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

[Zoe Carver via Phabricator via cfe-commits]

zoecarver added inline comments.


================
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())
----------------
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)? 


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D92361/new/

https://reviews.llvm.org/D92361