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

[thopre01 at gcc dot gnu.org]

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

--- Comment #21 from Thomas Preud'homme <thopre01 at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #20)
> (In reply to Thomas Preud'homme from comment #19)
> > (In reply to Richard Biener from comment #18)
> > > (In reply to Thomas Preud'homme from comment #17)
> > > > (In reply to rguenther@suse.de from comment #16)
> > > > > On Thu, 23 Mar 2017, thopre01 at gcc dot gnu.org wrote:
> > > > > > 
> > > > > > Funnily this led back to the Cortex-M0+ reduced testcase. With the patch in
> > > > > > comment #13 applied we can still see a difference in the push (one register
> > > > > > pushed Vs 0).
> > > > > 
> > > > > I can't reproduce zero pushes here I get three with/without 
> > > > > -fno-code-hoisting.  code hoisting hoists the two loads inside
> > > > > the switch before it so we have
> > > > 
> > > > Ooops my apologize, it needs more flags indeed. -O2 -funroll-all-loops shows
> > > > 2 registers pushed Vs 1 when -fno-code-hoisting is added.
> > > 
> > > Still can't reproduce.  I've configured with
> > > 
> > > /space/rguenther/src/svn/gcc-7-branch/configure
> > > --target=arm-suse-linux-gnueabi --disable-libstdcxx-pch
> > > --enable-languages=c,c++
> > > 
> > > and am using
> > > 
> > > ./cc1 -quiet cortex-m0plus_reproducer.c -O2 -mcpu=cortex-m0plus -mthumb -I
> > > include  -fdump-tree-pre-details -fdump-tree-crited  -fdump-tree-all
> > > -funroll-all-loops -fno-code-hoisting
> > 
> > Oh my bad, I was still not clear enough. I meant using the Cortex-M0+
> > testcase but build it for Cortex-M7. Hopefully with -mcpu=cortex-m7 you
> > should see a difference between with and without code hoisting.
> 
> I do.  Two pushes with code hoisting and three without.  Changed -mcpu above
> to -mcpu=cortex-m7.

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?