public Code(Label value)
{
this.Type = CodeType.Label;
this.Value = value.Index;
this.Flags = CodeFlags.None;
}
/// <summary>
/// Fix the label to the given label.
/// </summary>
public void Fix(Label fixTo, SourceSpan span)
{
this.ForwardsTo = fixTo;
this.Span = span;
}
/// <summary>
/// The main entry point into the code generator, this assembles a
/// complete, parsed source file.
/// </summary>
public CgResult CgProgram(ParseTreeNode node)
{
/*
* If it weren't for labels, this would be a piece of cake. All
* we'd have to do is produce cpu code for each instruction and
* join them all together.
*
* But we *do* have to deal with labels, so there. We could handle
* this by keeping track of a list of fixups; locations that contain
* a forward reference. Then, once we know what that label resolves
* to, we go back and do the fixups.
*
* However, I'd like to be able to do optimisation on the generated
* code; specifically, replacing jumps to nearby labels with
* relative jumps using add/sub pc, X. Doing this will cause the
* address of all subsequent code (and labels) to change.
*
* In that case, it's probably simpler to just leave all labels
* unresolved until *after* optimisation. But what happens when
* we do shorten a given instruction?
*
* When we shorten a given instruction, all subsequent fixups need
* to have their target address adjusted.
*
* An attractive alternative is to not keep a list of fixups;
* instead, generated code is widened such that we can store
* metadata for each word. This would include storing label
* references directly. The advantage of this is that moving
* code around is cheap.
*
* So that's what we'll do.
*
* For now, we'll just use a list to store the generated code.
* Ideally, we'd have a linked list of pages since we won't be
* doing a lot of cutting.
*/
var code = new List<Code>();
/*
* This array stores difference literals. We need to do this
* since difference literals can depend on labels but won't
* fit in a code.
*/
var diffLits = new List<Difference>();
/*
* We also need something to keep track of the defined labels.
* Since we need to be able to pack this into code entries,
* we'll stick them in a flat array.
*/
var labels = new List<Label>();
/*
* A way of mapping a label name to a label object would also
* be handy.
*/
var labelMap = new Dictionary<string, Label>();
/*
* In order to implement local labels, we need to track the
* last non-local label we saw.
*/
var lastGlobalLabel = InitialLabel;
/*
* We'll also need a function we can pass into the codegen
* functions to turn a label name into a label object.
*/
LabelLookup lookup = delegate(string name)
{
if (name.StartsWith("."))
name = lastGlobalLabel + name;
if (labelMap.ContainsKey(name))
return labelMap[name];
else
{
var index = (ushort)labels.Count;
var label = new Label(index, name);
labels.Add(label);
labelMap[name] = label;
return label;
}
};
/*
* And while we're on the subject: a function to write to the
* code list.
*/
CodeWriter write = delegate(Code c)
{
code.Add(c);
};
Func<Difference, Code> diffToCode = delegate(Difference diff)
{
var index = (ushort)diffLits.Count;
diffLits.Add(diff);
return new Code(CodeType.Difference, index);
};
/*
* Whack 'em in a context.
*/
var ctx = new CgContext
{
Lookup = lookup,
Write = write,
GetCurrentAddress = () => (ushort)code.Count,
EncodeDifference = diffToCode,
};
/*
* Ok, let's process those lines.
*/
foreach (var line in node.ChildNodes)
{
var labelNode = line.ChildNodes[0].ChildNodes.FirstOrDefault();
var instrNode = line.ChildNodes[1].ChildNodes.FirstOrDefault();
var labelName = labelNode != null ? labelNode.ChildNodes[0].Token.Text : null;
if (labelName != null)
{
if (!labelName.StartsWith("."))
lastGlobalLabel = labelName;
var label = lookup(labelName);
if (label.Fixed)
throw new CodegenException("Label '{0}' already defined at {1}.",
labelName, label.Span.Location);
if (instrNode != null && instrNode.Term.Name == "AddressFix")
{
var fix = EvalLiteralWord(instrNode.ChildNodes[0], ref ctx);
switch (fix.Type)
{
case CodeType.Literal:
label.Fix(fix.Value, labelNode.Span);
break;
case CodeType.Label:
label.Fix(labels[fix.Value], labelNode.Span);
break;
case CodeType.Difference:
throw new CodegenException("Labels cannot be fixed to difference literals");
default:
throw new UnexpectedGrammarException();
}
instrNode = null;
}
else
{
var addr = (ushort)code.Count;
label.Fix(addr, labelNode.Span);
}
}
if (instrNode != null)
CgOptInstruction(instrNode, ref ctx);
}
/*
* Let's add a few extra labels for fun. And usefulness.
*/
ctx.Lookup("__CODE_START").Fix(0, node.Span);
ctx.Lookup("__CODE_END").Fix((ushort)code.Count, node.Span);
/*
* We now have code generated for the whole program. What we
* we need to do now is go back and fill in the labels.
*
* Before that, let's just make sure that all labels were
* actually defined somewhere...
*/
foreach (var label in labels)
if (!label.Fixed)
throw new CodegenException("Label '{0}' never defined.", label.Name);
/*
* Right. We now have enough information to generate the final, complete
* binary.
*
* First of all, we'll fill in guesses for what the labels are attached to.
*/
foreach (var label in labels)
{
if (label.IsForwarded)
continue;
var addr = label.Value;
if (addr >= code.Count)
continue;
switch (code[addr].Type)
{
case CodeType.Instruction:
label.Type = LabelType.Code;
break;
case CodeType.Label:
if( (code[addr].Flags & CodeFlags.IsLiteral) == 0 )
goto default;
label.Type = LabelType.Data;
break;
case CodeType.Literal:
label.Type = LabelType.Data;
break;
default:
label.Type = LabelType.Unknown;
break;
}
}
/*
* Now we can produce the output image.
*/
Func<Code, ushort> evalCode = null;
evalCode = delegate(Code c)
{
switch (c.Type)
{
case CodeType.Label:
return labels[c.Value].Value;
case CodeType.Difference:
{
var diff = diffLits[c.Value];
return (ushort)(evalCode(diff.Target) - evalCode(diff.Base));
}
default:
return c.Value;
}
};
var image = new ushort[code.Count];
{
var i = 0;
foreach (var c in code)
{
image[i] = evalCode(c);
++i;
}
}
/*
* Done. Collect the results.
*/
labels.Sort((a, b) => a.Value - b.Value);
return new CgResult
{
Image = image,
Labels = labels,
};
}