[PATCH][RFC] Fix P1 PR77498

[Bin.Cheng]

On Fri, Mar 31, 2017 at 11:37 AM, Richard Biener <rguenther@suse.de> wrote:
> On Fri, 31 Mar 2017, Markus Trippelsdorf wrote:
>
>> On 2017.03.31 at 11:16 +0200, Richard Biener wrote:
>> > On Fri, 31 Mar 2017, Richard Biener wrote:
>> >
>> > > On Fri, 31 Mar 2017, Rainer Orth wrote:
>> > >
>> > > > Hi Christophe,
>> > > >
>> > > > > With this patch, the following testcase now fails on arm* targets:
>> > > > > gcc.dg/tree-ssa/pr71347.c scan-tree-dump-not optimized ".* = MEM.*;"
>> > > >
>> > > > same on Solaris/SPARC.
>> > >
>> > > I've reverted r241968 with (patch reverted).  It doesn't include
>> > > sparc, so please amend as you see fit.
>> >
>> > Ah, r241441.  I'll fixup myself then.
>>
>> It also fails on some X86 configurations:
>> https://gcc.gnu.org/ml/gcc-regression/2017-03/msg00237.html
>> https://gcc.gnu.org/ml/gcc-regression/2017-03/msg00238.html
>
> I see.  The test is somewhat strange (well, it's an IVOPTS test).  To
> simulate the effect of the PRE changes we could simply enable
> -fpredictive-commoning on it.
>
> Let's ask Bin what the testcase was supposed to test... (the testcase
> comment suggests that pcom is applied but it certainly wasn't before
> the xfails were removed).

It's supposed to test that both loads inside loop can be optimized,
X[i-1] by predcom or pre; X[1] by loop invariant.  When the test is
added, neither pre nor predcom (not at O2) can do this, but we have
another chance that pre + ivopts + dom together can eliminate X[i-1].
But this really depends on each pass does the right thing.  That's
also why it is added with ivopts change.  Back in time, it was ivopts
did "wrong".  Apparently, having a test on different passes is
fragile.  I will send a patch adding option "-fpredictive-commoning"
because predcom seems the right pass to do that.  Also given we are
considering enabling predcom at O2 for GCC 8.

Thanks,
bin
>
> Richard.