* add nested packed struct/union behavior tests
* use ptr_info.packed_offset rather than trying to duplicate the logic from Sema.structFieldPtrByIndex()
* use the container_ptr_info.packed_offset to account for non-aligned nested structs.
* dedup type.packedStructFieldBitOffset() and module.structPackedFieldBitOffset()
My previous change for reading / writing to unions at comptime did not handle
union field read/writes correctly in all cases. Previously, if a field was
written to a union, it would overwrite the entire value. This is problematic
when a field of a larger size is subsequently read, because the value would not
be long enough, causing a panic.
Additionally, the writing behaviour itself was incorrect. Writing to a field of
a packed or extern union should only overwrite the bits corresponding to that
field, allowing for memory reintepretation via field writes / reads.
I addressed these problems as follows:
Add the concept of a "backing type" for extern / packed unions
(`Type.unionBackingType`). For extern unions, this is a `u8` array, for packed
unions it's an integer matching the `bitSize` of the union. Whenever union
memory is read at comptime, it's read as this type.
When union memory is written at comptime, the tag may still be known. If so, the
memory is written using the tagged type. If the tag is unknown (because this
union had previously been read from memory), it's simply written back out as the
backing type.
I added `write_packed` to the `reinterpret` field of
`ComptimePtrMutationKit`. This causes writes of the operand to be packed - which
is necessary when writing to a field of a packed union. Without this, writing a
value to a `u1` field would overwrite the entire byte it occupied.
The final case to address was reading a different (potentially larger) field
from a union when it was written with a known tag. To handle this, a new kind of
bitcast was introduced (`bitCastUnionFieldVal`) which supports reading a larger
field by using a backing buffer that has the unwritten bits set to
undefined. The reason to support this (vs always just writing the union as it's
backing type), is that no reads to larger fields ever occur at comptime, it
would be strictly worse to have spent time writing the full backing type.
This fixes a panic in `unionAbiSize` when a 0-length array of a union is used as a struct field.
Because `resolveTypeLayout` does not resolve the `elem_ty` if `arrayLenIncludingSentinel` returns
0 for the array, the child union type is not guaranteed to have a resolved layout at this point.
Fixed this case by just returning 0 here.
This commit introduces the new `ref_coerced_ty` result type into AstGen.
This represents a expression which we want to treat as an lvalue, and
the pointer will be coerced to a given type.
This change gives known result types to many expressions, in particular
struct and array initializations. This allows certain casts to work
which previously required explicitly specifying types via `@as`. It also
eliminates our dependence on anonymous struct types for expressions of
the form `&.{ ... }` - this paves the way for #16865, and also results
in less Sema magic happening for such initializations, also leading to
potentially better runtime code.
As part of these changes, this commit also implements #17194 by
disallowing RLS on explicitly-typed struct and array initializations.
Apologies for linking these changes - it seemed rather pointless to try
and separate them, since they both make big changes to struct and array
initializations in AstGen. The rationale for this change can be found in
the proposal - in essence, performing RLS whilst maintaining the
semantics of the intermediary type is a very difficult problem to solve.
This allowed the problematic `coerce_result_ptr` ZIR instruction to be
completely eliminated, which in turn also simplified the logic for
inferred allocations in Sema - thanks to this, we almost break even on
line count!
In doing this, the ZIR instructions surrounding these initializations
have been restructured - some have been added and removed, and others
renamed for clarity (and their semantics changed slightly). In order to
optimize ZIR tag count, the `struct_init_anon_ref` and
`array_init_anon_ref` instructions have been removed in favour of using
`ref` on a standard anonymous value initialization, since these
instructions are now virtually never used.
Lastly, it's worth noting that this commit introduces a slightly strange
source of generic poison types: in the expression `@as(*anyopaque, &x)`,
the sub-expression `x` has a generic poison result type, despite no
generic code being involved. This turns out to be a logical choice,
because we don't know the result type for `x`, and the generic poison
type represents precisely this case, providing the semantics we need.
Resolves: #16512Resolves: #17194
SPIR-V doesn't support true element indexing, so we probably
need to switch over to isByRef like in llvm for this to work
properly. Currently a temporary is used, which at least
seems to work.
