Re: [PATCH][ARM] Enable code hoisting with -Os (PR80155)

[Wilco Dijkstra <Wilco.Dijkstra@arm.com>]

Hi Prathamesh,

> My only concern with the patch is that the issue isn't specific to
> code-hoisting.
> For this particular case (reproducible with pr77445-2.c), disabling
> jump threading
> doesn't cause the register spill with hoisting enabled.
> Likewise disabling forwprop3 and forwprop4 prevents the spill.
> The last time I tried, setting setting
> max-jump-thread-duplication-stmts to 20 and
> fsm-scale-path-stmts to 3, not only removed the spill but also
> resulted in 9 more hoistings,
> but regressed some other test on jump threading.
> So I was wondering whether we should look into fine-tuning relevant params,
> instead of disabling code hoisting entirely (we will end up regressing
> some test cases either way) ?

The point is that -fcode-hoisting is the optimization that made some benchmarks
run significantly slower compared to older GCC versions. It's been more than 2.5
years without progress, so let's just fix it now in the obvious way.

Yes, there are likely better solutions. And if someone writes an acceptable patch
then we can easily enable hoisting again. Surely it is far better to have a fixed
compiler *now* rather than wait a few more years for a solution that has the
same effect but which might not be trivially backportable?

Cheers,
Wilco