[llvm-dev] TargetMachine vs LLVMTargetMachine

[Eric Christopher via llvm-dev]

On Tue, Oct 3, 2017 at 2:37 PM Hal Finkel <hfinkel at anl.gov> wrote:

>
> On 10/03/2017 04:30 PM, Eric Christopher wrote:
>
>
>
> On Tue, Oct 3, 2017 at 8:54 AM Hal Finkel via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>>
>> On 10/02/2017 10:57 PM, Matthias Braun via llvm-dev wrote:
>>
>> The distinction between the LLVMTargetMachine and TargetMachine classes
>> has become somewhat muddy recently. So I created:
>>
>> https://reviews.llvm.org/D38482
>>
>> to clean things up. During review it was noted that we may rather merge
>> the two instead which looks like this:
>>
>> https://reviews.llvm.org/D38489
>>
>> We really should choose one of the two over the status quo. Some points
>> for the discussion:
>>
>> - I am not aware of any target that is actually implementing
>> TargetMachine only but not LLVMTargetMachine (I assume the C backend used
>> to do it before it was removed)
>> - Even when merging the two, I believe it is still possible to implement
>> a target without linking to lib/CodeGen by returning nullptr for the
>> various methods related to CodeGen.
>>
>>
>> If this is true, then it seems like we don't lose anything from doing
>> this (and the code gets cleaner). We don't currently have anything in tree
>> that
>>
>
> This was my suggestion of yesterday.
>
>
>> motivates the current split. It does seem useful to retain the ability to
>> have a direct-from-the-IR backend (mostly for translation into another IR).
>>
>>
> I'm dubious of this need, but as long as it doesn't add any overhead to
> the resultant code I'm good.
>
>
> The other thing that I asked in the other review thread, which I'll repeat
> here, is: does GlobalISel further reduce the motivation for this? The
> primary reason, as I understand it, for needing this kind of "direct"
> translation is to avoid going through type legalization. This can be
> important for software targeting FPGAs, for example, because the hardware
> can deal with arbitrary bit widths. With GlobalISel, can you go into
> CodeGen effectively without any legalization and then go on from there (if
> one needed such a thing)?
>
>
Not right now, no.

That said I'm uncertain how much we particularly care about keeping two
classes here when we can just change how initialization works to provide
nullptr versions of the local bits rather than having a separate overhead
class.

-eric



>  -Hal
>
>
>
> -eric
>
>
>>  -Hal
>>
>>
>> - The split would give some notion of an internal CodeGen interface and
>> an external interface visible to frontend/middleend etc.
>> - The code gets simpler when merging the two and we have to
>> document/explain less.
>>
>> - Matthias
>>
>>
>> _______________________________________________
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>>
>>
>> --
>> Hal Finkel
>> Lead, Compiler Technology and Programming Languages
>> Leadership Computing Facility
>> Argonne National Laboratory
>>
>> _______________________________________________
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>>
>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
>
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