Re: [PING][PATCH] define auto_vec copy ctor and assignment (PR 90904)

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

On Tue, Jul 6, 2021 at 5:06 PM Martin Sebor <msebor@gmail.com> wrote:
>
> Ping: https://gcc.gnu.org/pipermail/gcc-patches/2021-June/573968.html
>
> Any questions/suggestions on the final patch or is it okay to commit?

I don't remember seeing one (aka saying "bootstrapped/tested, OK to commit?"
or so) - and the link above doesn't have one.

So, can you re-post it please?

Thanks,
Richard.

> On 6/29/21 7:46 PM, Martin Sebor wrote:
> > On 6/29/21 4:58 AM, Richard Biener wrote:
> >> On Mon, Jun 28, 2021 at 8:07 PM Martin Sebor <msebor@gmail.com> wrote:
> >>>
> >>> On 6/28/21 2:07 AM, Richard Biener wrote:
> >>>> On Sat, Jun 26, 2021 at 12:36 AM Martin Sebor <msebor@gmail.com> wrote:
> >>>>>
> >>>>> On 6/25/21 4:11 PM, Jason Merrill wrote:
> >>>>>> On 6/25/21 4:51 PM, Martin Sebor wrote:
> >>>>>>> On 6/1/21 3:38 PM, Jason Merrill wrote:
> >>>>>>>> On 6/1/21 3:56 PM, Martin Sebor wrote:
> >>>>>>>>> On 5/27/21 2:53 PM, Jason Merrill wrote:
> >>>>>>>>>> On 4/27/21 11:52 AM, Martin Sebor via Gcc-patches wrote:
> >>>>>>>>>>> On 4/27/21 8:04 AM, Richard Biener wrote:
> >>>>>>>>>>>> On Tue, Apr 27, 2021 at 3:59 PM Martin Sebor <msebor@gmail.com>
> >>>>>>>>>>>> wrote:
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> On 4/27/21 1:58 AM, Richard Biener wrote:
> >>>>>>>>>>>>>> On Tue, Apr 27, 2021 at 2:46 AM Martin Sebor via Gcc-patches
> >>>>>>>>>>>>>> <gcc-patches@gcc.gnu.org> wrote:
> >>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>> PR 90904 notes that auto_vec is unsafe to copy and assign
> >>>>>>>>>>>>>>> because
> >>>>>>>>>>>>>>> the class manages its own memory but doesn't define (or
> >>>>>>>>>>>>>>> delete)
> >>>>>>>>>>>>>>> either special function.  Since I first ran into the
> >>>>>>>>>>>>>>> problem,
> >>>>>>>>>>>>>>> auto_vec has grown a move ctor and move assignment from
> >>>>>>>>>>>>>>> a dynamically-allocated vec but still no copy ctor or copy
> >>>>>>>>>>>>>>> assignment operator.
> >>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>> The attached patch adds the two special functions to
> >>>>>>>>>>>>>>> auto_vec
> >>>>>>>>>>>>>>> along
> >>>>>>>>>>>>>>> with a few simple tests.  It makes auto_vec safe to use in
> >>>>>>>>>>>>>>> containers
> >>>>>>>>>>>>>>> that expect copyable and assignable element types and passes
> >>>>>>>>>>>>>>> bootstrap
> >>>>>>>>>>>>>>> and regression testing on x86_64-linux.
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>> The question is whether we want such uses to appear since
> >>>>>>>>>>>>>> those
> >>>>>>>>>>>>>> can be quite inefficient?  Thus the option is to delete those
> >>>>>>>>>>>>>> operators?
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> I would strongly prefer the generic vector class to have the
> >>>>>>>>>>>>> properties
> >>>>>>>>>>>>> expected of any other generic container: copyable and
> >>>>>>>>>>>>> assignable.  If
> >>>>>>>>>>>>> we also want another vector type with this restriction I
> >>>>>>>>>>>>> suggest
> >>>>>>>>>>>>> to add
> >>>>>>>>>>>>> another "noncopyable" type and make that property explicit in
> >>>>>>>>>>>>> its name.
> >>>>>>>>>>>>> I can submit one in a followup patch if you think we need one.
> >>>>>>>>>>>>
> >>>>>>>>>>>> I'm not sure (and not strictly against the copy and assign).
> >>>>>>>>>>>> Looking around
> >>>>>>>>>>>> I see that vec<> does not do deep copying.  Making
> >>>>>>>>>>>> auto_vec<> do it
> >>>>>>>>>>>> might be surprising (I added the move capability to match
> >>>>>>>>>>>> how vec<>
> >>>>>>>>>>>> is used - as "reference" to a vector)
> >>>>>>>>>>>
> >>>>>>>>>>> The vec base classes are special: they have no ctors at all
> >>>>>>>>>>> (because
> >>>>>>>>>>> of their use in unions).  That's something we might have to
> >>>>>>>>>>> live with
> >>>>>>>>>>> but it's not a model to follow in ordinary containers.
> >>>>>>>>>>
