[llvm-dev] Inlining with different target features

[David Blaikie via llvm-dev]

On Mon, Aug 31, 2020 at 3:51 PM Thomas Lively <tlively at google.com> wrote:

> The difference is illustrated in this bug report:
> https://github.com/rust-lang/rust/issues/74320. Essentially, frontends
> like Rust want to allow users to opt-in to a feature by annotating a
> function that will use it. Your suggestion to require that all functions
> have the same feature set would prevent frontends from doing this sort of
> thing when targeting WebAsssembly. Which on one hand would be entirely
> reasonable, since annotating a single function with a feature misrepresents
> how WebAssembly features work, but on the other hand would make WebAssembly
> different from other targets, which I'd like to avoid.
>

Ah, then I'm not sure I understand "We already require separate builds of a
library for each feature set the library wants to support, so this wouldn't
make that story any worse." then. Is that statement ("already require
separate builds for a library for each feature set the library wants to
support") any different from, say x86? I guess for something like x86 you
could have one object built with one feature, another object built with
another feature, a 3rd that does a runtime test before dispatching to the
appropriate object - and all that in a single library.

So, yeah, that statement makes it a bit less versatile than the x86/other
target model - but it doesn't make it as bad as I'm suggesting. So what I'm
suggesting (that everything'd have to be rebuilt for any final target
combination of features) would make things worse, by the sounds of it?


>
> On Mon, Aug 31, 2020 at 2:13 PM David Blaikie <dblaikie at gmail.com> wrote:
>
>>
>>
>> On Mon, Aug 31, 2020 at 1:50 PM Thomas Lively via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> David,
>>>
>>> That's right, WebAssembly does not have a way to conditionally use a
>>> feature or even do runtime feature testing right now. It's on our roadmap
>>> of things to design and standardize, but it is still a long way off.
>>>
>>> > Another direction would be to require the features to be specified
>>> consistently for all components of the build, I guess - if that's the net
>>> effect anyway. Would make portable libraries difficult, though - because
>>> they'd be linked into different things with different feature sets and that
>>> would violate the invariant.
>>>
>>> I agree this would be reasonable. We already require separate builds of
>>> a library for each feature set the library wants to support, so this
>>> wouldn't make that story any worse.
>>>
>>
>> So if that's already the case, what would change between that state and
>> what I'm suggesting?
>>
>>
>>> The only reason I wouldn't want to enforce this is because that would be
>>> another way targeting Wasm would be different from targeting other
>>> platforms for frontends.
>>>
>>> Eric,
>>>
>>> Updating all the features up front would indeed make the most sense. I
>>> haven't seen a way for backends to specify LLVM IR passes that should be
>>> run early, though. Is that possible, or would frontends have to add this
>>> extra pass when targeting Wasm?
>>>
>>>
>>>
>>>
>>> On Mon, Aug 31, 2020 at 1:40 PM Eric Christopher <echristo at gmail.com>
>>> wrote:
>>>
>>>> Hi Thomas,
>>>>
>>>> I'd prefer not to change areInlineCompatible because I think it reads
>>>> fairly closely what is expected here (also see x86 for how this is used for
>>>> subset inlining calculations). I think if you plan on updating all of the
>>>> features for the functions to match you might just want to do that
>>>> initially rather than try to update them piecemeal after or during
>>>> inlining.
>>>>
>>>> Thoughts?
>>>>
>>>> -eric
>>>>
>>>> On Mon, Aug 17, 2020 at 4:49 PM Thomas Lively via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>> Hi llvm-dev,
>>>>>
>>>>> I recently updated the WebAssembly TargetTransformInfo to allow
>>>>> functions with different target feature sets to be inlined into each other,
>>>>> but I ran into an issue I want to get the community's opinion on.
>>>>>
>>>>> Since WebAssembly modules have to be validated before they are run, it
>>>>> only makes sense to talk about WebAssembly features at module granularity
>>>>> rather than function granularity. The WebAssembly backend even runs a pass
>>>>> that applies the union of all used features to each function. That means
>>>>> that ideally inlining for the WebAssembly target would be able to disregard
>>>>> features entirely, since they will all be the same in the end.
>>>>>
>>>>> However, right now I have to be more conservative than that and only
>>>>> allow a callee to be inlined into a caller if the callee has a subset of
>>>>> the caller's features. Otherwise, a target intrinsic might end up being
>>>>> used in a function that does not have the necessary target features
>>>>> enabled, which would cause a validation failure.
>>>>>
>>>>> The best solution I can think of for this problem would be to allow
>>>>> targets to opt-in to having a caller's feature set updated to include the
>>>>> callee's feature set when the callee is inlined into the caller. This could
>>>>> be implemented via a new TTI hook, but a more general solution might be to
>>>>> change the return type of `areInlineCompatible` to allow targets to control
>>>>> this behavior on a case-by-case basis. Does this general direction sound
>>>>> ok, and if so, would it be better to add a new hook or add functionality to
>>>>> the existing one?
>>>>>
>>>>> Thanks,
>>>>>
>>>>> Thomas
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
>>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>
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