[libc-commits] [PATCH] D71094: [libc] Add implementation of errno and define the other macros of errno.h.

[Alex Brachet via Phabricator via libc-commits]

abrachet added inline comments.


================
Comment at: libc/src/errno/llvmlibc_errno.h:14
+// public header.
+extern int &llvmlibc_errno;
+
----------------
sivachandra wrote:
> MaskRay wrote:
> > sivachandra wrote:
> > > abrachet wrote:
> > > > MaskRay wrote:
> > > > > I don't think this will work. In the test translation unit, the compiler does not know that `llvmlibc_errno` is bound to a thread-local variable.
> > > > > 
> > > > > Even in the definition site, it's not clear to me that a reference to a thread-local variable will guarantee to work.
> > > > Right, it must be `thread_local int &`. I've tested it only briefly, if the reference isn't `thread_local` it doesn't work. As far as I can tell `thread_local int &` works.
> > > > 
> > > > 
> > > Sigh yes. This declaration has to include `thread_local` for everything to work correctly. Will update in the next round.
> > But why do we need a thread_local reference in the first place? It is represented as a thread_local pointer that is initialized to the return address of a TLS wrapper function under the hood. You can check the codegen. The performance is bad.
> I did not consider codegen. So yes, the macro wins over the reference if you look at the codegen.
>  You can check the codegen. The performance is bad.
Yup I didn't realize it would be much worse. I agree that it is not worth the performance overhead and we should just go back to the macro.


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  https://reviews.llvm.org/D71094/new/

https://reviews.llvm.org/D71094