On Thu, Feb 6, 2020 at 11:06 AM Richard Biener
<richard.guent...@gmail.com> wrote:
>
> On Wed, Feb 5, 2020 at 4:55 PM Martin Sebor <mse...@gmail.com> wrote:
> >
> > On 2/5/20 1:19 AM, Richard Biener wrote:
> > > On Tue, Feb 4, 2020 at 11:02 PM Martin Sebor <mse...@gmail.com> wrote:
> > >>
> > >> On 2/4/20 2:31 PM, Jeff Law wrote:
> > >>> On Tue, 2020-02-04 at 13:08 -0700, Martin Sebor wrote:
> > >>>> On 2/4/20 12:15 PM, Richard Biener wrote:
> > >>>>> On February 4, 2020 5:30:42 PM GMT+01:00, Jeff Law <l...@redhat.com>
> > >>>>> wrote:
> > >>>>>> On Tue, 2020-02-04 at 10:34 +0100, Richard Biener wrote:
> > >>>>>>> On Tue, Feb 4, 2020 at 1:44 AM Martin Sebor <mse...@gmail.com>
> > >>>>>>> wrote:
> > >>>>>>>> PR 93519 reports a false positive -Wrestrict issued for an inlined
> > >>>>>> call
> > >>>>>>>> to strcpy that carefully guards against self-copying. This is
> > >>>>>> caused
> > >>>>>>>> by the caller's arguments substituted into the call during inlining
> > >>>>>> and
> > >>>>>>>> before dead code elimination.
> > >>>>>>>>
> > >>>>>>>> The attached patch avoids this by removing -Wrestrict from the
> > >>>>>> folder
> > >>>>>>>> and deferring folding perfectly overlapping (and so undefined)
> > >>>>>> calls
> > >>>>>>>> to strcpy (and mempcpy, but not memcpy) until much later. Calls to
> > >>>>>>>> perfectly overlapping calls to memcpy are still folded early.
> > >>>>>>>
> > >>>>>>> Why do we bother to warn at all for this case? Just DWIM here.
> > >>>>>> Warnings like
> > >>>>>>> this can be emitted from the analyzer?
> > >>>>>> They potentially can, but the analyzer is and will almost always
> > >>>>>> certainly be considerably slower. I would not expect it to be used
> > >>>>>> nearly as much as the core compiler.
> > >>>>>>
> > >>>>>> WHether or not a particular warning makes sense in the core compiler
> > >>>>>> or
> > >>>>>> analyzer would seem to me to depend on whether or not we can
> > >>>>>> reasonably
> > >>>>>> issue warnings without interprocedural analysis. double-free
> > >>>>>> realistically requires interprocedural analysis to be effective. I'm
> > >>>>>> not sure Wrestrict really does.
> > >>>>>>
> > >>>>>>
> > >>>>>>> That is, I suggest to simply remove the bogus warning code from
> > >>>>>> folding
> > >>>>>>> (and _not_ fail the folding).
> > >>>>>> I haven't looked at the patch, but if we can get the warning out of
> > >>>>>> the
> > >>>>>> folder that's certainly preferable. And we could investigate
> > >>>>>> deferring
> > >>>>>> self-copy removal.
> > >>>>>
> > >>>>> I think the issue is as usual, warning for code we'll later remove as
> > >>>>> dead. Warning at folding is almost always premature.
> > >>>>
> > >>>> In this instance the code is reachable (or isn't obviously
> > >>>> unreachable).
> > >>>> GCC doesn't remove it, but provides benign (and reasonable) semantics
> > >>>> for it(*). To me, that's one aspect of quality. Letting the user know
> > >>>> that the code is buggy is another. I view that as at least as
> > >>>> important
> > >>>> as folding the ill-effects away because it makes it possible to fix
> > >>>> the problem so the code works correctly even with compilers that don't
> > >>>> provide these benign semantics.
> > >>> If you look at the guts of what happens at the point where we issue the
> > >>> warning from within gimple_fold_builtin_strcpy we have:
> > >>>
> > >>>> DCH_to_char (char * in, char * out, int collid)
> > >>>> {
> > >>>> int type;
> > >>>> char * D.2148;
> > >>>> char * dest;
> > >>>> char * num;
> > >>>> long unsigned int _4;
> > >>>> char * _5;
> > >>>>
> > >>>> ;; basic block 2, loop depth 0
> > >>>> ;; pred: ENTRY
> > >>>> ;; succ: 4
> > >>>>
> > >>>> ;; basic block 4, loop depth 0
> > >>>> ;; pred: 2
> > >>>> ;; succ: 5
> > >>>>
> > >>>> ;; basic block 5, loop depth 0
> > >>>> ;; pred: 4
> > >>>> ;; succ: 6
> > >>>>
> > >>>> ;; basic block 6, loop depth 0
> > >>>> ;; pred: 5
> > >>>> if (0 != 0)
> > >>>> goto <bb 7>; [53.47%]
> > >>>> else
> > >>>> goto <bb 8>; [46.53%]
> > >>>> ;; succ: 7
> > >>>> ;; 8
> > >>>>
> > >>>> ;; basic block 7, loop depth 0
> > >>>> ;; pred: 6
> > >>>> strcpy (out_1(D), out_1(D));
> > >>>> ;; succ: 8
> > >>>>
> > >>>> ;; basic block 8, loop depth 0
> > >>>> ;; pred: 6
> > >>>> ;; 7
> > >>>> _4 = __builtin_strlen (out_1(D));
> > >>>> _5 = out_1(D) + _4;
> > >>>> __builtin_memcpy (_5, "foo", 4);
> > >>>> ;; succ: 3
> > >>>>
> > >>>> ;; basic block 3, loop depth 0
> > >>>> ;; pred: 8
> > >>>> return;
> > >>>> ;; succ: EXIT
> > >>>>
> > >>>> }
> > >>>>
> > >>>
> > >>> Which shows the code is obviously unreachable in the case we're warning
> > >>> about. You can't see this in the dumps because it's exposed by
> > >>> inlining, then cleaned up before writing the dump file.
