aaron.ballman added inline comments.
================
Comment at: clang/include/clang/Basic/AttrDocs.td:1416-1417
+not significant. This allows global constants with the same contents to be
+merged. This can break global pointer identity, i.e. two different globals have
+the same address.
+
----------------
aeubanks wrote:
> aaron.ballman wrote:
> > What happens for tentative definitions where the value isn't known? e.g.,
> > ```
> > [[clang::unnamed_addr]] int i1, i2;
> > ```
> >
> > What happens if the types are similar but not the same?
> > ```
> > [[clang::unnamed_addr]] signed int i1 = 32;
> > [[clang::unnamed_addr]] unsigned int i2 = 32;
> > ```
> >
> > Should we diagnose taking the address of such an attributed variable so
> > users have some hope of spotting the non-conforming situations?
> >
> > Does this attribute have impacts across translation unit boundaries
> > (perhaps only when doing LTO) or only within a single TU?
> >
> > What does this attribute do in C++ in the presence of constructors and
> > destructors? e.g.,
> > ```
> > struct S {
> > S();
> > ~S();
> > };
> >
> > [[clang::unnamed_addr]] S s1, s2; // Are these merged and there's only one
> > ctor/dtor call?
> > ```
> globals are only mergeable if they're known to be constant and have the same
> value/size. this can be done at compile time only if the optimizer can see
> the constant values, or at link time
>
> so nothing would happen in any of the cases you've given.
>
> but yeah that does imply that we should warn when the attribute is used on
> non const, non-POD globals. I'll update this patch to do that
>
> as mentioned in the description, we actually do want to take the address of
> these globals for table-driven parsing, but we don't care about identity
> equality
> globals are only mergeable if they're known to be constant and have the same
> value/size. this can be done at compile time only if the optimizer can see
> the constant values, or at link time
>
> so nothing would happen in any of the cases you've given.
Ahhhh that's good to know. So I assume we *will* merge these?
```
struct S {
int i, j;
float f;
};
[[clang::unnamed_addr]] const S s1 = { 1, 2, 3.0f };
[[clang::unnamed_addr]] const S s2 = { 1, 2, 3.0f };
[[clang::unnamed_addr]] const S s3 = s2;
```
> but yeah that does imply that we should warn when the attribute is used on
> non const, non-POD globals. I'll update this patch to do that
Thank you, I think that will be more user-friendly
> as mentioned in the description, we actually do want to take the address of
> these globals for table-driven parsing, but we don't care about identity
> equality
Yeah, I still wonder if we want to diagnose just the same -- if the address is
never taken, there's not really a way to notice the optimization, but if the
address is taken, you basically get UB (and I think we should explicitly
document it as such). Given how easy it is to accidentally take the address of
something (like via a reference in C++), I think we should warn by default, but
still have a warning group for folks who want to live life dangerously.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D158223/new/
https://reviews.llvm.org/D158223
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