[LLVMdev] A "backend" is ... ?

[Robinson, Paul]

I haven't run out of popcorn watching the "Upstreaming PNaCl's
IR simplification passes" thread but one thing struck me that
I would really like clarified.  It feels like a tangential 
issue so I thought I'd ask it separately.

In the very same message, I see this:

>>>> Some background:  There are two related use cases for these
>>>> IR simplification passes:
>>>>
>>>>  1) Simplifying the task of writing a new LLVM backend.  This
>>>> is Emscripten's use case.  The IR simplification passes reduce
>>>> the number of cases a backend has to handle, so they would be
>>>> useful for anyone else creating a new backend.
>>>
>>> If these simplify writing a backend, why wouldn't the patches
>>> include commensurate simplifications to LLVM's backends? That
>>> would both give them an in-tree customer, and more immediate
>>> value to the community and project as a whole.
>>
>> I'd also like to add:
>> If these simplify writing a backend, should there be commensurate
>> changes to any relevant documentation for getting started writing
>> backends? (we don't have much such documentation though...)
>
> Very much so, yes.

And this...

>>>> * Calling conventions lowering:  ExpandVarArgs and ExpandByVal
>>>> lower varargs and by-value argument passing respectively.
>>>> They would be useful for any backend that doesn't want to
>>>> implement varargs or by-value calling conventions.
>>>
>>> Why wouldn't these be applicable to existing backends? What is
>>> hard about the existing representations?

And this...

>>>>    * PromoteIntegers legalizes integer types (e.g. i30 is
>>>> converted to i32).
>>>
>>> Does it split up too-wide integers? Do we really want another
>>> integer legalization framework in LLVM? I am actually interested
>>> in doing (partial) legalization in the IR during lowering
>>> (codegenprep time) in order to simplify the backend, but I don't
>>> think we should develop such a framework independently of the
>>> legalization currently used in the backends.

...all of which clearly presume multiple backends exist;
and yet I then see this:

>>>> * Module-level lowering:  This implements, at the IR level,
>>>> functionality that is traditionally provided by "ld".  e.g.
>>>> ExpandCtors lowers llvm.global_ctors to the __init_array_start
>>>> and __init_array_end symbols that are used by C libraries at
>>>> startup.
>>>
>>> This doesn't make any sense to me. The IR representation is
>>> strictly simpler. It is trivially lowered in a backend. I don't
>>> understand what this would benefit.
>>
>> It might be simpler to do in the backend, but I think that the
>> point is that it is a recurring cost in every backend; in
>> particular for backends written by people starting out/playing
>> around with LLVM (i.e. potential future contributors), where
>> any potential performance loss is acceptable for the sake of
>> simplifying things.
>
> I don't understand this at all.
>
> We have a *target independent* backend. There is only one, so
> there should be no recurring cost.

So we have lots of backends, conjuring up new ones is common
enough that we desire better documentation for doing it, the
goal of simplifying these backends is moderately worthy, and..
"there is only one" backend.

Uh... what?

Yours in ignorance and confusion,
--paulr