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Document the new discriminant design

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Richard Feldman 2022-05-20 22:03:04 -04:00
parent 7465b82a04
commit 2daf001786
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13
bindgen/src/bindgen_rs.rs
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@ -186,6 +186,19 @@ pub union {name} {{"#
TargetInfo::from(&target_lexicon::Triple::host()),
);
// An old design, which ended up not working out, was that the tag union
// was a struct containing two fields: one for the `union`, and another
// for the discriminant.
//
// The problem with this was alignment; e.g. if you have one variant with a
// RocStr in it and another with an I128, then the `union` has a size of 32B
// and the discriminant is right after it - making the size of the whole struct
// round up to 48B total, since it has an alignment of 16 from the I128.
//
// However, Roc will generate the more efficient thing here: the whole thing will
// be 32B, and the discriminant will appear at offset 24 - right after the end of
// the RocStr. The current design recognizes this and works with it, by representing
// the entire structure as a union and manually setting the tag at the appropriate offset.
write!(
buf,
r#"