[cfe-dev] libc++ is not using always_inline anymore!

[Mehdi AMINI via cfe-dev]

Le dim. 11 nov. 2018 à 10:25, Louis Dionne <ldionne at apple.com> a écrit :

>
>
> On Nov 11, 2018, at 00:08, Louis Dionne via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>
> On Nov 10, 2018, at 23:14, Mehdi AMINI <joker.eph at gmail.com> wrote:
>
>
>
> Le mar. 30 oct. 2018 à 15:57, Louis Dionne via cfe-dev <
> cfe-dev at lists.llvm.org> a écrit :
>
>>
>> On Oct 30, 2018, at 18:00, David Blaikie <dblaikie at gmail.com> wrote:
>>
>> Awesome!
>>
>> What are the new semantics? That this ABI stability guarantee is provided
>> by hiding the functions in each user so they can't be deduplicated with
>> anotehr user's copy? (what about other copies that are from the same build?
>> I guess even those won't get coalesced/collapsed together? Would that be
>> useful to support?)
>>
>>
>> There are currently two modes (in LLVM trunk, and that is the plan for
>> LLVM 8 too):
>>
>> 1. (the default) All TUs linked together inside the same final linked
>> image need to have use the same libc++ version. Inline functions are
>> ODR-merged across TUs like they normally are. In this mode, we don't use
>> any funny attribute to control linkage (neither always_inline nor
>> internal_linkage).
>>
>> 2. (can be opted-in) TUs can be linked together even if they use
>> different headers of libc++. This is achieved by using internal_linkage on
>> implementation detail functions of libc++. Those functions are local to a
>> TU and they are NOT ODR-merge across TUs. This results in more code
>> duplication than option (1).
>>
>>
>> I assume this doesn't change the defaults, but does it make it any easier
>> for users who don't need the ABI stability guarantee? (or was it already
>> easy/no change here?)
>>
>>
>> It actually does change the default. However, it depends of what ABI
>> guarantee you're talking about.
>>
>> 1. The ABI stability of the shared objects is always (and has always
>> been, and will most likely always) be guaranteed. The only way to change
>> that is to explicitly use the _LIBCPP_ABI_UNSTABLE macro, which says "use
>> all the ABI breaking features". This obviously only works if you're also
>> linking against a library that was built to provide that ABI. This ability
>> to use the unstable ABI has been provided for a long time, it wasn't the
>> default, and it still isn't the default -- my change is completely
>> orthogonal to that.
>>
>> 2. The "ABI stability" of static archives is a different matter. The
>> question here is whether you can link programs against static archives
>> built with different versions of libc++. The answer used to be YES by
>> default, not it is NO by default. If you want to retain that ability, you
>> need to use the `_LIBCPP_HIDE_FROM_ABI_PER_TU` macro. And also please give
>> us a heads up so we know someone is using it.
>>
>
> In general I'm worried of "undefined behavior" that isn't caught by a
> tool, ideally at build time otherwise at runtime. I would really encourage
> to not introduce any default behavior where you can't provide an easy
> detection mechanism to the user.
>
>
> Can you please expand on what you mean here? Are you referring to the
> potential for ODR violations if someone links TUs built against different
> versions of the libc++ headers? If so, that situation exists for every
> single C++ library out in the wild.
>
> When you provide a system library / core components, you can (should) hold
higher standard than other convenience library that the user opt in in my
opinion.
Also the "the others aren't better" seems to me like a fairly lame excuse.


> More specifically, what I mean here is that anyone relying on this
> guarantee is walking an incredibly thin line
>

I don't know what you mean here.


> , and so I think it is reasonable for such users to explicitly opt into
> the guarantee.
>

You're missing the point: as a user building my application / libraries
using Xcode for instance, I am not aware of what guarantee the compiler
provides in this kind of area. This is incredibly subtle. How do I know
what I need to know to "opt into the guarantee"?
This is not much different than undefined behavior in the language (expect
less documented and more arcane), and we learned with time that dealing
with undefined behavior for the average user is incredibly difficult.

So back to the point: safety and usability should come first when you
supply libraries and tools to users. This is on the same line as an "API
should be easy to use and hard to misuse". If you don't have an easy way to
find/detect the problem that you can widespread as part of your
environment/tools, then you created an "API that is too easy to misuse".
(I know this is not really an API discussion here, but that's not much
different: this is a contract with a client).

I argue that whenever we're introducing such behavior, we're not delivering
a good user experience.

I think that Chandler presented good foundations to approach this at CppCon
2016 in the talk “Garbage In, Garbage Out: Arguing about Undefined
Behavior...": https://www.youtube.com/watch?v=yG1OZ69H_-o

Especially, since we're going from a "wider" to a "narrower" contract here,
this slide is particularly relevant and represent the concern I raised:
https://youtu.be/yG1OZ69H_-o?t=1279

Best,

-- 
Mehdi
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