[LLVMdev] Final added to parser<bool>
Reed Kotler
reed.kotler at imgtec.com
Thu Mar 19 09:34:36 PDT 2015
On 03/19/2015 09:24 AM, David Blaikie wrote:
>
>
> On Thu, Mar 19, 2015 at 9:18 AM, Reed Kotler <reed.kotler at imgtec.com
> <mailto:reed.kotler at imgtec.com>> wrote:
>
> Well, you are an mclinker contributor
>
>
> Me personally? Not that I know of.
Sorry. I thought i had seen your name in an mclinker commit.
>
> and Google uses mclinker
>
>
> So I've been told, though I hadn't even heard of mclinker until this
> email thread.
It's another linker (we don't have enough of them :) ) . It's used to
run on mobile devices
and has been designed with that criteria in mind. (At least that is my
understanding).
> and now it's broken as the result of your change.
>
>
> This is a pretty standard state of affairs for LLVM-using projects,
> LLVM's APIs change continuously.
Probably then this is a configuration issue I need to take up with
mclinker folks.
Their build instructions have you just download tip of tree from svn and
that clearly is not guaranteed to work.
Probably their wiki should list the version from LLVM to download.
> I still don't see any justification to making a change in a public
> interface that is used by other non LLVM projects
> to fix some issue with clang warnings. People should be able to
> derive from those classes. I can't understand
> your reasoning as to why these classes must be final.
>
>
> Because if they're not, it's easy to write bugs with them - especially
> in non-tree projects, strangely enough. (if you derive from this type,
> then happen to own those derived objects polymorphically, you'll
> invoke undefined behavior - by making the type 'final' it removes the
> ability to write such bugs).
Isn't that a problem with the design of this class? (or these classes)?
> I was kind of surprised that there are no mclinker build bots that
> would have picked this up right away.
>
>
> On 03/19/2015 09:08 AM, David Blaikie wrote:
>>
>>
>> On Thu, Mar 19, 2015 at 9:05 AM, Reed Kotler
>> <reed.kotler at imgtec.com <mailto:reed.kotler at imgtec.com>> wrote:
>>
>> On 03/19/2015 08:55 AM, David Blaikie wrote:
>>>
>>>
>>> On Thu, Mar 19, 2015 at 4:30 AM, Reed Kotler
>>> <Reed.Kotler at imgtec.com <mailto:Reed.Kotler at imgtec.com>> wrote:
>>>
>>> One could argue that mclinker is doing something good or
>>> not by how it's using this class
>>> but I don't see the need for parser<bool> to be final.
>>> That is a subjective opinion that mclinker needs to
>>> be changed.
>>>
>>> I think that "final" was added to some of these command
>>> line classes to avoid some kind of clang warning.
>>>
>>> That seems wrong to me that the tools are dictating
>>> something in an API.
>>>
>>>
>>> Well the warning's a pretty good one - and dictates a way to
>>> design an API safely to avoid the possibility of a bad delete.
>>>
>>> It's not uncommon to have a polymorphic hierarchy that
>>> doesn't require polymorphic ownership. To ensure that
>>> polymorphic ownership doesn't sneak in, we have a warning to
>>> ensure the class either has an interface compatible with
>>> polymorphic ownership (has a virtual dtor) or disallows
>>> polymorphic ownership (has a protected dtor in any non-final
>>> base).
>>>
>>> There is a narrower warning that only diagnoses this at the
>>> use of polymorphic destruction (-Wdelete-non-virtual-dtor)
>>> but we've not needed to fallback to that in LLVM so far &
>>> I'd mildly rather avoid doing so. The
>>> protected-final-or-virtual pattern seems fairly
>>> elegant/self-documenting to me.
>>>
>>> There may be other valid reasons to create derived classes.
>>>
>>> I think we should remove the "final" attribute on those
>>> classes and fix the clang warning issue in some
>>> better way than changing the API.
>>>
>>> If mclinker had been part of the llvm project tools tree
>>> that would not have been allowed even.
>>>
>>>
>>> Or we might've decided that the use in mclinker was bad &
>>> should be fixed - I haven't really looked at it to consider
>>> the details of its use, to be honest.
