[PATCH] Add optional __Bfloat16 support

Copy + paste the RFC from x86-64-abi group. It’s recommended to post discusstions there.


Pass and return __Bfloat16 values in XMM registers.

__Bfloat16 (BF16) is a new floating-point format that can accelerate machine learning (deep learning training, in particular) algorithms.
It’s first introduced by Intel AVX-512 extension called AVX-512_BF16. __Bfloat16 has 8 bits of exponent and 7 bits of mantissa and it’s different from _Float16.

Currently __bfloat16 is a typedef of short, which creates a problem where the compiler does not raise any alarms if it is used to add, subtract, multiply or divide, but the result of the calculation is actually meaningless.
To solve this problem, a real scalar type __Bfloat16 needs to be introduced. It is mainly used for intrinsics, not available for C standard operators. __Bfloat16 will also be used for movement like passing parameter, load and store, vector initialization, vector shuffle, and .etc. It creates a need for a corresponding psABI.

x86-64-ABI/low-level-sys-info.tex | 10 +++++++±-
1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/x86-64-ABI/low-level-sys-info.tex b/x86-64-ABI/low-level-sys-info.tex
index a8b69db…ba8db0d 100644
— a/x86-64-ABI/low-level-sys-info.tex
+++ b/x86-64-ABI/low-level-sys-info.tex
@@ -302,6 +302,12 @@ be used to represent the type, is a family of integer types.
This permits the use of these types in allocated arrays using the common
sizeof(Array)/sizeof(ElementType) pattern.

+\subsubsection{Special Types}
+The \code{__Bfloat16} type uses a 8-bit exponent and 7-bit mantissa.
+It is used for \code{BF16} related intrinsics, it cannot be
+used with standard C operators.
\subsubsection{Aggregates and Unions}

Structures and unions assume the alignment of their most strictly
@@ -563,8 +569,8 @@ The basic types are assigned their natural classes:
\item Arguments of types (signed and unsigned) \code{_Bool}, \code{char},
\code{short}, \code{int}, \code{long}, \code{long long}, and
pointers are in the INTEGER class.
-\item Arguments of types \code{_Float16}, \code{float}, \code{double},

  • \code{_Decimal32},
    +\item Arguments of types \code{_Float16}, \code{__Bfloat16}, \code{float},
  • \code{double}, \code{_Decimal32},
    \code{_Decimal64} and \code{__m64} are in class SSE.
    \item Arguments of types \code{__float128}, \code{_Decimal128}
    and \code{__m128} are split into two halves. The least significant