[llvm-dev] TargetSelect.h and layering

[Chandler Carruth via llvm-dev]

FWIW, I think the end state we'll end up wanting is what you describe in
your email: fine grained dependencies and something like libLLVM{AllTargets,
NativeTarget}{AsmPrinters,AsmParsers,Descs,Disassemblers,Infos}

I think the "Native" thing can be solved by having a CMake (and
llvm-config) level alias that points to a specific single target library.
Then I think you could actually build lib/Target/All/... directory tree
that provides the "all" libraries and links everything together.

Last but not least, I think in this world we'd want each of the narrow,
specific interfaces to be *inside* the individual target libraries rather
than squeezed into a single header file.

But this is a lot of churn and work. So I'm not seeing a huge problem if it
is ust too much churn and work and you make the header a textual header for
now. I'd document this super clearly in the header and lift it up a
directory to live alongside our other textual headers like LinkAllPasses.h

On Thu, Dec 7, 2017 at 2:10 AM Eric Christopher <echristo at gmail.com> wrote:

> My only alternate ideas are:
>
> a) To heck with this only a single target thing.
> b) Autogenerate the entire file and library support as part of the build
> and have the various functions "defined" in the appropriate libraries.
>
> I don't really think a) is feasible, and b) is a bit of a stretch too. :\
>
> -eric
>
> On Mon, Dec 4, 2017 at 5:37 PM David Blaikie <dblaikie at gmail.com> wrote:
>
>> Ping - any further/other thoughts from folks? I'm not /too/ fussed, but
>> generally like the idea of lib layering being simple/clear/obvious, but
>> understand these are sort of the degenerate/worst case.
>>
>> On Tue, Nov 28, 2017 at 12:12 PM David Blaikie <dblaikie at gmail.com>
>> wrote:
>>
>>> On Tue, Nov 28, 2017 at 11:27 AM Reid Kleckner <rnk at google.com> wrote:
>>>
>>>> On Tue, Nov 28, 2017 at 11:23 AM, David Blaikie via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>> Alternatively we can really say this header is a textual header - it's
>>>>> included generally only once in a whole program, the functions are called
>>>>> only once, etc. Though that does seem a little unfortunate on principle but
>>>>> not much practical problem with it, I think. It'd be nice in theory to be
>>>>> able to depend on the right library, have that bring in the right
>>>>> dependencies, etc.
>>>>>
>>>>
>>>> As designed, TargetSelect.h doesn't fit neatly into the normal way of
>>>> arranging libraries.
>>>>
>>>
>>> Not sure about that - yeah, it's a bit of the degenerate case, for sure.
>>>
>>> But in a build system like Google's, where a lib has other lib
>>> dependencies (whereas in the LLVM CMake build it seems libs don't depend on
>>> other libs - so every executable has to explicitly list its transitive lib
>>> dependencies) it's pretty nice to have these little libraries explicitly in
>>> the build graph - much like we have those synthetic library targets in the
>>> CMake rules, so it's easy to depend on the right/full things.
>>>
>>> (but because the CMake lib rules for LLVM don't actually describe lib
>>> dependencies, I think even 'fixing' this in upstream LLVM wouldn't make the
>>> dep situation better - the synthetic targets would just have to expand to
>>> the underlying libs + the wrapper/selector lib as well)
>>>
>>>
>>>> I'd mark it textual and leave it alone.
>>>>
>>>
>>> Yeah, maybe... just makes me a bit sad to have inline functions that
>>> can't be trivially out-of-lined if/when desired, because they layering
>>> isn't fully/correctly represented in the build system. Modular codegen's
>>> been a good justification to flush out & fix several of these tricksy
>>> layering violations in LLVM already.
>>>
>>>
>>>>
>>>> Alternatively, we could make AllTargetsDescs and AllTargetsInfos and
>>>> all the other synthetic libraries in CMake into real libaries and sink the
>>>> bodies of these inline functions into each tiny little library. Doesn't
>>>> seem quite worth it, though.
>>>>
>>>
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