Script for Laboratory: Designing embedded ASIPs - CES

eplacement can be performed. The condition in this example uses some macros (IS_CHAR,
…) and it refers to a node of the source-subtree (add) and accesses some fields of this node
(Type).
With the COST keyword the user can define how expensive a rule shall be. For example the
cost can reflect the execution time of the code that has to be executed to implement the
behavior of this rule. While compilation of the application the code selector determines all
rule combinations that are able to cover the whole LIR with rules and then it chooses the
cheapest coverage concerning the COST clauses.
The last keyword in the example rule is EMIT. If a coverage of the LIR is found, chosen and
scheduled, then the emit parts of all rules are considered. The emit parts contain plain C-code,
but they can also access the properties of the named parts in the rule header. A usual emit part
for the above given rule would look like:
EMIT {
fprintf(outfile, “add %s, %s, %s\n”, rd, rs0, rs1);
}
The parameter outfile is expected to be a file, that was opened elsewhere and the parameters
rd, rs0 and rs1 will be replaced by the real register names, after the register allocator has
selected them. In our ASIP Meister examples the emit part looks different, because the
assembly instructions are not immediately written into a file, but they are written into a
temporary data structure, that takes care about inserting the NOP instructions before the result
is written to a file. This data structure can be accessed by the different versions of the
addInsInfo (speak: Add Instruction Information) methods, like shown in the example above.
8.2.2 Typical problems while creating the CoSy compiler
While creating the CoSy compiler a tool chain is executed step by step. In this chapter only
the 3 main point of this tool chain will be mentioned, as those 3 main points are also the main
point for the typical problems. The entry to this tool chain is ASIP Meister, as it creates the
software description for your current CPU. In the next step this software description is
transformed into CoSy rules by the tool “meistercg”, which is created by the developers of
ASIP Meister. The last step is compiling the CoSy compiler with the automatically created
rules.
The typical problems in the first step (ASIP Meister software creation) are explained in
Chapter 4. For the second step with “meistercg” there are very few problems. One example is
an if-then-else construction in the behavior description of an assembly instruction. While
ASIP Meister accepts this, it is not implemented in “meistercg” yet. Newer versions might
support such behavior descriptions, but the current version will abort. The error message
refers to a specific file (usually the instruction_set.arch in the dlx_basis.sw directory) that
manually has to be changed (i.e. delete the corresponding assembly instruction with the ifthen-else)
to solve the problem.
The most problems will appear in the final stage, the compilation of the CoSy compiler itself.
The reasons for those problems are typically falsely created rules from “meistercg”. The most
kinds of those falsely created rules are meanwhile automatically detected and fixed by a part
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