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

[William Tambe <tambewilliam@gmail.com>]

On Wed, Feb 23, 2022 at 1:26 AM <stefan@franke.ms> wrote:
>
> > -----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.
>

Would it be possible and safe to accomplish above transformation using
TARGET_MACHINE_DEPENDENT_REORG ? If yes, what functions can be used to
move an instruction ?

> Stefan
>