/// <summary>
/// Test two conditions, and conditionally branch vs. fall through:
/// either branch to target or fall through to subsequent code;
/// conditions are and'ed together: both together constitute "true".
/// Another way of thinking of this is condition conjunction (both).
/// </summary>
/// <param name="left">
/// Initial/left test condition.
/// </param>
/// <param name="right">
/// Initial/right test condition -- possibly short-circuted.
/// </param>
/// <param name="target">
/// Conditional branch target (or else fall thru).
/// </param>
/// <param name="reverse">
/// false --> normal case, fall through on both true, otherwise on false: branch to target
/// true --> reversed case, fall through at least one false, otherwise on true: branch to target
/// </param>
public void EvalBoth(AbstractSyntaxTree left, AbstractSyntaxTree right, BranchTargetLabel target, bool reverse)
{
// NB: "Both" means evaluating left/first and right/second operands in the normal way.
// Each operand is evaluated to branch around the then-part or loop-body on condition false,
// and fall thru into the then-part or loop-body on condition true (both sub conditions being true).
left.GenerateCodeForConditionalBranchWithPrettyPrint(this, target, reverse);
right.GenerateCodeForConditionalBranchWithPrettyPrint(this, target, reverse);
}
public void PlaceLabelHere(BranchTargetLabel label)
{
if (_labelList [label.Id])
{
throw new AssertionFailedException("label already defined: L" + label.Id);
}
_labelList [label.Id] = true;
OutLine(String.Format("L{0}:", label.Id));
}
/// <summary>
/// Test two conditions, and conditionally branch vs. fall through:
/// either branch to target or fall through to subsequent code;
/// conditions are or'ed together: either one constitutes "true".
/// Another way of thinking of this is condition disjunction (i.e. either).
/// </summary>
/// <param name="left">
/// Initial/left test condition.
/// </param>
/// <param name="right">
/// Initial/right test condition -- possibly short-circuted.
/// </param>
/// <param name="target">
/// Conditional branch target (or else fall thru).
/// </param>
/// <param name="reverse">
/// false --> normal case, fall through to following code on at least one true, otherwise on false: branch to target
/// true --> reversed case, fall through both false, otherwise on true: branch to target
/// </param>
public void EvalEither(AbstractSyntaxTree left, AbstractSyntaxTree right, BranchTargetLabel target, bool reverse)
{
// NB: "Either" means evaluating left/first operand, and on true, taking branch around the right/second operand
// and into the fall thru code of the then-part or loop-body!
// This requires use of an intermediate label, and a reversal of the evaluation condition, but only for the left/first operand.
// The second operand is evaluated as per normal, take brach around the then-part or loop-body on condition false.
var around = CreateLabel();
left.GenerateCodeForConditionalBranchWithPrettyPrint(this, around, !reverse);
right.GenerateCodeForConditionalBranchWithPrettyPrint(this, target, reverse);
PlaceLabelHere(around);
}
public void GenerateBinaryCompareConditionalBranch(Operator op, BranchTargetLabel label, bool reverse)
{
//Handle the Relational Operators: LessThan,LessOrEqual,GreaterThan,GreaterOrEqual,EqualEqual,NotEqual,
var branchOpcode = "";
switch (op)
{
case Operator.LessThan:
branchOpcode = reverse ? "B.LT" : "B.GE";
break;
case Operator.LessOrEqual:
branchOpcode = reverse ? "B.LE" : "B.GT";
break;
case Operator.GreaterThan:
branchOpcode = reverse ? "B.GT" : "B.LE";
break;
case Operator.GreaterOrEqual:
branchOpcode = reverse ? "B.GE" : "B.LT";
break;
case Operator.EqualEqual:
branchOpcode = reverse ? "B.EQ" : "B.NE";
break;
case Operator.NotEqual:
branchOpcode = reverse ? "B.NE" : "B.EQ";
break;
default:
throw new AssertionFailedException("Operator is not one of the relational operators: " + op.ToString());
}
OutLine(String.Format(" {0}{2}L{1}", branchOpcode, label.Id, _argumentPosition.Substring(branchOpcode.Length)));
}
public void GenerateUnaryTestConditionalBranch(BranchTargetLabel label, bool reverse)
{
var opcode = reverse ? "B.TRUE" : "B.FALSE";
OutLine(String.Format(" {0}{2}L{1}", opcode, label.Id, _argumentPosition.Substring(opcode.Length)));
}
public void GenerateUnconditionalBranch(BranchTargetLabel label)
{
var opcode = "JUMP";
OutLine(String.Format(" {0}{2}L{1}", opcode, label.Id, _argumentPosition.Substring(opcode.Length)));
}
public virtual void GenerateCodeForConditionalBranchWithPrettyPrint(CodeGenContext context, BranchTargetLabel label, bool reverse)
{
context.PrettyPrint(this); // prints && operator
GenerateCodeForConditionalBranch(context, label, reverse);
}
/// <summary>
/// Generates code for the given tree node, with the purpose of accomplishing a conditional branch to target
/// or falling thru to whatever comes after this generated code.
