We've got a big one here! This commit reworks how we represent pointers
in the InternPool, and rewrites the logic for loading and storing from
them at comptime.
Firstly, the pointer representation. Previously, pointers were
represented in a highly structured manner: pointers to fields, array
elements, etc, were explicitly represented. This works well for simple
cases, but is quite difficult to handle in the cases of unusual
reinterpretations, pointer casts, offsets, etc. Therefore, pointers are
now represented in a more "flat" manner. For types without well-defined
layouts -- such as comptime-only types, automatic-layout aggregates, and
so on -- we still use this "hierarchical" structure. However, for types
with well-defined layouts, we use a byte offset associated with the
pointer. This allows the comptime pointer access logic to deal with
reinterpreted pointers far more gracefully, because the "base address"
of a pointer -- for instance a `field` -- is a single value which
pointer accesses cannot exceed since the parent has undefined layout.
This strategy is also more useful to most backends -- see the updated
logic in `codegen.zig` and `codegen/llvm.zig`. For backends which do
prefer a chain of field and elements accesses for lowering pointer
values, such as SPIR-V, there is a helpful function in `Value` which
creates a strategy to derive a pointer value using ideally only field
and element accesses. This is actually more correct than the previous
logic, since it correctly handles pointer casts which, after the dust
has settled, end up referring exactly to an aggregate field or array
element.
In terms of the pointer access code, it has been rewritten from the
ground up. The old logic had become rather a mess of special cases being
added whenever bugs were hit, and was still riddled with bugs. The new
logic was written to handle the "difficult" cases correctly, the most
notable of which is restructuring of a comptime-only array (for
instance, converting a `[3][2]comptime_int` to a `[2][3]comptime_int`.
Currently, the logic for loading and storing work somewhat differently,
but a future change will likely improve the loading logic to bring it
more in line with the store strategy. As far as I can tell, the rewrite
has fixed all bugs exposed by #19414.
As a part of this, the comptime bitcast logic has also been rewritten.
Previously, bitcasts simply worked by serializing the entire value into
an in-memory buffer, then deserializing it. This strategy has two key
weaknesses: pointers, and undefined values. Representations of these
values at comptime cannot be easily serialized/deserialized whilst
preserving data, which means many bitcasts would become runtime-known if
pointers were involved, or would turn `undefined` values into `0xAA`.
The new logic works by "flattening" the datastructure to be cast into a
sequence of bit-packed atomic values, and then "unflattening" it; using
serialization when necessary, but with special handling for `undefined`
values and for pointers which align in virtual memory. The resulting
code is definitely slower -- more on this later -- but it is correct.
The pointer access and bitcast logic required some helper functions and
types which are not generally useful elsewhere, so I opted to split them
into separate files `Sema/comptime_ptr_access.zig` and
`Sema/bitcast.zig`, with simple re-exports in `Sema.zig` for their small
public APIs.
Whilst working on this branch, I caught various unrelated bugs with
transitive Sema errors, and with the handling of `undefined` values.
These bugs have been fixed, and corresponding behavior test added.
In terms of performance, I do anticipate that this commit will regress
performance somewhat, because the new pointer access and bitcast logic
is necessarily more complex. I have not yet taken performance
measurements, but will do shortly, and post the results in this PR. If
the performance regression is severe, I will do work to to optimize the
new logic before merge.
Resolves: #19452Resolves: #19460
This change allows struct field inits to use layout information
of their own struct without causing a circular dependency.
`semaStructFields` caches the ranges of the init bodies in the `StructType`
trailing data. The init bodies are then resolved by `resolveStructFieldInits`,
which is called before the inits are actually required.
Within the init bodies, the struct decl's instruction is repurposed to refer
to the field type itself. This is to allow us to easily rebuild the inst_map
mapping required for the init body instructions to refer to the field type.
Thanks to @mlugg for the guidance on this one!
This removes the strategy where union with different active
fields would be generated, and instead simply pointer casts
the active field type where required. This also allows removing
spv.ptrType and using self.ptrType instead, and allows caching
all union types (because there is only the canonical one).
There are a couple concepts here worth understanding:
Key.UnionType - This type is available *before* resolving the union's
fields. The enum tag type, number of fields, and field names, field
types, and field alignments are not available with this.
InternPool.UnionType - This one can be obtained from the above type with
`InternPool.loadUnionType` which asserts that the union's enum tag type
has been resolved. This one has all the information available.
Additionally:
* ZIR: Turn an unused bit into `any_aligned_fields` flag to help
semantic analysis know whether a union has explicit alignment on any
fields (usually not).
* Sema: delete `resolveTypeRequiresComptime` which had the same type
signature and near-duplicate logic to `typeRequiresComptime`.
- Make opaque types not report comptime-only (this was inconsistent
between the two implementations of this function).
* Implement accepted proposal #12556 which is a breaking change.
This implements the semantics as discussed in today's compiler meeting,
where the alignment of pointers to fields of default-layout unions
cannot exceed the field's alignment.
Resolves: #15878
