[Bug tree-optimization/80155] [7 regression] Performance regression with code hoisting enabled

[rguenther at suse dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80155

--- Comment #25 from rguenther at suse dot de <rguenther at suse dot de> ---
On Fri, 7 Apr 2017, thopre01 at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80155
> 
> --- Comment #24 from Thomas Preud'homme <thopre01 at gcc dot gnu.org> ---
> (In reply to rguenther@suse.de from comment #23)
> > On Fri, 7 Apr 2017, thopre01 at gcc dot gnu.org wrote:
> > 
> > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80155
> > > 
> > > --- Comment #22 from Thomas Preud'homme <thopre01 at gcc dot gnu.org> ---
> > > (In reply to Thomas Preud'homme from comment #21)
> > > > 
> > > > I can see this behavior for Cortex-M0+ indeed but the results are different
> > > > for Cortex-M7 for me:
> > > > 
> > > > <with code hoisting>
> > > > % arm-none-linux-gnueabi-gcc -S ~/cortex-m0plus_reproducer.c -O2
> > > > -mcpu=cortex-m7 -mthumb -fdump-tree-pre-details -fdump-tree-crited 
> > > > -fdump-tree-all -funroll-all-loops
> > > > 
> > > > % grep push cortex-m0plus_reproducer.s
> > > >         push    {r4, r5}
> > > > 
> > > > 
> > > > <without code hoisting>
> > > > % arm-none-linux-gnueabi-gcc -S ~/cortex-m0plus_reproducer.c -O2
> > > > -mcpu=cortex-m7 -mthumb -fdump-tree-pre-details -fdump-tree-crited 
> > > > -fdump-tree-all -funroll-all-loops -fno-code-hoisting
> > > > 
> > > > % grep push cortex-m0plus_reproducer.s
> > > >         push    {r4}
> > > > 
> > > > Would you mind checking again?
> > > 
> > > btw, this toolchain was built on 2017-04-04 in case that's the issue.
> > 
> > Ok, I can reproduce it now.  With the proposed patch to fix the other
> > case it regresses with -fno-code-hoisting though (comment #9 plus fix).
> > 
> > Note that code-hosting is simply doing it's job here, hoisting common
> > code out of a switch.
> 
> I understand that. Code is smaller so it's definitely worthwhile at -Os. But it
> sometimes lead to worse performance due to more spilling.

But there's no good way to estimate that.  We have tons of transforms
that have the issue of being "sometimes worse"