> >>>>>>>>>> I don't think we have to live with it anymore, now that we're
> >>>>>>>>>> writing C++11.
> >>>>>>>>>>
> >>>>>>>>>>> The auto_vec class was introduced to fill the need for a
> >>>>>>>>>>> conventional
> >>>>>>>>>>> sequence container with a ctor and dtor.  The missing copy
> >>>>>>>>>>> ctor and
> >>>>>>>>>>> assignment operators were an oversight, not a deliberate
> >>>>>>>>>>> feature.
> >>>>>>>>>>> This change fixes that oversight.
> >>>>>>>>>>>
> >>>>>>>>>>> The revised patch also adds a copy ctor/assignment to the
> >>>>>>>>>>> auto_vec
> >>>>>>>>>>> primary template (that's also missing it).  In addition, it adds
> >>>>>>>>>>> a new class called auto_vec_ncopy that disables copying and
> >>>>>>>>>>> assignment as you prefer.
> >>>>>>>>>>
> >>>>>>>>>> Hmm, adding another class doesn't really help with the confusion
> >>>>>>>>>> richi mentions.  And many uses of auto_vec will pass them as vec,
> >>>>>>>>>> which will still do a shallow copy.  I think it's probably better
> >>>>>>>>>> to disable the copy special members for auto_vec until we fix
> >>>>>>>>>> vec<>.
> >>>>>>>>>
> >>>>>>>>> There are at least a couple of problems that get in the way of
> >>>>>>>>> fixing
> >>>>>>>>> all of vec to act like a well-behaved C++ container:
> >>>>>>>>>
> >>>>>>>>> 1) The embedded vec has a trailing "flexible" array member with
> >>>>>>>>> its
> >>>>>>>>> instances having different size.  They're initialized by memset
> >>>>>>>>> and
> >>>>>>>>> copied by memcpy.  The class can't have copy ctors or assignments
> >>>>>>>>> but it should disable/delete them instead.
> >>>>>>>>>
> >>>>>>>>> 2) The heap-based vec is used throughout GCC with the
> >>>>>>>>> assumption of
> >>>>>>>>> shallow copy semantics (not just as function arguments but also as
> >>>>>>>>> members of other such POD classes).  This can be changed by
> >>>>>>>>> providing
> >>>>>>>>> copy and move ctors and assignment operators for it, and also for
> >>>>>>>>> some of the classes in which it's a member and that are used with
> >>>>>>>>> the same assumption.
> >>>>>>>>>
> >>>>>>>>> 3) The heap-based vec::block_remove() assumes its elements are
> >>>>>>>>> PODs.
> >>>>>>>>> That breaks in VEC_ORDERED_REMOVE_IF (used in gcc/dwarf2cfi.c:2862
> >>>>>>>>> and tree-vect-patterns.c).
> >>>>>>>>>
> >>>>>>>>> I took a stab at both and while (1) is easy, (2) is shaping up to
> >>>>>>>>> be a big and tricky project.  Tricky because it involves using
> >>>>>>>>> std::move in places where what's moved is subsequently still used.
> >>>>>>>>> I can keep plugging away at it but it won't change the fact that
> >>>>>>>>> the embedded and heap-based vecs have different requirements.
> >>>>>>>>>
> >>>>>>>>> It doesn't seem to me that having a safely copyable auto_vec needs
> >>>>>>>>> to be put on hold until the rats nest above is untangled.  It
> >>>>>>>>> won't
> >>>>>>>>> make anything worse than it is.  (I have a project that depends on
> >>>>>>>>> a sane auto_vec working).
> >>>>>>>>>
> >>>>>>>>> A couple of alternatives to solving this are to use std::vector or
> >>>>>>>>> write an equivalent vector class just for GCC.
> >>>>>>>>
> >>>>>>>> It occurs to me that another way to work around the issue of
> >>>>>>>> passing
> >>>>>>>> an auto_vec by value as a vec, and thus doing a shallow copy, would
> >>>>>>>> be to add a vec ctor taking an auto_vec, and delete that.  This
> >>>>>>>> would
> >>>>>>>> mean if you want to pass an auto_vec to a vec interface, it
> >>>>>>>> needs to
> >>>>>>>> be by reference.  We might as well do the same for operator=,
> >>>>>>>> though
> >>>>>>>> that isn't as important.
> >>>>>>>
> >>>>>>> Thanks, that sounds like a good idea.  Attached is an implementation
> >>>>>>> of this change.  Since the auto_vec copy ctor and assignment have
> >>>>>>> been deleted by someone else in the interim, this patch doesn't
> >>>>>>> reverse that.  I will propose it separately after these changes
> >>>>>>> are finalized.
> >>>>>>>
