On 6/29/21 8:43 AM, Jason Merrill wrote: > On 6/28/21 2:07 PM, Martin Sebor wrote: >> On 6/28/21 2:07 AM, Richard Biener wrote: >>> On Sat, Jun 26, 2021 at 12:36 AM Martin Sebor 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 >>>>>>>>>>> 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 >>>>>>>>>>>>> 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. > > The c_parser_declaration_or_fndef case is rather ugly: the vec is passed > by value, but then the modifications in c_finish_omp_declare_simd modify > the original vec. > > We could keep the same semantic problem and make it more blatant by > changing to const vec& and doing a const_cast in > c_finish_omp_declare_simd before modifying the vec. > > Do the other two have the same problem? Yes, the functions that take a vec by value and are passed an auto_vec "by reference" (the result of to_vec()) modify the auto_vec. This is the "bug" this patch is designed to keep from happening by accident, while letting the API clients do it intentionally. Changing these APIs to take a const vec& while still letting them modify the argument by casting away the constness seems even more surprising to me than the current by-value style. I do think it should be fixed but I'd have been more comfortable handling that separately. Attached is a (near) minimal change along these lines to c_parser_declaration_or_fndef and its callers. The logic isn't exactly the same as the original but no tests fail. If this is the direction we want to go in I can see about making an analogous change to the other two similar functions in the patch. Let me know. Martin