Re: [Iverilog-devel] Partial fix for pr1867161

[Stephen W. <st...@ic...>]

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Cary R. wrote:
> --- Stephen Williams <st...@ic...> wrote:
>> Actually, I think there was an obscure reason for the way it is
>> currently. I recall that it's current behavior is used to do some
>> specialized part selects in place. For example, if a smaller
>> load width N is given, then only the first N bits are loaded.
>> This is a common case. It might be that %load/vp0 is not used
>> that way, but I'm pretty sure that %load/v is.
> 
> I didn't change %load/v so it should be safe. I'll double check %load/vp0
> to make sure it's not used in this manner. If it is I think we will have
> to add a parameter to pass the value and increase the size of the opcode
> table or do the part select a different way. Do you have a preference?

You mean increasing the size of the vvp_code_s struct? This is
the fixed size of each instruction in the code stream, so increasing
this will increase proportional to the number of instructions the
run-time use. This might not be significant in the end.

> To me increasing the size of the opcode table is the most straight forward
> way to solve this if it is a problem. I don't think the increased table
> size is that significant.

It is true that there are some instructions that are just plain
bizarre in the lengths taken to avoid adding another operand. Maybe
it's time to add another operand slot just for that reason:-/ It
would mean updating the rather intense opcode table, but that's
just grunt work.

Another way to handle this that doesn't increase the size of
the table might me to create a bit_idx2[4] member in the second
union of the table that has 4 small integers. Instructions that
are heavily laden with index numbers could take advantage of this
new operand format, would remain compact, and would make lots of
better use of word registers.

- --
Steve Williams                "The woods are lovely, dark and deep.
steve at icarus.com           But I have promises to keep,
http://www.icarus.com         and lines to code before I sleep,
http://www.picturel.com       And lines to code before I sleep."
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