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[BasicCABI] Pass compiler-rt 128-bit return values in memory #223

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[BasicCABI] Pass compiler-rt 128-bit return values in memory #223

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40 changes: 26 additions & 14 deletions BasicCABI.md
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Expand Up @@ -173,23 +173,35 @@ making a copy of any indirectly passed data that the callee should not modify.
Similarly, types can either be returned directly from WebAssembly functions or
returned indirectly via a pointer parameter prepended to the parameter list.

Type | Parameter | Result |
-----------------------------|---------------|----------|
scalar[1] | direct | direct |
empty struct or union | ignored | ignored |
singleton struct or union[2] | direct | direct |
other struct or union[3] | indirect | indirect |
array | indirect | N/A |

[1] `long long double` and `__int128` are passed directly as two `i64` values.
Signed 8 and 16-bit scalars are sign-extended, and unsigned 8 and 16-bit
scalars are zero-extended before passing or returning.

[2] Any struct or union that recursively (including through nested structs,
Type | Parameter | Result |
-----------------------------|---------------|--------------------|
scalar | direct[1] | direct/indirect[2] |
empty struct or union | ignored | ignored |
singleton struct or union[3] | direct | direct |
other struct or union[4] | indirect | indirect |
array | indirect | N/A |

[1] In case of parameters, `long long double`, `__int128`, and `__float128` are
passed directly as two `i64` values. In case of return values, for
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[1] appears once in the "Parameter" column, so I'm confused about seeing "in case of return values" here. Should that part be in [2]?

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Right, it should go to [2]. Given that whether we are gonna do this or not is unclear, will fix it later (if we go this route)

non-compiler-rt functions, they are passed directly as two `i64` values if
[multivalue](https://github.com/WebAssembly/multi-value) is enabled and
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I don't think this is true is it? Enable multi-value doesn't change the ABI, right?

We do have an experimental multi-value ABI but its not official, and we probably don't want to be mentioning it at all here.

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Will fix this once we decide what to do with the feature flag / experimental flag / ABI.

indirectly passed via a pointer parameter otherwise.
Signed 8 and 16-bit scalars are sign-extended, and unsigned 8 and 16-bit scalars
are zero-extended before passing or returning.

[2] In case of return values for compiler-rt functions, `long long double`,
`__int128`, and `__float128` are indirectly passed via a pointer parameter that
points to the value in memory, regardless of whether
[multivalue](https://github.com/WebAssembly/multi-value) is enabled or not, to
make the compiler-rt ABI uniform across the features.
Signed 8 and 16-bit scalars are sign-extended, and unsigned 8 and 16-bit scalars
are zero-extended before passing or returning.

[3] Any struct or union that recursively (including through nested structs,
unions, and arrays) contains just a single scalar value and is not specified to
have greater than natural alignment.

[3] This calling convention was defined before
[4] This calling convention was defined before
[multivalue](https://github.com/WebAssembly/multi-value) was standardized. A new
default calling convention that changes this behavior and takes advantage of
multivalue may be introduced in the future.
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