[llvm-dev] Should we split llvm Support and ADT?

Chandler Carruth via llvm-dev llvm-dev at lists.llvm.org
Wed Jul 5 18:46:03 PDT 2017

Having watched a similar library go through this exact evolution, I really
doubt we want to make any split around "things known to be in C++ in the
future"... It turns out that this is nearly impossible to predict and
precludes a tremendous amount of useful utilities.

For example, there is no indication that the range helpers LLVM provides
will ever end up in C++'s standard library, but they certainly seem useful
for the demangler (the concrete use case cited).

What is the concrete problem with just linking the support library, in all
its glory, into the demangler? Why *shouldn't* we do that? I feel like that
has gotten lost (for me)....

On Wed, Jul 5, 2017 at 6:27 PM Zachary Turner via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> You mentioned that a good line to draw is one where we're adding things
> that are *known* to be added to c++ future. How strictly do we want to
> enforce this? There are lots of things have equally broad utility, but
> aren't necessarily known to be added to c++ in the future.
> For example, all of MathExtras and StringExtras, many member functions of
> StringRef that are not in string_view, etc. can we still have these in the
> top level compatibility library?
> We could still aim for interfaces that 1-to-1 match STL, but it would nice
> if we could have some equally low level extras to enhance these classes
> On Wed, Jul 5, 2017 at 5:44 PM Chris Lattner <clattner at nondot.org> wrote:
>> Sure, I guess that splitting the arrayref/stringref headers out is a fine
>> first step.
>> -Chris
>> On Jul 5, 2017, at 5:07 PM, Zachary Turner <zturner at google.com> wrote:
>> Re-writing StringRef / ArrayRef etc to use the exact same API is a good
>> idea long term, but there's a lot of ugly messy details that need to be
>> dealt with.  There's thousands of uses of take_front / drop_front, etc that
>> have to be converted.  Then there's some methods that aren't in string_view
>> at all, like consume_integer(), consume_front(), etc that would have to be
>> raised up to global functions in StringExtras.  All of this can certainly
>> be done, but it's going to be a *ton* of churn and hours spent to get it
>> all STL-ified.
>> Do you consider this a blocker for doing such a split?  Would it make
>> sense to do it incrementally where we first just move StringRef et all
>> wholesale, and then incrementally work to STL-ify the interface?
>> On Wed, Jul 5, 2017 at 5:01 PM Chris Lattner <clattner at nondot.org> wrote:
>>> Yes, that proposal makes sense to me: the split would be between things
>>> that *are* known to be subsumed into later versions of C++, and therefore
>>> are a compatibility library.
>>> What do you think about this as an implementation approach:
>>>  - Rewrite StringRef (et al) to use the exact same APIs as
>>> std::string_view.  Keep the StringRef name for now.
>>>  - When cmake detects that C++’17 mode is supported, the build would set
>>> a -D flag.
>>>  - StringRef.h would just include the C++’17 header and typedef
>>> StringRef to that type.
>>>  - When we start requiring C++’17, someone can
>>> “StringRef”->RAUW(“std::string_view”) and nuke the header.
>>> This allows us to have a clean path out of these custom types, and makes
>>> it very clear that these headers are compatibility shims that go away in
>>> the future.  It also makes it clear what the division is.
>>> -Chris
>>> On Jul 5, 2017, at 10:38 AM, Zachary Turner <zturner at google.com> wrote:
>>> So, here is an example of where I think a split would be really helpful.
>>> https://reviews.llvm.org/D34667
>>> This code would benefit vastly even from just being able to use
>>> StringRef and ArrayRef.  We have other cases as well where we export some
>>> code that cannot depend on the rest of LLVM.
>>> Thinking about it some, StringRef, ArrayRef, and various other things
>>> like STLExtras and iterator.h basically can be summarized as "things that
>>> are either already already planned for, or wouldn't be entirely out of
>>> place in the STL itself".  For example, StringRef is std::string_view.
>>> ArrayRef is std::array_view.  iterator_facade_base is a better version of
>>> std::iterator.
>>> So I would drop my suggestion to split the libraries in such a way that
>>> it might benefit TableGen, and instead re-word my suggestion to include
>>> only classes such as StringRef, ArrayRef, and other STL-like utilities that
>>> can benefit utilities like our demangler etc that cannot depend on the rest
>>> of LLVM.  If and when we ever require C++17 for building LLVM (a long ways
>>> away, obviously, but we might as well be forward thinking), we would
>>> certainly be able to use std::string_view and std::array_view in the
>>> demangler.  So splitting things in a way such as this makes long term sense
>>> IMO.
>>> On Sun, Jun 4, 2017 at 10:50 AM Zachary Turner <zturner at google.com>
>>> wrote:
>>>> Fair enough, i sort of regret mentioning that specific method of
>>>> splitting originally.
>>>> For the record, i think any splitting should make sense on its own
>>>> merit without considering tablegen, and hopefully the end result of
>>>> "tablegen eventually depends on less stuff" would happen naturally
>>>> On Sun, Jun 4, 2017 at 10:37 AM Chris Lattner <clattner at nondot.org>
>>>> wrote:
>>>>> > On May 26, 2017, at 5:47 PM, Zachary Turner via llvm-dev <
>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>> >
>>>>> > Changing a header file somewhere and having to spend 10 minutes
>>>>> waiting for a build leads to a lot of wasted developer time.
>>>>> >
>>>>> > The real culprit here is tablegen.  Can we split support and ADT
>>>>> into two - the parts that tablegen depends on and the parts that it doesn’t?
>>>>> In all the comments downthread, I think there is one thing that hasn't
>>>>> been mentioned: doing a split like this makes tblgen evolution more
>>>>> difficult.  If libsupport was split into “used by tblgen” and “not used by
>>>>> tblgen” sections, and then a new tblgen feature needs to use other parts of
>>>>> libsupport, they’d have to be moved into the “used by tblgen” directory.
>>>>> Splitting libsupport as a whole out into its own llvm subproject has
>>>>> come up many times though, and does make a lot of sense.
>>>>> -Chris
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