AlexVlx wrote:
> @efriedma-quic was kind enough to have a call where we discussed this a bit
> more. I'll update tomorrow with a potential way forward, for the group's
> consideration.
Following up, here's a possible approach to making progress, broken down in
phases, (@efriedma-quic can correct me if I am misrepresenting any of these):
1. Have what is proposed here as an initial step, with the addition that we
issue warnings on unguarded uses of builtins / ASM (similar to what
`__builtin_available` / `@available` do), and we clean-up non-extern functions
that become unreachable as a consequence of predicate expansion (i.e. `foo` can
only be called from within this module, and it was only being called from a
predicate guarded block, which was removed);
2. Add attribute based checking for predicate guarded areas:
- Functions can be annotated either with the existing `target` attribute
or with a new `target_can_invoke` (name up for bike-shedding) attribute;
- Within a predicate guarded scope, if we encounter contradictions, e.g.
we call a `target("gfx9000")` function, or a
`target_can_invoke(builtin_only_on_gfx9000)`, within a
`__builtin_amdgcn_processor_is("gfx8999")`, that is an error
- This should reward users that go through the effort of annotating their
functions, making it much harder to write bugs
- I'm not entirely sure how to do this well yet (nested guarded regions,
where to track the currently active guard etc.), and it probably needs a bit
more design, hence why it's a different phase
- It is a pre-requisite for any attempt at making these general, rather
than target specific
3. In relation with generalisation, if we go in that direction (i.e. other
targets are interested / we think there's merit into hoisting these into
generic Clang builtins), we will have to look at whether or not we want a
different IR representation (possibly / probably along the lines of what has
been discussed here), for cases where a target must run some potentially
disruptive optimisations before and cannot just do the expansion right after
Clang.
https://github.com/llvm/llvm-project/pull/134016
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