[PATCH] D96838: Add GNU attribute 'retain'

[Fangrui Song via Phabricator via cfe-commits]

MaskRay marked an inline comment as done.
MaskRay added inline comments.


================
Comment at: clang/include/clang/Basic/Attr.td:2655
+
+def Retain : InheritableAttr {
+  let Spellings = [GCC<"retain">];
----------------
MaskRay wrote:
> aaron.ballman wrote:
> > rjmccall wrote:
> > > MaskRay wrote:
> > > > aaron.ballman wrote:
> > > > > MaskRay wrote:
> > > > > > aaron.ballman wrote:
> > > > > > > MaskRay wrote:
> > > > > > > > aaron.ballman wrote:
> > > > > > > > > Should this be a target-specific attribute as it only has effects on ELF targets?
> > > > > > > > As I understand it, GCC `retain` is not warned on unsupported targets.
> > > > > > > > 
> > > > > > > > Regardless of GCC's choice, I think not having a `warning: unknown attribute 'retain' ignored [-Wunknown-attributes]` diagnostic makes sense. `retain` will be usually used together with `used`. In Clang, `used` already has "retain" semantics on macOS and Windows (I don't know what they do on GCC; GCC folks want orthogonality for ELF and I agree). If `retain` is silently ignored on macOS and Windows, then users don't need to condition compile for different targets.
> > > > > > > The other side of that is a user who writes only `retain` and expects it to do something when it's actually being silently ignored. While they may usually use it in conjunction with `used`, I'm more worried about the situation where the user is possibly confused.
> > > > > > `retain` without `used` can be used on some external linkage definitions, as well as internal linkage definitions which are referenced by live sections. I agree there could be some confusion, but hope with mccall's suggestion (thanks) the documentation is clear.
> > > > > If `retain` without `used` is an expected usage pattern, then I think we need a diagnostic when we ignore the `retain` attribute. I don't think it is reasonable to expect users to read the documentation because they won't know that they've misused the attribute when we silently ignore it.
> > > > > 
> > > > > Alternatively, would it be possible to make `retain` useful on all targets? e.g., when `retain` is used by itself on a declaration compiled for macOS or Windows, the backend does whatever it would normally do for `used`?
> > > > There is the normal behavior: `__attribute__((retain)) static void f1() {} // expected-warning {{unused function 'f1'}}`.
> > > > 
> > > > Sema.cpp:1227 has the unused diagnostics. There are already many different versions of diagnostics. Do you suggest another diagnostic line for `used is needed`? If you think so, I'll need to figure out how to do that...
> > > > 
> > > > Added some tests leveraging the existing diagnostic code.
> > > We could definitely support retain-without-used with the ELF semantics on all targets if we think they're useful.
> > I was thinking more along the lines of:
> > ```
> > def Retain : InheritableAttr, TargetSpecificAttr<???> {
> >   ...
> > }
> > ```
> > so that on targets where `retain` is ignored, the user gets an unknown attribute diagnostic as with other target-specific attributes. However, I see now that the attribute being passed along to the backend in all situations and so it isn't really a target-specific attribute in the same way as `MSNoVTable` (etc) is. It's not that `retain` isn't sensible on those targets, it's that different backends will want to handle the attribute differently and that might include it being a harmless noop. Is that a correct understanding? If so, then I don't think we need to make it target-specific or otherwise diagnose it.
> This is the case where LangRef definition reflects the state on Mach-O but not on ELF (and not on PE-COFF before 2018-01).
> 
> https://reviews.llvm.org/rG99f479abcf2c9b36daad04eb91cd0aafa659bb1d (CC @compnerd) (2018-01) is the commit. We could let `!retain` (if we choose to use `!retain` as the representation) lower to similar `/INCLUDE:` directives in the `.drectve` section.
> However, I see now that the attribute being passed along to the backend in all situations and so it isn't really a target-specific attribute in the same way as MSNoVTable (etc) is.

Yes. It depends on whether the backend can translate `!retain` into something affecting linker garbage collection behavior.
On ELF, it works via `SHF_GNU_RETAIN` (requires bleeding-edge toolchain).
On other binary formats, it is currently a no-op. Further notes:

On COFF, I can make `!retain` work by using `/INCLUDE:` for non-local-LLVM-linkage definitions.
On macOS, the maintainer can decide whether it is useful to set the S_ATTR_NO_DEAD_STRIP symbol attribute for `!retain`.

>  If so, then I don't think we need to make it target-specific or otherwise diagnose it.

Thanks! Marks as resolved.


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  https://reviews.llvm.org/D96838/new/

https://reviews.llvm.org/D96838