[PATCH] D157762: [WIP] Implement [[msvc::no_unique_address]]

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added inline comments.


================
Comment at: clang/include/clang/Basic/Attr.td:1797
 
+def NoUniqueAddressMSVC : InheritableAttr, TargetSpecificAttr<TargetMicrosoftCXXABI> {
+  let Spellings = [CXX11<"msvc", "no_unique_address", 201803>];
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erichkeane wrote:
> I think I would combine this and the one above as a single attribute.  Then, only 'add' it in SemaDeclAttr.cpp when the spelling is the appropriate one for the current target.  IMO, msvc::no_unique_address should ONLY be valid in MSVC mode, else we encourage its propagation.
> 
> We don't need it to be a 'TargetSpecificAttr', just do the 'ignored' warning in the handler in SemaDeclAttr.td.
> 
> The accessor that we need is to just query which spelling.
That's the approach I was thinking of, but then we lose some of the declarative expressiveness from Attr.td, which is unfortunate. But this is a pretty unique situation and so perhaps it's reasonable and we can try to think of a tablegen approach once we get a second attribute in the same situation.


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Comment at: clang/lib/AST/Decl.cpp:4523-4524
 bool FieldDecl::isPotentiallyOverlapping() const {
-  return hasAttr<NoUniqueAddressAttr>() && getType()->getAsCXXRecordDecl();
+  return (hasAttr<NoUniqueAddressAttr>() ||
+          hasAttr<NoUniqueAddressMSVCAttr>()) &&
+         getType()->getAsCXXRecordDecl();
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akhuang wrote:
> aaron.ballman wrote:
> > dblaikie wrote:
> > > Having to check both of these in several places seems problematic - can we wrap that up somewhere? (or, maybe ideally, is there a way for `msvc::no_unique_address` to map to the actual NoUniqueAddressAttr as a different spelling of the same thing?)
> > This was why I was hoping we could merge the two in Attr.td, but I'm not certain that will be easy.
> What does merging the two in Attr.td mean? Could we just put the two spellings in one attribute, or would that make it impossible for clang-cl to ignore the [[no_unique_address]] spelling
We can have multiple syntactic spellings for the same semantic attribute (e.g., `__attribute__((foo))` and `__attribute__((bar))` can both map to a single `FooBarAttr` AST node), and we have "accessors" on the AST node that let you tell which spelling was used: https://github.com/llvm/llvm-project/blob/90ecadde62f30275c35fdf7928e3477a41691d21/clang/include/clang/Basic/Attr.td#L4095

The suggestion Erich and I are thinking of is:

1) Add the additional spelling to `NoUniqueAddress`.
2) Add accessors to differentiate the spellings.
3) Remove the `TargetSpecificAttr` from `NoUniqueAddress`, manually implement those checks in an attribute handler in SemaDeclAttr.cpp.

Then, anywhere you care about the attribute in general, you can look for `isa<NoUniqueAddress>`, and anywhere you care about which spelling, you can use `cast<NoUniqueAddress>(A)->isMSVC()` (or whatever you name the accessors).


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D157762/new/

https://reviews.llvm.org/D157762