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Document the C ABI for _BitInt and other types. #240

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Dec 10, 2024
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Document the C ABI for _BitInt and other types. #240

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9 changes: 9 additions & 0 deletions BasicCABI.md
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Expand Up @@ -38,6 +38,8 @@ General type | C Type | `sizeof` | Alignment (bytes) | Wasm Value Type
Integer | `unsigned long` | 4 | 4 | i32
Integer | `long long` / `signed long long` | 8 | 8 | i64
Integer | `unsigned long long` | 8 | 8 | i64
Integer | `__int128_t` | 16 | 16 | (none)
Integer | `__uint128_t` | 16 | 16 | (none)
Pointer | *`any-type *`* / *`any-type (*)()`* | 4 | 4 | i32
Floating point | `float` | 4 | 4 | f32
Floating point | `double` | 8 | 8 | f64
Expand All @@ -56,6 +58,13 @@ General type | C Type | `sizeof` | Alignment (bytes) | Wasm Value Type
Otherwise, if a compiler supports such an `enum`, it would use `i64`
for the `enum`.
* Emscripten uses 8 byte alignment for `long double`.
* `__int128_t` and `__uint128_t` are passed as two `i64`s in little-endian
order, and are otherwise represented in the ABI as a struct containing two
`i64` fields, in little-endian order.
* `_Complex` types are represented in the ABI as a struct containing two
corresponding floating-point fields, real and imaginary.
* `_BitInt(N)` types are supported up to width 128 and are represented as the
smallest same-signedness Integer type with at least as many bits.

**Aggregates and Unions**

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