[SAoC 2021] Replace druntime Hooks with Templates: Milestone 1, Week 4
teodor.dutu at gmail.com
Fri Oct 15 22:36:42 UTC 2021
The PR that replaces the `_d_arrayctor` and `_d_arraysetctor`
hooks was blocked
by a bug in the compiler's backend, which was posted as an issue
[here](https://issues.dlang.org/show_bug.cgi?id=22372). The issue
was eventually fixed, which has allowed the aforementioned PR to
make some progress.
However, another bug was revealed, whereby the `_d_arrayctor`
function](https://github.com/dlang/druntime/blob/34b31e2b7ac6198255bca8d6d318a4f481b05604/src/core/internal/array/construction.d#L24) turned out to be incorrect. When instantiating the function with `const` or `immutable` array type as argument, the compiler issued warnings when its return value was ignored, because `_d_arrayctor` is also a strongly pure function. This means that the call to it seemingly has no effect. Furthermore, this false impression might make the compiler remove the call to it altogether.
A temporary and hackish solution was to add a pointer type
parameter to `_d_arrayctor`, implemented by [this
PR](https://github.com/dlang/druntime/pull/3587). This solution
turns `_d_arrayctor` from being **strongly pure** to being
**weakly pure**. As a result, its call cannot be optimised out by
We're still looking for a more elegant solution to the issue
above, as discussed in [this
thread](https://firstname.lastname@example.org). One alternative is to change `_d_arrayctor`'s signature so that its 2 parameters are of different types - say `T1` and `T2` - so that `Unqual!T1 == Unqual!T2`. I am currently working on this change.
In the meantime, I improved the error messages for the
`_d_arrayctor` and `_d_arraysetctor` templates in druntime. For
an [older PR](https://github.com/dlang/druntime/pull/3582), I had
to resort to using some rather austere error messages because of
the requirement that `_d_arrayctor` be `@nogc`.
However, following my mentor Edi's
[advice](https://github.com/dlang/druntime/pull/3582#discussion_r725480441), I implemented `@nogc` versions of the array utils functions inside `core/internal/util/array.d` and updated `_d_arrayctor` to use `enforceRawArraysConformableNogc`, in [this PR](https://github.com/dlang/druntime/pull/3583).
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