Re: [PATCH, PR68373 ] Call scev_const_prop in pass_parallelize_loops::execute

[Richard Biener <rguenther at suse dot de>]

On Thu, 19 Nov 2015, Tom de Vries wrote:

> On 17/11/15 23:20, Tom de Vries wrote:
> > [ was: Re: [PATCH, 10/16] Add pass_oacc_kernels pass group in passes.def ]
> > 
> > Hi,
> > 
> > Consider test-case test.c, with a use of the final value of the
> > iteration variable (return i):
> > ...
> > unsigned int
> > foo (int *a, unsigned int n)
> > {
> >    unsigned int i;
> >    for (i = 0; i < n; ++i)
> >      a[i] = 1;
> > 
> >    return i;
> > }
> > ...
> > 
> > Compiled with:
> > ...
> > $ gcc -S -O2 test.c -ftree-parallelize-loops=2 -fdump-tree-all-details
> > ...
> > 
> > Before parloops, we have:
> > ...
> >   <bb 4>:
> >    # i_12 = PHI <0(3), i_10(5)>
> >    _5 = (long unsigned int) i_12;
> >    _6 = _5 * 4;
> >    _8 = a_7(D) + _6;
> >    *_8 = 1;
> >    i_10 = i_12 + 1;
> >    if (n_4(D) > i_10)
> >      goto <bb 5>;
> >    else
> >      goto <bb 6>;
> > 
> >    <bb 5>:
> >    goto <bb 4>;
> > 
> >    <bb 6>:
> >    # i_14 = PHI <n_4(D)(4), 0(2)>
> > ...
> > 
> > Parloops will fail because:
> > ...
> > phi is n_2 = PHI <n_4(D)(4)>
> > arg of phi to exit:   value n_4(D) used outside loop
> >    checking if it a part of reduction pattern:
> >    FAILED: it is not a part of reduction....
> > ...
> > [ note that the phi looks slightly different. In
> > gather_scalar_reductions -> vect_analyze_loop_form ->
> > vect_analyze_loop_form_1 -> split_loop_exit_edge we split the edge from
> > bb4 to bb6. ]
> > 
> > This patch uses scev_const_prop at the start of parloops.
> > scev_const_prop first also splits the exit edge, and then replaces the
> > phi with a assignment:
> > ...
> >   final value replacement:
> >    n_2 = PHI <n_4(D)(4)>
> >    with
> >    n_2 = n_4(D);
> > ...
> > 
> > This allows parloops to succeed.
> > 
> > And there's a similar story when we compile with -fno-tree-scev-cprop in
> > addition.
> > 
> > Bootstrapped and reg-tested on x86_64.
> > 
> > OK for stage3/stage1?
> 
> The patch has been updated to do the final value replacement only for the loop
> that parloops is processing, as suggested in review comment at
> https://gcc.gnu.org/ml/gcc-patches/2015-11/msg02166.html .
> 
> That means the patch is now also required for the kernels patch series.
> 
> Bootstrapped and reg-tested on x86_64.
> 
> OK for stage 3 trunk?

Ok.  Please mention tree-optimization/68373 in the changelog.

Thanks,
Richard.

> Thanks,
> - Tom
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nuernberg)