[llvm-dev] TargetSelect.h and layering

[David Blaikie via llvm-dev]

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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