On Tue, May 21, 2019 at 4:13 AM Martin Sebor <mse...@gmail.com> wrote:
>
> On 5/20/19 3:16 AM, Richard Biener wrote:
> > On Mon, May 20, 2019 at 10:16 AM Marc Glisse <marc.gli...@inria.fr> wrote:
> >>
> >> On Mon, 20 May 2019, Richard Biener wrote:
> >>
> >>> On Sun, May 19, 2019 at 6:16 PM Marc Glisse <marc.gli...@inria.fr> wrote:
> >>>>
> >>>> Hello,
> >>>>
> >>>> 2 pieces:
> >>>>
> >>>> - the first one handles the case where the denominator is negative. It
> >>>> doesn't happen often with exact_div, so I don't handle it everywhere, but
> >>>> this one looked trivial
> >>>>
> >>>> - handle the case where a pointer difference is cast to an unsigned type
> >>>> before being compared to a constant (I hit this in std::vector). With
> >>>> some
> >>>> range info we could probably handle some non-constant cases as well...
> >>>>
> >>>> The second piece breaks Walloca-13.c (-Walloca-larger-than=100 -O2)
> >>>>
> >>>> void f (void*);
> >>>> void g (int *p, int *q)
> >>>> {
> >>>> __SIZE_TYPE__ n = (__SIZE_TYPE__)(p - q);
> >>>> if (n < 100)
> >>>> f (__builtin_alloca (n));
> >>>> }
> >>>>
> >>>> At the time of walloca2, we have
> >>>>
> >>>> _1 = p_5(D) - q_6(D);
> >>>> # RANGE [-2305843009213693952, 2305843009213693951]
> >>>> _2 = _1 /[ex] 4;
> >>>> # RANGE ~[2305843009213693952, 16140901064495857663]
> >>>> n_7 = (long unsigned intD.10) _2;
> >>>> _11 = (long unsigned intD.10) _1;
> >>>> if (_11 <= 396)
> >>>> [...]
> >>>> _3 = allocaD.1059 (n_7);
> >>>>
> >>>> and warn.
> >>>
> >>> That's indeed to complicated relation of _11 to n_7 for
> >>> VRP predicate discovery.
> >>>
> >>>> However, DOM3 later produces
> >>>>
> >>>> _1 = p_5(D) - q_6(D);
> >>>> _11 = (long unsigned intD.10) _1;
> >>>> if (_11 <= 396)
> >>>
> >>> while _11 vs. _1 works fine.
> >>>
> >>>> [...]
> >>>> # RANGE [0, 99] NONZERO 127
> >>>> _2 = _1 /[ex] 4;
> >>>> # RANGE [0, 99] NONZERO 127
> >>>> n_7 = (long unsigned intD.10) _2;
> >>>> _3 = allocaD.1059 (n_7);
> >>>>
> >>>> so I am tempted to say that the walloca2 pass is too early, xfail the
> >>>> testcase and file an issue...
> >>>
> >>> Hmm, there's a DOM pass before walloca2 already and moving
> >>> walloca2 after loop opts doesn't look like the best thing to do?
> >>> I suppose it's not DOM but sinking that does the important transform
> >>> here? That is,
> >>>
> >>> Index: gcc/passes.def
> >>> ===================================================================
> >>> --- gcc/passes.def (revision 271395)
> >>> +++ gcc/passes.def (working copy)
> >>> @@ -241,9 +241,9 @@ along with GCC; see the file COPYING3.
> >>> NEXT_PASS (pass_optimize_bswap);
> >>> NEXT_PASS (pass_laddress);
> >>> NEXT_PASS (pass_lim);
> >>> - NEXT_PASS (pass_walloca, false);
> >>> NEXT_PASS (pass_pre);
> >>> NEXT_PASS (pass_sink_code);
> >>> + NEXT_PASS (pass_walloca, false);
> >>> NEXT_PASS (pass_sancov);
> >>> NEXT_PASS (pass_asan);
> >>> NEXT_PASS (pass_tsan);
> >>>
> >>> fixes it?
> >>
> >> I will check, but I don't think walloca uses any kind of on-demand VRP, so
> >> we still need some pass to update the ranges after sinking, which doesn't
> >> seem to happen until the next DOM pass.
> >
> > Oh, ok... Aldy, why's this a separate pass anyways? I think similar
> > other warnigns are emitted from RTL expansion? So maybe we can
> > indeed move the pass towards warn_restrict or late_warn_uninit.
>
> I thought there was a preference to add new middle-end warnings
> into passes of their own rather than into existing passes. Is
> that not so (either in general or in this specific case)?
The preference was to add them not into optimization passes. But
of course having 10+ warning passes, each going over the whole IL
is excessive. Also each of the locally computing ranges or so.
Given the simplicity of Walloca I wonder why it's not part of another
warning pass - since it's about tracking "sizes" again there are plenty
that fit ;)
> From my POV, the main (only?) benefit of putting warnings in their
> own passes is modularity. Are there any others?
>
> The biggest drawback I see is that it makes it hard to then share
> data across multiple passes. The sharing can help not just
> warnings (reduce both false positive and false negative rates) but
> also optimization. That's why I'm merging the strlen and sprintf
> passes, and want to eventually also look into merging
> the -Wstringop-overflow warnings there (also emitted just before
> RTL expansion. Did I miss any downsides?
When things fit together they are fine to merge obviously.
One may not like -Warray-bounds inside VRP but it really "fits".
OTOH making a warning part of an optimization pass naturally
limits its effect to when the specific optimization is enabled.
In theory it's possible to do -Warray-bounds at -O0 - we are in
SSA form after all - but of course you don't want to enable VRP at -O0.
> I don't know if there's the -Walloca pass would benefit from merging
> with any of the others or vice versa, but superficially it seems like
> it might be worth thinking about integrating the -Walloc-larger-than
> warnings into the -Walloca pass, if only to keep similar functionality
> in the same place.
-Wrestrict and all the format warning stuff seems related as well.
> > I also see that the Og pass pipeline misses the second walloca pass
> > completely (and also the warn_restrict pass).
>
> That's worth fixing.
>
> Martin
>
> >
> > Given code sinkings obvious effects on SSA value-range representation
> > it may make sense to add another instance of that pass earlier.
> >
> >
> >
> >>
> >> --
> >> Marc Glisse
>
