inference: track reaching defs for slots

[topolarity]

This PR implements the "Path-Convergence Criterion" for SSA / ϕ-nodes as part of type-inference.

The VarState.ssadef field corresponds to the "reaching definition" of the slot in SSA form, which allows us to conveniently reason about the identity of a slot across multiple program points. If the reaching def is equal at two program points, then the slot contents are guaranteed to be egal (i.e. x₀ === x₁)

Tasks:

• Split StateRefinement change to separate PR
• Split VarTable plumbing to separate PR
• Remove old Conditional invalidation logic
• Add tests

Fixes #55548 (alternative to #55551), by having Conditional remember the reaching definition that it narrows.

[aviatesk]

Thanks, this looks much better than my PR in general.
Still I would like to add the test case from it, i.e.

# JuliaLang/julia#55548: invalidate stale slot wrapper types in `ssavaluetypes`
_issue55548_proj1(a, b) = a
function issue55548(a)
    a = Base.inferencebarrier(a)::Union{Int64,Float64}
    if _issue55548_proj1(isa(a, Int64), (a = Base.inferencebarrier(1.0)::Union{Int64,Float64}; true))
        return a
    end
    return 2
end
@test Float64 <: Base.infer_return_type(issue55548, (Int,))

[aviatesk]

@nanosoldier runbenchmarks("inference", vs=":master")

[topolarity]

Thanks for the test case and review @aviatesk

fwiw, there's also a precision regression that I encountered w/ this PR:

function foo()
    value = @noinline rand((true, false, 1))
    is_bool = value isa Bool
    if inferencebarrier(@noinline rand(Bool))
        value = 1.0
        is_bool = false
    end
    # at this merge point, only the "false half" of the Conditional should be invalidated
    return is_bool ? value : false
end
@assert only(Base.return_types(foo, ())) === Bool # fails w/ PR

but I think this can be mostly fixed by widening the Conditional at the merge point and giving it a new .ssadef to match

[nanosoldier]

Your benchmark job has completed - possible performance regressions were detected. A full report can be found here.

[vtjnash]

bump? are we able to land this and backport to v1.11/v1.10?

[serenity4]

As discussed with @topolarity, I will take over this PR and finish it so we can have a fix for the underlying issue.

It seems like most of the core functionality works well at the moment, so I believe it's mostly a matter of finishing up the design and related refactors, and address performance regressions.

[serenity4]

@nanosoldier runbenchmarks("inference", vs=":master")

[nanosoldier]

Your benchmark job has completed - possible performance regressions were detected. A full report can be found here.

[serenity4]

@nanosoldier runbenchmarks("inference", vs=":master")

[nanosoldier]

Your benchmark job has completed - possible performance regressions were detected. A full report can be found here.

[serenity4]

@nanosoldier runbenchmarks("inference", vs=":master")

[nanosoldier]

Your benchmark job has completed - possible performance regressions were detected. A full report can be found here.