> > >>
> > >> In the specific case of the bug the code is of course eliminated
> > >> because it's guarded by the if (s != d). I was referring to
> > >> the general (unguarded) case of:
> > >>
> > >> char *s = "", *p;
> > >>
> > >> int main (void)
> > >> {
> > >> p = strcpy (s, s);
> > >> puts (p);
> > >> }
> > >>
> > >> where GCC folds the assignment 'p = strcpy(s, s);' to effectively
> > >> p = s; That's perfectly reasonable but it could equally as well
> > >> leave the call alone, as it does when s is null, for instance.
> > >>
> > >> I think folding it away is not only reasonable but preferable to
> > >> making the invalid call, but it's done only rarely. Most of
> > >> the time GCC does emit the undefined access (it does that with
> > >> calls to library functions as well as with direct stores and
> > >> reads). (I am hoping we can change that in the future so that
> > >> these kinds of problems are handled with some consistency.)
> > >>
> > >>>
> > >>> ISTM this would be a case we could handle with the __builtin_warning
> > >>> stuff.
> > >>>
> > >>> I think the question is do we want to do anything about it this cycle?
> > >>>
> > >>>
> > >>> If so, I think Martin's approach is quite reasonable. It disables
> > >>> folding away the self-copies from gimple-fold and moves the warning
> > >>> into the expander. So if there's such a call in the IL at expansion
> > >>> time we get a warning (-O0).
> > >>>
> > >>> I'd hazard a guess that the diagnostic was added to the strlen pass to
> > >>> capture the missed warning when we're optimizing and the self-copy has
> > >>> survived until that point. There's a couple issues that raises though.
> > >>>
> > >>> First, it's insufficient. DSE (for example) can do self-copy removal,
> > >>> so it needs the same handling. There may be others places too.
> > >>>
> > >>> Second, if the code becomes unreachable after strlen, then we've got
> > >>> new false positive issues.
> > >>>
> > >>> It's the classic problems we have with all middle end based warnings.
> > >>>
> > >>> But I could live with those if we can show that using __builtin_warning
> > >>> to handle this stuff in gcc-11 works... ISTM we emit the
> > >>> __builtin_warning call before any self-copy like that, whenever we
> > >>> happen to spot them. They'll naturally get removed if the path becomes
> > >>> unreachable. We'd warn during expansion for calls to
> > >>> __builtin_warning. We could even optionally warn when removing a call
> > >>> to __bulitin_warning.
> > >>>
> > >>> Thoughts?
> > >>
> > >> The patch has pretty much the same effect as emitting __builtin_warning
> > >> from the folder would. It defers the folding until much later, and if
> > >> the code isn't eliminated, it issues a warning and folds the call away.
> > >
> > > But it affects subsequent optimizations - the call is more expensive
> > > in any size heuristics, it posses an (alias-set zero) memory write
> > > barrier (unless you start to optimize no-op copies in the alias oracle),
> > > it is a _call_ - passes like the vectorizer are not happy about a call.
> > > It prevents SRA of the accessed object, ...
> >
> > This is a strcpy call copying over itself. It's undefined code,
> > and so hardly anything that's common or so performance sensitive
> > to make a noticeable difference.
> >
> > > So no, leaving in the call is _not_ equivalent to sticking in a
> > > __builtin_warning() call (or however we actually implement it,
> > > I'd prefer a stmt in the "debug" category so it's simply ignored
> > > or elided by most passes by means of existing code).
> > >
> > > That said, I'd prefer to not do anything about this bug. Iff then
> > > in the inliner try doing a CFG cleanup before folding stmts
> > > (it's doing delayed folding anyway). But not for GCC 10.
> > > One could also mark stmts with no-warning before the inliner
> > > folds them (and then mark back) to avoid those classes of
> > > folding warnings.
> >
> > I think this would very unfortunate for GCC 10. The user's code
> > is clearly correct -- they take pains to avoid the overlapping
> > copy by guarding against it just before it -- and GCC simply emits
> > an invalid warning because of how it does inlining. All that will
> > be accomplished by not fixing it is we will release a worse quality
> > compiler than we otherwise can, unnecessarily eroding our users'
> > confidence in the value of GCC's diagnostic.
> >
> > I'll look into your suggestions for the inliner in stage 1 but
> > please reconsider for GCC 10.
>
> One possibility is the attached but that adds an extra CFG cleanup
> (also untested but on the testcase). Another possibility would be
> to re-do fold_marked_statements in terms of cleanup_control_flow_pre (),
> recognizing unreachable regions on the way and simply avoid folding
> stmts in blocks that will be then removed by the pending CFG cleanup.
>
> I'll see whether I can cook that up quickly.Like this. Indenting not fixed and untested apart from on the testcase. Disadvantage is we're walking _all_ blocks (at least skipping stmts). Technically the iteration scheme should probably change to push edges rather than edge iterators so we can avoid the repeated find_taken_edge call. Anyway, just a prototype. Note SSA propagators / VN avoid to fold stmts that are obviously unreachable so it somewhat makes sense to beat the inliner to do the same. Richard. > > Martin
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