/// </summary>
/// <param name="context">
/// where the generated code is output to, and some helper functions
/// </param>
/// <param name="label">
/// the desired target of the conditional branch
/// </param>
/// <param name="reverse">
/// the reverse parameter's meaning:
///
/// false --> use the normal if-then-style evaluation:
/// take branch on condition false and fall thru on condition true
///
/// true --> use the reverse of the normal sense of if-then evaluation:
/// take branch on condition true and fall thru on condition false
/// one might use this, if for some reason
/// we had choosen to swap the code ordering of the then-part and else-part
/// or, we need to evaluate something involving negation
/// or, we're evaluating a do ... while instead of while.
///
/// NB: The normal approach for an if-then-statement, e.g. in:
/// if ( a ) { b=1; }
/// is that we evaluate "a" and
/// conditionally branch: take branch around the then-part on "a" being false, or else,
/// fall thru to then-part (e.g. { b=1; } ) on "a" being true.
///
/// *If* statements and *While* statements and *For* statements evaluate their condition to
/// take the branch (around the then or the loop) on the expressed condition being false, and
/// fall-thru (into the then-part or loop-body) on it being true.
///
/// Sometimes we need to reverse things, e.g.:
/// To accomplish negation operator as in
/// if ( ! a ) { b=1; }
/// We would start by evaluating "! a" as with any if-statemetn: for conditional branch with reverse=false,
/// (meaning take branch on false, and fall thru on true)
/// However, the "!" will be handled by reversing the evaluation condition (for the further evaluation of "a")
/// rather than generating any actual negation code.
///
/// Also, *do while* statements evaluate the conditional for branch oppositely than regular while:
/// take branch (backward, to repeat the loop) on true, and fall-thru (exiting the loop) on false.
/// So, *do while* should evaluate its conditional expression using reverse=true.
///
/// Further note that the short-circut boolean || operator requires
/// evaluation of the left/first operand under reversed conditions:
/// this in order to branch around the right/second operand (short-circuting it).
/// So, to accomplish the fall thru to the then-part or loop-body,
/// the left/first operand is evaluated to take branch (condition true) into the then-part
/// and the right/second operand is evaluated to take branch (condition false) around the then-part
///
/// Combinations of &&, || and ! will reverse evaluation conditions as needed.
///
/// See the overrides:
/// ShortCircutAndTreeNode.GenerateCodeForConditionalBranch, and,
/// ShortCircutOrTreeNode.GenerateCodeForConditionalBranch
///
/// </param>
protected virtual void GenerateCodeForConditionalBranch(CodeGenContext context, BranchTargetLabel label, bool reverse)
{
//
// This implementation is used by any operator that does not customize the evaluation for conditional branch:
// This default implementation generates the value to be tested, and then does a test & branch on that value.
//
GenerateCodeForValue/*WithPrettyPrint*/ (context, EvaluationIntention.Value); // pretty print already done at this level
context.GenerateUnaryTestConditionalBranch(label, reverse);
// Note: ! && || operators can generate substantially better code using custom implementation rather than this generic one.
// Negation is discussed above:
// rather than generating code to negate a boolean and test & branch on that,
// negation simply evaluates the negated expression with reversed branch condition.
// The short-circut operators also need not generate a boolean value to be tested & branched on,
// and can instead directly branch on the conditions of the left-hand and right-hand side expressions.
}
