Hi Bob
> * Given that ABIArgInfo::Expand will only be used for unions that are
> homogeneous, does it really make sense to put code in
> CodeGenTypes::GetExpandedTypes and CodeGenFunction::ExpandTypeToArgs to
> search through members of a union to find the largest field? For the current
> usage, it should be sufficient to just grab the first one.
No. Consider e.g.
union {
float bar[3];
struct {
float baz[4];
};
};
Here we should select float[4] as proper h. aggregate.
> * More seriously, I don't see how you can get away with asserting on unions
> in CodeGenFunction::ExpandTypeFromArgs. Am I missing something?
Yes. I missed this point. I have a testcase and working on the fix.
> * I don't much like the code duplication for the "expand a field" code in
> ExpandTypeToArgs. If you can get away with just grabbing the first field of
> a union, I think you could unify that into a single loop over the fields and
> just exit after the first iteration in the case of a union. Otherwise, maybe
> you could factor that code out into a separate function.
+1
> * When I first implemented this, I intentionally limited it for C++ to
> aggregate-like types. Can you give an example of a non-aggregate-like C++
> type that you think should be passed as a homogeneous aggregate? Certainly
> anything with a vtable pointer isn't going to work.
Easily. Consider e.g.
class Vector2 {
float32x2_t vec;
};
Vector2 foo(const Vector2 &v) { return v; }
Vector2 is not C++ aggregate, because it contains private fields.
However, it's h. aggregate per ARM specs, because check for
homogeneity should be performed w/o language-specific restrictions.
> * I'm not surprised that aggregate-like C++ types were not handled correctly,
> since I didn't have any way to test this stuff. But, I'm curious: what was
> broken?
Bunch of examples where NEON vectors were wrapped inside Vector-like
classes with operators overloaded to do vector ops.
> * The big thing still missing here is that there is no logic to check how
> many VFP registers have already been used for other arguments. When deciding
> whether to pass an argument as a homogeneous aggregate, one of the criteria
> is that the entire aggregate has to fit into the remaining unused argument
> registers, right?
This is correct. I thought about this as a next step.
--
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University
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