JuliaLang / julia / #37876

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Commit Message

inference: model partially initialized structs with `PartialStruct` (#55297)

There is still room for improvement in the accuracy of `getfield` and
`isdefined` for structs with uninitialized fields. This commit aims to
enhance the accuracy of struct field defined-ness by propagating such
struct as `PartialStruct` in cases where fields that might be
uninitialized are confirmed to be defined. Specifically, the
improvements are made in the following situations:
1. when a `:new` expression receives arguments greater than the minimum
number of initialized fields.
2. when new information about the initialized fields of `x` can be
obtained in the `then` branch of `if isdefined(x, :f)`.

Combined with the existing optimizations, these improvements enable DCE
in scenarios such as:
```julia
julia> @noinline broadcast_noescape1(a) = (broadcast(identity, a); nothing);

julia> @allocated broadcast_noescape1(Ref("x"))
16 # master
0  # this PR
```

One important point to note is that, as revealed in
JuliaLang/julia#48999, fields and globals can revert to `undef` during
precompilation. This commit does not affect globals. Furthermore, even
for fields, the refinements made by 1. and 2. are propagated along with
data-flow, and field defined-ness information is only used when fields
are confirmed to be initialized. Therefore, the same issues as
JuliaLang/julia#48999 will not occur by this commit.

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38 existing lines in 6 files now uncovered.

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