Re: Split instruction and immediate stream

On Sun, 23 Mar 2025 17:54:19 +0000, Robert Finch wrote:

On 2025-03-23 8:12 a.m., Marcus wrote:

Den 2025-03-23 kl. 04:06, skrev Robert Finch:

On 2025-03-22 11:04 a.m., Thomas Koenig wrote:

Marcus <m.delete@this.bitsnbites.eu> schrieb:

It does not quite work yet. Constants are placed and code is moved,
but the linked program does not have the correct references yet.
>
Experimental, but looking like things will work.
>

>
Although I have not tried any of these techniques, here are my thoughts.
>
Why not always place the constant next to (right after) the instruction
that references it, instead of at an offset within the cache line?
>

That is a very good idea. It is the same thing almost as using a
variable length instruction.
>
LB650 uses a smaller constant packet (16-bits) than the instruction. So,
instructions would need to be able to be aligned at 16-bit boundaries.
LB650 instruction are fixed 32-bit. There is also the possibility of
sharing the same constants, although slim.

Consider stack pointer displacements:: local x is always [SP,offset(X)]
since these are mostly 16-bit displacements, they are already optimal.
But consider local-static data:: displacement( x ) varies with IP,
unless the compiler 'eats' an instruction to load the address--which
is generally not that great of an idea.
Then consider extern-global data:: you are likely using 32-bit
(tiny-medium model) or 64-bit (large and huge model). The 32-bit
form is arguably IP-relative, the 64-bit form can be argued either
way.
My overall argument is that there are unlikely to be "all that many"
exactly equal constants in a dozen instructions that is that cache
line.

The effect should be very similar, but now you have a simpler offset
(it's always zero) and you eliminate the problem with having to keep
track of where the constants are in order to prevent the PC/IP from
running into the constant area.
>

I wish I had thought of that last night. But I have coded things now.

Never let a done (but mediocre) solution prevent better solutions--
that is for your Boss to decide.

Got the compiler / assembler going. The listings are a few percent
shorter than the PowerPC. It may be due to bugs yet. I think the
difference may be the PowerPC burns up bits using pairs of instructions
for high/low halves of the constant.
>
The vbcc compiler for the PowerPC was modified.