AW: Make GCC move instructions between a multi-cycle instruction and the next instruction that depends on its result.

[<stefan@franke.ms>]

> -----Ursprüngliche Nachricht-----
> Von: Gcc-help <gcc-help-bounces+bebbo=bejy.net@gcc.gnu.org> Im
> Auftrag von Segher Boessenkool
> Gesendet: Mittwoch, 23. Februar 2022 01:23
> An: William Tambe <tambewilliam@gmail.com>
> Cc: gcc-help <gcc-help@gcc.gnu.org>
> Betreff: Re: Make GCC move instructions between a multi-cycle instruction
> and the next instruction that depends on its result.
> 
> On Tue, Feb 22, 2022 at 03:15:55PM -0600, William Tambe via Gcc-help
> wrote:
> [snip]
> > > Without above transformation, `add %4 %7` would cause the cpu to
> > > wait on `div %4 %5` when it could have executed instructions that do
> > > not depend on the result of "div".
> >
> > How to implement above transformation such that GCC moves instructions
> > between a multi-cycle instruction and the next instruction that
> > depends on its result ?
> 
> GCC has a pretty advanced instruction scheduler.  You can start looking at
> <https://gcc.gnu.org/onlinedocs/gccint/Scheduling.html> for example?
> 

You should also look at the md files defining an automaton -->
"define_automaton". 

There you model the pipelines, latency etc.p.p. for the cpu. 
Then annotate the insns in the cpu md file with the types from the
automaton.
That information can be used by the scheduler and you may still need to
implement some of the scheduler hooks.

Start looking at a simple cpu.

Stefan