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Assembler with metaprogramming features
Author:  Date: 2017-08-14 22:33
Agner wrote:
I think it is convenient to have preprocessing directives like #include and #if resolved before the assembler begins to interpret anything else because programmers are used to this concept and it is completely transparent what happens. If we would use some kind of metaprogramming for the same purpose then it becomes less obvious at what stage in the process these directives are executed.

This is a valid point, I still have to think about that.

My idea was to use metaprogramming as an extension to the preprocessor. For example, variables defined on meta-level could be treated as preprocessor-macros by the actual assembler. This policy also solves the ambiguity problem:
Agner wrote:

I have encountered a problem with this strategy, though. Consider the following code, where % means define an assemble-time variable:

%A = R1
%B = R2
A = B

This is ambiguous. It could generate the runtime instruction R1 = R2, or it could change the assemble-time variable A to R2.
If the symbol always gets set in the meta-code and always gets inserted in the assembler code, this example would always produce:
R1 = R2
We usually do not know the content of a register at assemble time. For that reason, we should be able to automaticly make a symbol an alias when defining with a register. Function arguments should also be able to be defined as aliases automaticly.


Agner wrote:

A possible solution to this dilemma is to define a symbol for alias names. For example:
& parameter1 = R0
could define parameter1 as an alias for register R0. This would be useful for function parameters and local variables, but it would generate further confusion with three different prefixes: #, %, &
This is a trade-off that has to be made. How many features do we need and how much complexity is still easy enough to use?
My vision was usage of the metaprogramming-features in a template-like fashion. For example, this is a template for an engine written in python, called mako. It outputs html-code:
<%inherit file="base.html"/>
<%
rows = [[v for v in range(0,10)] for row in range(0,10)]
%>
<table>
% for row in rows:
${makerow(row)}
% endfor
</table>

<%def name="makerow(row)">
<tr>
% for name in row:
<td>${name}</td>\
% endfor
</tr>
</%def>


My suggestion is aiming for a fairly simple code, maybe at the price of a more complex interpretation. I really don't know if this would be a better way than the current way of using different prefixes. It could also cause more problems with debugging, as you mentioned. There also might be many other problems I'm currently not seeing, but it is an idea nonetheless.

 
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