>> There are lots of valid reasons to want to be able to derive
>> classes; that's why C++ lets you do that. :)
>>
>> I don't think that LLVM judging MCLinker coding is relevant
>> here.
>>
>>
>> It seems like it is to me - when refactoring code we look at the
>> use cases and judge what the right way to handle those use cases
>> is. One of those ways is to fix/change/remove the use-cases that
>> seem problematic/unnecessary/incorrect.
>>
>> Lots of projects use LLVM now and changing the API in this way
>> does not have justification other than to deal with some
>> clang warning issues that can be solved in other ways.
>>
>>
>> I think the practices that the clang warning enforces/encourages
>> are good ones, otherwise I would've fixed/changed/disabled the
>> warning. We've used these practices/this warning for a while now
>> & it seems to suit LLVM's use cases pretty well so far.
>>
>> I don't see any justification for deciding once and for all
>> that there are no valid reasons for deriving from these classes
>> and to use a c++ feature to enforce that.
>>
>> As I said earlier, had mclinker been in the tools
>> subdirectory of LLVM then the change the CommandLine.h would
>> not have even been allowed.
>>
>>
>>
>>
>>>
>>> ________________________________________
>>> From: Simon Atanasyan [simon at atanasyan.com
>>> <mailto:simon at atanasyan.com>]
>>> Sent: Wednesday, March 18, 2015 9:35 PM
>>> To: Reed Kotler
>>> Cc: David Blaikie; Simon Atanasyan; Rich Fuhler;
>>> llvmdev at cs.uiuc.edu <mailto:llvmdev at cs.uiuc.edu>
>>> Subject: Re: [LLVMdev] Final added to parser<bool>
>>>
>>> Hi Reed,
>>>
>>> The FalseParser has a rather strange design. It's
>>> purpose to parse
>>> options like -no-to-do-something and always return false
>>> even if a
>>> user provides -no-to-do-something=no. That should be
>>> fixed on the
>>> MCLinker side.
>>>
>>> On Thu, Mar 19, 2015 at 5:14 AM, reed kotler
>>> <rkotler at mips.com <mailto:rkotler at mips.com>> wrote:
>>> >
>>> //===----------------------------------------------------------------------===//
>>> > // FalseParser
>>> >
>>> //===----------------------------------------------------------------------===//
>>> > class FalseParser : public parser<bool> {
>>> > public:
>>> > explicit FalseParser(Option &O) : parser<bool>(O) { }
>>> >
>>> > // parse - Return true on error.
>>> > bool parse(cl::Option& O, StringRef ArgName,
>>> StringRef Arg, bool& Val) {
>>> > if (cl::parser<bool>::parse(O, ArgName, Arg, Val))
>>> > return false;
>>> > Val = false;
>>> > return false;
>>> > }
>>> > };
>>> >
>>> > I don't know the history of this. I'm just starting to
>>> do some mclinker work
>>> > to add the new mips r6 relocations to it.
>>> >
>>> >
>>> > On 03/18/2015 07:00 PM, David Blaikie wrote:
>>> >
>>> >
>>> >
>>> > On Wed, Mar 18, 2015 at 6:48 PM, reed kotler
>>> <rkotler at mips.com <mailto:rkotler at mips.com>> wrote:
>>> >>
>>> >> Hi David,
>>> >>
>>> >> Is there a reason that we need to have "final" for
>>> parser<bool> ???
>>> >
>>> >
>>> > Clang has a (reasonable) warning for types with
>>> virtual functions and a
>>> > non-virtual dtor. This warning is suppressed if the
>>> dtor is protected or the
>>> > class is final (since in the first case it's clear
>>> that the user intends not
>>> > to destroy objects via base pointers, only derived
>>> ones - and in the second
>>> > case there's no risk of derived classes, so public
>>> access to the dtor is
>>> > safe even without virtual dispatch.
>>> >
>>> > Since the parser hierarchy never needed polymorphic
>>> destruction (all
>>> > instances are concrete instances of derived classes
>>> owned and destroyed
>>> > directly, not via base pointers) this seemed like a
>>> fine way to structure
>>> > the API.
>>>
>>> --
>>> Simon Atanasyan
>>>
>>>
>>
>>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150319/1bc60015/attachment.html>
More information about the llvm-dev
mailing list