llvm dev - Jul 2017 - Should we split llvm Support and ADT?

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
<mailto: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
<mailto: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 <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 <mailto: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 <mailto:clattner at nondot.org>> wrote:
>> 
>> > On May 26, 2017, at 5:47 PM, Zachary Turner via llvm-dev
<llvm-dev at lists.llvm.org <mailto: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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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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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
>>>>>
>>>>>
>>>>>
>>>
>> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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> On Jul 5, 2017, at 6:26 PM, Zachary Turner <zturner at google.com>
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.
In my opinion, this should be very strictly enforced.  We need a black and white
test for “what goes where”.  The “already accepted into a future standard”
metric satisfies that.

I’d even go so far as to say that stuff in StringRef that isn’t in string_view
(e.g. the atoi stuff) should be split out (e.g. to StringExtras) before the
move.

-Chris