[llvm-dev] TargetMachine vs LLVMTargetMachine

[Hal Finkel via llvm-dev]

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 <mailto: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)?

The other motivation used to be to avoid register allocation, thus 
keeping the infinite set of virtual registers, but we've already 
addressed that issue.

  -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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