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druntime: Implement va_arg for WebAssembly #5111

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druntime: Implement va_arg for WebAssembly #5111

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18 changes: 18 additions & 0 deletions runtime/druntime/src/core/stdc/stdarg.d
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Original file line number Diff line number Diff line change
Expand Up @@ -180,6 +180,24 @@ else version (DigitalMars)
*/
version (GNU)
T va_arg(T)(ref va_list ap); // intrinsic
else version (WebAssembly){
pragma(LDC_va_arg)
T ldc_va_arg(T)(ref va_list ap);
T va_arg(T)(ref va_list ap)
{
static if (__traits(isScalar, T) || is(T == U*, U))
{
return ldc_va_arg!T(ap);
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@kinke kinke Mar 31, 2026

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We don't use this LLVM intrinsic anywhere else, it's basically useless and only works for trivial types (if at all), where the implementation is trivial anyway (your else branch most likely).

After glancing at https://github.com/ldc-developers/ldc/blob/master/gen/abi/wasm.cpp and the last paragraph in https://github.com/WebAssembly/tool-conventions/blob/main/BasicCABI.md#function-arguments-and-return-values, I think the else branch might really suffice.

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@kinke kinke Mar 31, 2026

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So if the else branch suffices, I'd say incorporate it in the generic va_arg template below, as for the other archs.

}
else
{
ap = ap.alignUp!(T.alignof);
auto p = cast(T*) ap;
ap += T.sizeof.alignUp;
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@kinke kinke Mar 31, 2026

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I don't think the size needs to be aligned up here; this will be taken care of by the alignUp!(T.alignof) for the next argument. If I understand the spec paragraph correctly:

Arguments are arranged in order in the buffer, with each argument placed at the lowest appropriately aligned address after the previous argument.

return *p;
}
}
}
else
T va_arg(T)(ref va_list ap)
{
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