[PATCH] D121768: [X86][tablgen] Auto-generate trivial fields and trivial interfaces for target features

[Kan Shengchen via Phabricator via llvm-commits]

skan added a comment.

In D121768#3392554 <https://reviews.llvm.org/D121768#3392554>, @craig.topper wrote:

> In D121768#3392153 <https://reviews.llvm.org/D121768#3392153>, @skan wrote:
>
>> In D121768#3391810 <https://reviews.llvm.org/D121768#3391810>, @tmatheson wrote:
>>
>>>> Your method can not get rid of non-trivial feature like useAA, which has an virtual interface.
>>>
>>> I gave an example above of exactly this:
>>>
>>>   bool enablePostRAScheduler() const override { return usePostRAScheduler(); }
>>>
>>>
>>>
>>>> In addition, as a LLVM backend developer, I often need to hack these interfaces to do some performance tuning or testing work, so the flexibility is quite important.
>>>
>>> Could you be more specific? I can't imagine how consistent getter/field naming prevents performance or testing work.
>>>
>>> Still seeing no justification for the added complexity of 2 new fields to SubtargetFeature, 3 new SubtargetFeature subclasses, in order to avoid a couple of function renames. We already have custom predicates.
>>
>> Let me tell the scenario:
>>
>> The features defined in TD file have dependencies between each other. The simpliest case is that both feature A and feature B depends on feature C due to any historic HW reason. When we disable C by passing
>> knobs like `-mattr=-C` or `-mnoC`, feature A and B will be disabled too.  However, it's possible either A or B does not depends on C in SW, namely, compiler could emit ISA in A while not emitting ISA in C.
>> Handwritten (non-trivial) interface gives us an quick way to untie such dependencies. If we'd like to disable C w/o affecting A and B, we can simply write such code
>>
>>   bool hasC() const { return false; }
>>
>> That's the flexibility and we could have more complicated cases.
>
> Since you would have to edit both the .td to make it non-trivial and then edit the .h file. Couldn't you instead rename the field in the .td so the autogenerated getter has a different name, and then add the custom getter using the old name?

It's not straightforward compared to my method. As I said, this is the simpliest case, non-trival interfaces allow us to untie and retie the dependencies in more complicated cases.  Another example is that we could add a knob like `-force-noC` to the backend, then define the interface like

  bool hasC() const { return C && !ForceNoC; }

so that we can control the dependency w/o building two compilers.


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