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Asserts InternPool keys have unique representations #35914

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andrewrk merged 2 commits from masonremaley/intern-pool-unique-key-reprs into master 2026年06月25日 20:38:16 +02:00 AGit

Asserts InternPool keys have unique representations, resolving an inline TODO. I also added asserts that these types are extern compatible so as to guarantee stability across compilations.

While I was at it I also made a small improvement to hasUniqueRepresentation WRT unions. Not necessary for this PR could be separated.

SpirvType was failing both of these assertions. I resolved this by making it extern, and adding explicitly zeroed padding.

I didn't yet add test coverage that type equality works on reified SPIRV types, as zirReifySpirvType returns unique values for each type regardless of the hash. See #35915 for more info.

Asserts InternPool keys have unique representations, resolving an inline TODO. I also added asserts that these types are extern compatible so as to guarantee stability across compilations. While I was at it I also made a small improvement to `hasUniqueRepresentation` WRT unions. Not necessary for this PR could be separated. `SpirvType` was failing both of these assertions. I resolved this by making it extern, and adding explicitly zeroed padding. I didn't yet add test coverage that type equality works on reified SPIRV types, as `zirReifySpirvType` returns unique values for each type regardless of the hash. See #35915 for more info.
MasonRemaley changed title from (削除) Adds unions support to hasUniqueRepresentation (削除ここまで) to Asserts InternPool keys have unique representations 2026年06月24日 08:25:19 +02:00
@ -903,0 +906,4 @@
if(!hasUniqueRepresentation(field_type))returnfalse;
}
returntrue;
},
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I don't think this is correct for unions with auto layout. They can either have an explicit enum tag or an implicit safety tag, both of which can have padding bits. The check for each field should probably be @sizeOf(field_type) == @sizeOf(T) to allow extern unions and plain auto unions in unsafe release modes.

I guess we could also consider a more comprehensive check for tagged unions to accept the ones where hasUniqueRepresentation(tag_type) and @sizeOf(tag_type) + @sizeOf(field_type) == @sizeOf(T), this isn't possible for safety-tagged plain unions though since the safety tag cannot be accessed via @typeInfo.

Also there should be the same check for packed unions like there is for packed structs.

I don't think this is correct for unions with `auto` layout. They can either have an explicit enum tag or an implicit safety tag, both of which can have padding bits. The check for each field should probably be `@sizeOf(field_type) == @sizeOf(T)` to allow `extern` unions and plain `auto` unions in unsafe release modes. I guess we could also consider a more comprehensive check for tagged unions to accept the ones where `hasUniqueRepresentation(tag_type) and @sizeOf(tag_type) + @sizeOf(field_type) == @sizeOf(T)`, this isn't possible for safety-tagged plain unions though since the safety tag cannot be accessed via `@typeInfo`. Also there should be the same check for `packed` unions like there is for `packed` structs.
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Agree that the version in the PR is incorrect, however:

I guess we could also consider a more comprehensive check for tagged unions to accept the ones where [...]

This actually wouldn't be correct even for explicitly-tagged unions, because it turns out that the design of Zig does not require implementations to use the given tag type for the in-memory representation of a tagged union. For instance, a union(enum(u16)) with 256 or fewer fields could be represented in memory with a 1-byte tag---and this choice has no observable effects to the user, so is legal.

This means that the only auto-layout unions you can really let through here are the ones you initially suggested, where @sizeOf(FieldType) == @sizeOf(UnionType) for all fields.

Agree that the version in the PR is incorrect, however: > I guess we could also consider a more comprehensive check for tagged unions to accept the ones where [...] This actually wouldn't be correct even for explicitly-tagged unions, because it turns out that the design of Zig does not require implementations to use the given tag type for the in-memory representation of a tagged union. For instance, a `union(enum(u16))` with 256 or fewer fields *could* be represented in memory with a 1-byte tag---and this choice has no observable effects to the user, so is legal. This means that the only auto-layout unions you can really let through here are the ones you initially suggested, where `@sizeOf(FieldType) == @sizeOf(UnionType)` for all fields.
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Good catch, not sure how I forgot that tags exist! I modified the check to be @sizeOf(field_type) == @sizeOf(T) and added handling for packed unions.

Good catch, not sure how I forgot that tags exist! I modified the check to be `@sizeOf(field_type) == @sizeOf(T)` and added handling for packed unions.
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andrewrk requested changes 2026年06月25日 20:24:58 +02:00
Dismissed
@ -900,6 +900,15 @@ pub inline fn hasUniqueRepresentation(comptime T: type) bool {
return@sizeOf(T)==sum_size;
},
.@"union"=>|info|{
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auto layout unions have non-well-defined memory layout

auto layout unions have non-well-defined memory layout
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Regardless of what the language specification guarantees, if the size of the union exactly equals the size of all fields, then we know the union layout has no unused/padding bits (which is effectively what this function is trying to figure out). I could be down for saying that we should ignore the "actual" layout here and just return false for any type with ill-defined layout, but that is not the current behaviour on e.g. struct types, so what Mason has implemented here is consistent with the current behaviour.

Regardless of what the language specification guarantees, if the size of the union exactly equals the size of all fields, then we know the union layout has no unused/padding bits (which is effectively what this function is trying to figure out). I could be down for saying that we should ignore the "actual" layout here and just return `false` for any type with ill-defined layout, but that is not the current behaviour on e.g. `struct` types, so what Mason has implemented here is consistent with the current behaviour.
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I see, I suppose that is an OK definition of this function. This might bite us in the ass later though when we try to introduce tooling to find unsound operations on memory.

I see, I suppose that is an OK definition of this function. This might bite us in the ass later though when we try to introduce tooling to find unsound operations on memory.
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Moving convo to #35944

Moving convo to #35944
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Thanks for making a followup issue!

Thanks for making a followup issue!
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