Re: [9fans] FP register usage in Plan9 assembler

[erik quanstrom <quanstro@quanstro.net>]

On Tue Feb  2 08:43:47 PST 2016, sstallion@gmail.com wrote:
> Hi Giacomo,
>
> It's probably worth mentioning that learning assembly using the Plan 9
> assembler is probably a bad idea. *a makes heavy use of pseudo
> instructions and registers and unless you're well versed in its
> quirks, can be very confusing when looking at more common assembly
> dialects. Many instructions are directly encoded in the instruction
> stream, largely due to the fact that it is more difficult than it
> should be to extend the assembler as architectures evolve*. More
> mechanically, Plan 9 makes use of a loader, which causes a number of
> operations you would expect to be present in the assembler to be
> deferred until later.

hi, steve!

i've found the plan 9 assemblers to be straight-forward enough.  i love
the consistency from one architecture to another.  i like the fact that
there are so few meta-directives, and i do like the fact that the linker
will do the obvious rearrangments of plan 9 assembly, including automatic
delay slot filling, and other such fiddly bits.

obviously, this is just an opinion, so ymmv.

i think this is off the original point, but as to modifying the assmbler.
to add a new instruction, the linker, assembler and libmach need modification.
typically this is a matter of adding a line to each one for assembly, linking,
and disassembly, respecively.  it's worth looking at the addition of the ymm,
and then zmm registers and those patterns to 6[al] and libmach.  this is an
example of adding a new instruction encoding to an existing arch.

> * The diff to update support for ARMv7-A to 5a came in at over 2800
> lines; this was to add a handful of instructions.

do you perhaps mean the linker?

the pi kernels, which supports v6 (original) and v7 (pi2) rely on small asm
files for arch-specific functions.  i think you provided some explaination of
why this approach would not work for v5, but unfortunately, i don't remember
it.

- erik