[llvm-dev] TargetSelect.h and layering

[Eric Christopher via llvm-dev]

Yep. Here too.


On Thu, Dec 7, 2017, 9:13 AM Chandler Carruth <chandlerc at gmail.com> wrote:

> Yes, "all" is "all configured" I think throughout this discussion. Sorry
> for that confusion.
>
> On Thu, Dec 7, 2017 at 6:09 PM Robinson, Paul <paul.robinson at sony.com>
> wrote:
>
>> Could you guys clarify one thing for me?  It sounds like the idea is that
>> the current model of configuring the selection of targets would go away, to
>> be replaced by an all-or-native-only switch.  Sometimes I like to configure
>> X86+ARM because that reduces rebuild time and still gets me the vast
>> majority of debug-info tests.
>>
>> Or maybe you're using "all" as a shorthand for "all configured targets"
>> which would be just fine.
>>
>> Thanks
>>
>> --paulr
>>
>>
>>
>> *From:* llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] *On Behalf Of *Chandler
>> Carruth via llvm-dev
>> *Sent:* Wednesday, December 06, 2017 5:14 PM
>> *To:* Eric Christopher
>> *Cc:* llvm-dev; Richard Smith
>> *Subject:* Re: [llvm-dev] TargetSelect.h and layering
>>
>>
>>
>> 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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