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From: Robert Finch <robfi680@gmail.com>
Newsgroups: comp.arch
Subject: Re: Split instruction and immediate stream
Date: Sat, 8 Mar 2025 18:07:53 -0500
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On 2025-03-08 9:21 a.m., Thomas Koenig wrote:
> There was a recent post to the gcc mailing list which showed
> interesting concept of dealing with large constants in an ISA:
> Splitting a the instruction and constant stream.  It can be found
> at https://github.com/michaeljclark/glyph/ , and is named "glyph".
> 
> I think the problem the author is trying to solve is better addressed by
> My 66000 (and I would absolutely _hate_ to write an assembler for it).
> Still, I thought it worth mentioning.

Found that post interesting.

As outlined, the immediate base register requires a double-wide link 
register. This may be okay for code with 32b addresses running in a 
64-bit machine. But otherwise would probably need to go through another 
GPR to manage the immediate base register. It is potentially more 
instructions in the function prolog / epilog code. And more instructions 
at function call.

I think splitting the code and constant into separate streams requires 
another port(s) on the I$. The port may already be present if 
jump-through-table, JTT, is supported.

I guess that the constant tables for a subroutine would be placed either 
before or after a subroutine. I would not use the constant tables for 
all constants. Small constants are better encoded directly in the 
instruction. That means using bits to select between small constants or 
relative addresses.

I think it is better to use a constant prefix / postfix instruction to 
encode larger constants in the instruction stream. Or use a wider 
instruction format. In Q+ constant postfixes can be used to override a 
register spec, allowing immediate constants to be used with many more 
instructions.