[llvm-dev] Inlining with different target features

[Thomas Lively via llvm-dev]

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
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