Re: [PATCH 2/3] tree-cfg: do not duplicate returns_twice calls

[Richard Biener <richard.guenther@gmail.com>]

On Tue, Jul 12, 2022 at 10:10 PM Alexander Monakov <amonakov@ispras.ru> wrote:
>
>
> Apologies for the prolonged silence Richard, it is a bit of an obscure topic,
> and I was unsure I'd be able to handle any complications in a timely manner.
> I'm ready to revisit it now, please see below.
>
> On Mon, 17 Jan 2022, Richard Biener wrote:
>
> > On Fri, Jan 14, 2022 at 7:21 PM Alexander Monakov <amonakov@ispras.ru> wrote:
> > >
> > > A returns_twice call may have associated abnormal edges that correspond
> > > to the "second return" from the call. If the call is duplicated, the
> > > copies of those edges also need to be abnormal, but e.g. tracer does not
> > > enforce that. Just prohibit the (unlikely to be useful) duplication.
> >
> > The general CFG copying routines properly duplicate those edges, no?
>
> No (in fact you say so in the next paragraph). In general I think they cannot,
> abnormal edges are a special case, so it should be the responsibility of the
> caller.
>
> > Tracer uses duplicate_block so it should also get copies of all successor
> > edges of that block.  It also only traces along normal edges.  What it might
> > miss is abnormal incoming edges - is that what you are referring to?
>
> Yes (I think its entire point is to build a "trace" of duplicated blocks that
> does not have incoming edges in the middle, abnormal or not).
>
> > That would be a thing we don't handle in duplicate_block on its own but
> > that callers are expected to do (though I don't see copy_bbs doing that
> > either).  I wonder if we can trigger this issue for some testcase?
>
> Oh yes (in fact my desire to find a testcase delayed this quite a bit).
> When compiling the following testcase with -O2 -ftracer:
>
> __attribute__((returns_twice))
> int rtwice_a(int), rtwice_b(int);
>
> int f(int *x)
> {
>         volatile unsigned k, i = (*x);
>
>         for (k = 1; (i = rtwice_a(i)) * k; k = 2);
>
>         for (; (i = rtwice_b(i)) * k; k = 4);
>
>         return k;
> }
>
> tracer manages to eliminate the ABNORMAL_DISPATCHER block completely, so
> the possibility of transferring control back to rtwice_a from rtwice_b
> is no longer modeled in the IR. I could spend some time "upgrading" this
> to an end-to-end miscompilation, but I hope you agree this is quite broken
> already.
>
> > The thing to check would be incoming abnormal edges in
> > can_duplicate_block_p, not (only) returns twice functions?
>
> Unfortunately not, abnormal edges are also used for computed gotos, which are
> less magic than returns_twice edges and should not block tracer I think.

I think computed gotos should use regular edges, only non-local goto should
use abnormals...

I suppose asm goto also uses abnormal edges?

Btw, I don't see how they in general are "less magic".  Sure, we have an
explicit receiver (the destination label), but we can only do edge inserts
if we have a single computed goto edge into a block (we can "move" the
label to the block created when splitting the edge).

> This implies patch 1/3 [1] unnecessary blocks sinking to computed goto targets.
> [1] https://gcc.gnu.org/pipermail/gcc-patches/2022-January/588498.html
>
> How would you like to proceed here? Is my initial patch ok?

Hmm, so for returns twice calls duplicate_block correctly copies the call
and redirects the provided incoming edge to it.  The API does not
handle adding any further incoming edges - the caller would be responsible
for this.  So I still somewhat fail to see the point here.  If tracer does not
handle extra incoming edges properly then we need to fix tracer?  This
also includes non-local goto (we seem to copy non-local labels just
fine - wasn't there a bugreport about this!?).

So I think can_duplicate_block_p is the wrong place to fix (the RTL side
would need a similar fix anyhow?)

Richard.

> Alexander
>
> >
> > Richard.
> >
> > > gcc/ChangeLog:
> > >
> > >         * tree-cfg.c (gimple_can_duplicate_bb_p): Reject blocks with
> > >         calls that may return twice.
> > > ---
> > >  gcc/tree-cfg.c | 7 +++++--
> > >  1 file changed, 5 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/gcc/tree-cfg.c b/gcc/tree-cfg.c
> > > index b7fe313b7..a99f1acb4 100644
> > > --- a/gcc/tree-cfg.c
> > > +++ b/gcc/tree-cfg.c
> > > @@ -6304,12 +6304,15 @@ gimple_can_duplicate_bb_p (const_basic_block bb)
> > >      {
> > >        gimple *g = gsi_stmt (gsi);
> > >
> > > -      /* An IFN_GOMP_SIMT_ENTER_ALLOC/IFN_GOMP_SIMT_EXIT call must be
> > > +      /* Prohibit duplication of returns_twice calls, otherwise associated
> > > +        abnormal edges also need to be duplicated properly.
> > > +        An IFN_GOMP_SIMT_ENTER_ALLOC/IFN_GOMP_SIMT_EXIT call must be
> > >          duplicated as part of its group, or not at all.
> > >          The IFN_GOMP_SIMT_VOTE_ANY and IFN_GOMP_SIMT_XCHG_* are part of such a
> > >          group, so the same holds there.  */
> > >        if (is_gimple_call (g)
> > > -         && (gimple_call_internal_p (g, IFN_GOMP_SIMT_ENTER_ALLOC)
> > > +         && (gimple_call_flags (g) & ECF_RETURNS_TWICE
> > > +             || gimple_call_internal_p (g, IFN_GOMP_SIMT_ENTER_ALLOC)
> > >               || gimple_call_internal_p (g, IFN_GOMP_SIMT_EXIT)
> > >               || gimple_call_internal_p (g, IFN_GOMP_SIMT_VOTE_ANY)
> > >               || gimple_call_internal_p (g, IFN_GOMP_SIMT_XCHG_BFLY)
> > > --
> > > 2.33.1
> > >
> >