[PATCH] D136919: [X86][RFC] Change mangle name of __bf16 from u6__bf16 to DF16b

[Phoebe Wang via Phabricator via cfe-commits]

pengfei added a comment.

In D136919#3904925 <https://reviews.llvm.org/D136919#3904925>, @RKSimon wrote:

> What are the rules on this? Do we just handle this as an ABI breaking change and document it in the release notes - or do we need to provide any auto-upgrade path (with a warning?)?

TBH, I don't have a good idea. It supposes to be Itanium ABI problem which I know little about. @ldionne WDYT?
I also put an RFC in the discourse <https://discourse.llvm.org/t/rfc-using-mangle-name-df16b-for-x86-bf16-type/66256>, but no response so far.
The GCC proposal <https://gcc.gnu.org/pipermail/gcc-patches/2022-September/602062.html> suggests for ARM/AArch64 as well. They should have the same problem like us since they used `u6__bf16` for several versions, but I don't know any progress so far.
I want to get some feedback by this and I can discuss my GCC colleagues to see if we can change it back in GCC if this is not a good direction. There are two reasons I get so far:

1. It is a ABI breaking change to LLVM and other targets;
2. A mangled name of `__bf16` will be demanged as `std::bfloat_t`, which might confuse the user;

Welcome for suggestions~


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