnumUnderlyingType() : byValue.Type;
foreach (var vd in byValue.ValueAndDecision)
{
var value = vd.Key;
var decision = vd.Value;
var constantValue = ConstantValue.Create(value, underlyingSwitchType.SpecialType);
var constantExpression = new BoundLiteral(_factory.Syntax, constantValue, underlyingSwitchType);
var label = _factory.GenerateLabel("case+" + value);
var switchLabel = new BoundSwitchLabel(_factory.Syntax, label, constantExpression, constantValue);
var forValue = ArrayBuilder <BoundStatement> .GetInstance();
LowerDecisionTree(byValue.Expression, decision, forValue);
if (!decision.MatchIsComplete)
{
forValue.Add(_factory.Goto(noValueMatches));
}
var section = new BoundSwitchSection(_factory.Syntax, ImmutableArray.Create(switchLabel), forValue.ToImmutableAndFree());
switchSections.Add(section);
}
var rewrittenSections = switchSections.ToImmutableAndFree();
MethodSymbol stringEquality = null;
if (underlyingSwitchType.SpecialType == SpecialType.System_String)
{
_localRewriter.EnsureStringHashFunction(rewrittenSections, _factory.Syntax);
stringEquality = _localRewriter.UnsafeGetSpecialTypeMethod(_factory.Syntax, SpecialMember.System_String__op_Equality);
}
// The BoundSwitchStatement requires a constant target when there are no sections, so we accomodate that here.
var constantTarget = rewrittenSections.IsEmpty ? noValueMatches : null;
var switchStatement = new BoundSwitchStatement(
_factory.Syntax, null, _factory.Convert(underlyingSwitchType, byValue.Expression),
constantTarget,
ImmutableArray <LocalSymbol> .Empty, ImmutableArray <LocalFunctionSymbol> .Empty,
rewrittenSections, noValueMatches, stringEquality);
// The bound switch statement implicitly defines the label noValueMatches at the end, so we do not add it explicitly.
switch (underlyingSwitchType.SpecialType)
{
case SpecialType.System_Boolean:
// boolean switch is handled in LowerBooleanSwitch, not here.
throw ExceptionUtilities.Unreachable;
case SpecialType.System_String:
case SpecialType.System_Byte:
case SpecialType.System_Char:
case SpecialType.System_Int16:
case SpecialType.System_Int32:
case SpecialType.System_Int64:
case SpecialType.System_SByte:
case SpecialType.System_UInt16:
case SpecialType.System_UInt32:
case SpecialType.System_UInt64:
{
// emit knows how to efficiently generate code for these kinds of switches.
_loweredDecisionTree.Add(switchStatement);
break;
}
default:
{
// other types, such as float, double, and decimal, are not currently
// handled by emit and must be lowered here.
_loweredDecisionTree.Add(LowerNonprimitiveSwitch(switchStatement));
break;
}
}
LowerDecisionTree(byValue.Expression, byValue.Default);
}
