[llvm] [X86] Lower scalar bf16 arithmetic on AVX10.2 via packed ops (PR #212245)

[Simon Pilgrim via llvm-commits]

================
@@ -34513,6 +34522,37 @@ SDValue X86TargetLowering::LowerOperation(SDValue Op, SelectionDAG &DAG) const {
   }
 }
 
+/// Lower a scalar bf16 arithmetic node whose type is being soft-promoted.
+///
+/// AVX10.2 has no scalar bf16 arithmetic instructions, and bf16 is a
+/// soft-promoted-half type, so scalar ops would otherwise be promoted to f32.
+/// Instead widen each operand to a v8bf16 vector, perform the legal packed
+/// operation, and extract the low element afterwards.
+///
+/// Each operand is reinterpreted as f16 before building the vector. bf16 has no
+/// scalar register class of its own, but f16 does (FR16X), and bf16/f16 share
+/// the same bits and vector register class (VR128X). Going through f16 keeps the
+/// value in an XMM register: bf16->f16 bitcast, f16->v8f16 SCALAR_TO_VECTOR
+/// (a free COPY_TO_REGCLASS), and v8f16->v8bf16 bitcast are all free. Feeding a
+/// scalar bf16 to SCALAR_TO_VECTOR directly would instead route through bf16's
+/// i16 soft-promote carrier, forcing xmm<->GPR vmovw roundtrips.
+///
+/// Returns a bf16-typed value.
----------------
RKSimon wrote:

+1

https://github.com/llvm/llvm-project/pull/212245