Re: AArch64 bfloat16 mangling

[Richard Sandiford <richard.sandiford@arm.com>]

Jakub Jelinek <jakub@redhat.com> writes:
> On Thu, Mar 09, 2023 at 05:14:11PM +0000, Richard Sandiford wrote:
>> We decided to keep the current mangling of __bf16 and use it for
>> std::bfloat16_t too.  __bf16 will become a non-standard arithmetic type.
>> This will be an explicit diversion from the Itanium ABI.
>> 
>> I think that's equivalent to your (2) without the part about following
>> the Itanium ABI.
>
> I'm afraid I have no idea how can the above work though.
>
> Diversion from the Itanium ABI is doable, we have various examples
> where we mangle things differently, say on powerpc* where
> long double is mangled as g if it is IBM double double format (i.e.
> Itanium __float128) while for long double the spec says e, and
> as u9__ieee128 if it is IEEE quad.  __float128 also mangles as
> u9__ieee128 and so does __ieee128.
>
> The problem is if __bf16 needs to be treated differently from
> decltype (0.0bf16) aka std::bfloat16_t (the former being a non-standard
> arithmetic type, the latter being C++23 extended floating-point type,
> then they need to be distinct types.  And distinct types need to
> mangle differently.  Consider
> #include <stdfloat>
> template <typename T>
> void bar () {}
> void baz ()
> {
>   bar<__bf16> ();
>   bar<decltype (0.0bf16)> ();
>   bar<std::bfloat16_t> ();
> }
> If __bf16 is distinct from the latter two which are the same type,
> then it will instantiate bar twice, for both of those types, but
> if they are mangled the same, will emit two functions with the same
> name and assembler will reject it (or LTO might ICE etc.).
>
> Note, e.g.
> void foo (__float128, __ieee128, long double, _Float128) {}
> template <typename T>
> void bar () {}
> void baz ()
> {
>   bar <__float128> ();
>   bar <__ieee128> ();
>   bar <long double> ();
> }
> works on powerpc64le-linux with -mlong-double-128 -mabi=ieeelongdouble
> because __float128, __ieee128 and long double types are in that case
> the same type, not distinct, so e.g. bar is instantiated just once
> (only _Float128 mangles differently above).  With
> -mlong-double-128 -mabi=ibmlongdouble __float128 and __ieee128 are
> the same (non-standard) type, while long double mangles differently
> (g) and _Float128 too, so bar is instantiated twice.
>
> So, either __bf16 should be also extended floating-point type
> like decltype (0.0bf16) and std::bfloat16_t and in that case
> it is fine if it mangles u6__bf16, or __bf16 will be a distinct
> type from the latter two,

Yeah, the former is what I meant.  The intention is that __bf16 and
std::bfloat16_t are the same type, not distinct types.

Richard

> __bf16 non-standard arithmetic type
> while the latter two extended floating-point types, but then
> they need to mangle differently, most likely u6__bf16 vs. DF16b.
>
> 	Jakub