> >>>>>>> My approach was to 1) disable the auto_vec to vec conversion,
> >>>>>>> 2) introduce an auto_vec::to_vec() to make the conversion possible
> >>>>>>> explicitly, and 3) resolve compilation errors by either changing
> >>>>>>> APIs to take a vec by reference or callers to convert auto_vec to
> >>>>>>> vec explicitly by to_vec().  In (3) I tried to minimize churn while
> >>>>>>> improving the const-correctness of the APIs.
> >>>>>>
> >>>>>> What did you base the choice between reference or to_vec on?  For
> >>>>>> instance, it seems like c_parser_declaration_or_fndef could use a
> >>>>>> reference, but you changed the callers instead.
> >>>>>
> >>>>> I went with a reference whenever I could.  That doesn't work when
> >>>>> there are callers that pass in a vNULL, so there, and in assignments,
> >>>>> I used to_vec().
> >>>>
> >>>> Is there a way to "fix" the ugliness with vNULL?  All those functions
> >>>> should be able to use const vec<>& as otherwise they'd leak memory?
> >>>> Can't we pass vNULL to a const vec<>&?
> >>>
> >>> vNULL can bind to a const vec& (via the vec conversion ctor) but
> >>> not to vec&.  The three functions that in the patch are passed
> >>> vNULL modify the argument when it's not vNULL but not otherwise.
> >>> An alternate design is to have them take a vec* and pass in
> >>> a plain NULL (or nullptr) instead of vNULL.  That would require
> >>> some surgery on the function bodies that I've been trying to
> >>> avoid in the first pass.
> >>
> >> But I wonder if since you now identified them they could be massaged
> >> prior to doing the change.
> >>
> >> I do hope we end up not needing .to_vec () after all, if no users
> >> remain ;)
> >
> > I'd be happy to if none remained.  I see how to eliminate those in
> > calls to functions like c_parser_declaration_or_fndef() (done in
> > the attached revision of the patch), but no easy way to get rid
> > of those that replace other implicit conversions, like all those
> > assignments to the vec members of the ipa_call_arg_values ctor.
> > If it's appropriate to std::move those then that would get rid
> > of the .to_vec () call.  I'm not familiar with the code but I
> > have the impression it might be meant more as a reference to
> > some "remote" object (an instance of ipa_auto_call_arg_values?)
> > If that's right then making the vec members auto_vec references
> > (or pointers) would be one way to "fix" this.
> >
> >>> Functions that don't leak memory now shouldn't leak with these
> >>> changes, and conversely, those that do will still leak.  The patch
> >>> doesn't change that (as far as I know).
> >>
> >> It just occurs to me those cases could pass auto_vec<>() by reference
> >> instead
> >> of vNULL?  So if the vector is modified then it's released afterwards?
> >> That would fix the memleak.
> >
> > I see what you mean.  A function that modified the unnamed vec
> > temporary constructed from vNULL then the modified vector would
> > leak.  I don't think the functions the patch touches do that but
> > I've removed the vNULL conversion from all of them.  There are
> > many others that pass vNULL to a vec arguments that that the patch
> > doesn't touch but those would be worth a closer look at some point.
> >
> > Attached is a revised patch with these changes (a superset of
> > those I sent in response to Jason's question), tested on x86_64.
> >
> > Martin
> >
> >>
> >>> Going forward I think it's possible to replace most uses of vNULL
> >>> in GCC with direct initialization (e.g., vec<T> v{ }).  Those that
> >>> can't be readily replaced are the ones where vNULL is passed as
> >>> an argument to functions taking a vec by value.  Those could be
> >>> changed to avoid vNULL too, but it would take a different approach
> >>> and more effort.  I'm not against it but I'd rather decouple those
> >>> changes from this already sizeable patch.
> >>>
> >>> Martin
> >>>
> >>>>
> >>>> Richard.
> >>>>
> >>>>>
> >>>>> Martin
> >>>>>
> >>>
> >
>