m68k compound instructions

[Maxim Kuvyrkov <maxim at codesourcery dot com>]

Hello Andreas,
Hello Jeff,

I have recently started on implementing processor pipeline model for 
m68k target.  At this point I am trying to figure out the best way of 
annotating instructions in m68k.md, so that appropriate cpu unit 
reservations could be assigned to each one of them.

The problem is that about half of all define_insns in m68k.md expands 
into different sequences of asm instructions, and the sequences 
themselves can vary in length.  Does such an approach to write a machine 
description allow to fine tune emitted code for a particular processor 
or is it mostly a legacy of m68k being one of the first ports?

Considering the issue itself, there are two paths to follow:
1. Preserve the machine description the way it is and workaround the 
issue with compound insns somehow.
2. Simplify machine description by splitting all compound insns into 
simple equivalents.

Option '1' will likely lead to some complex scheme of annotating 
compound instructions with reservations like "insn1_wait_insn2_insn3", 
which won't be very manageable in the future.  Also, obfuscated design 
won't contribute to ease of tuning of the scheduler.

Option '2' looks more favorable to me, though implies greater initial 
effort of simplifying complex machine description without loosing 
precious bits of fine tuning.  Following this path define_split will be 
my best friend and helper.

Which option would you prefer?  Or maybe I'm just missing something?


Thanks,

Maxim