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.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D96838/new/
https://reviews.llvm.org/D96838
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