[PATCH] D92361: [trivial-abi] Support types without a copy or move constructor.

[Zoe Carver via Phabricator via cfe-commits]

zoecarver added a comment.

In D92361#2459655 <https://reviews.llvm.org/D92361#2459655>, @rjmccall wrote:

> In D92361#2459513 <https://reviews.llvm.org/D92361#2459513>, @zoecarver wrote:
>
>>> I think that as long as the class leaves a copy/move constructor defaulted, there's no need for a new trivial_abi attribute.
>>
>> Sorry, I'm not sure I follow. Could you elaborate a bit or provide an example? What do you mean by "new" trivial_abi attribute?
>
> Sorry, I mean that I think Akira's example should be passed directly.  It shouldn't require its own trivial_abi attribute in order to get the treatment.

No worries. I understand now. The problem we're discussing actually has nothing to do with the trivial-abi attribute. We just need to make sure that non-trival-abi types are not affected by this change (and it appears they were).



================
Comment at: clang/lib/Sema/SemaDeclCXX.cpp:6502
+  // except that it has a non-trivial member *with* the trivial_abi attribute.
+  for (auto Base : D->bases()) {
+    if (auto CxxRecord = Base.getType()->getAsCXXRecordDecl())
----------------
Quuxplusone wrote:
> zoecarver wrote:
> > ahatanak wrote:
> > > zoecarver wrote:
> > > > ahatanak wrote:
> > > > > It looks like this patch changes the way `D` is passed in the following code:
> > > > > 
> > > > > ```
> > > > > struct B {
> > > > >   int i[4];
> > > > >   B();
> > > > >   B(const B &) = default;
> > > > >   B(B &&);
> > > > > };
> > > > > 
> > > > > struct D : B {
> > > > >   D();
> > > > >   D(const D &) = default;
> > > > >   D(D &&) = delete;
> > > > > };
> > > > > 
> > > > > void testB(B a);
> > > > > void testD(D a);
> > > > > 
> > > > > void testCallB() {
> > > > >   B b;
> > > > >   testB(b);
> > > > > }
> > > > > 
> > > > > void testCallD() {
> > > > >   D d;
> > > > >   testD(d);
> > > > > }
> > > > > ```
> > > > > 
> > > > > `B` cannot be passed in registers because it has a non-trivial move constructor, whereas `D` can be passed in registers because the move constructor is deleted and the copy constructor is trivial.
> > > > > 
> > > > > I'm not sure what the best way to handle this is, but I just wanted to point this out.
> > > > Hmm. Good catch. One way to fix this would be to simply create a `HasPassableSubobject` variable and add that to the conditions below (instead of returning false here). But, it seems that `D` isn't passed by registers (even though, maybe it should be) on ToT: https://godbolt.org/z/4xevW5 
> > > > 
> > > > Given that, do you think it's OK to return false here, or should I update this patch to use the logic I just described (even though that would be a nfc)? 
> > > The argument is byval, so `D` is passed directly. If you remove `-O3` and add `-target aarch64`, you'll see that `[2 x i64]` is being passed
> > Ah, I see now. Great. Thanks. I'll update the patch. 
> Akira's example is legal C++ with no Clang-specific attributes, so its behavior is governed by the appropriate platform's ABI doc — there exists one correct answer.
> At least on x86-64 with the Itanium ABI, GCC and ICC and Clang ToT all agree on the answer: `B` and `D` have exactly the same passing convention. If your patch breaks that, that's a problem.
> Contrariwise, it appears that `B` and `D` have different passing conventions on "armv8" according to Clang, and the same passing convention on "ARM64" according to GCC: https://godbolt.org/z/j9jzYG
> Of course if the programmer adds `[[clang::trivial_abi]]` to one of them, then all bets are off, standards-wise, and you're free to figure out a way to pass it in registers if you want to. But otherwise I think you have to follow what the ABI says.
> Bear in mind that I don't really know ABIs other than Itanium/x86-64. Maybe the problem here is that other platforms don't have well-defined ABIs and so we get to make one up? Maybe everyone except me is already aware that that's what we're doing? :)
> At least on x86-64 with the Itanium ABI, GCC and ICC and Clang ToT all agree on the answer: B and D have exactly the same passing convention. If your patch breaks that, that's a problem.

IIUC (and that's a big "if") being trivial for the purposes of a call, does not mean it will _always_ be passed through registers. (For example, we'd never pass a 512-byte type through registers.) And, I don't think they do have the same calling convention, though they are both passed indirectly. As Akira rightly pointed out, on ToT `D` has the `byval` attribute indicating that it _could_ be passed directly (according to the ABI/standard at least). While it might not be an observable change in the assembly, my patch (or at least the current version on phab) removes the byval argument. I have a local version that fixes this, though, and I'll upload that shortly. 


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