Helper functions to use arm matrix multiplication instructions in a way that is testable on other platofrms

ops/ops-inl.h

@@ -1788,6 +1788,59 @@ static HWY_INLINE VF8 Reduce8(DF df, VF x_0, VF x_1, VF x_2, VF x_3, VF x_4,
   return hn::Load(df8, buf);
 }
 
+template <class DN, class VI8, class VI32, HWY_IF_I32_D(DN)>
+HWY_API VI32 PerBlock2x2MatMulMaybeEmulate(DN dn, VI8 a, VI8 b, VI32 c) {
+#if HWY_NATIVE_PER_BLOCK_2X2_MATMUL_INT8
+  return hn::PerBlock2x2MatMul(dn, a, b, c);
+#else
+  const hn::Repartition<int8_t, DN> di8;
+  constexpr size_t kMaxN = hn::MaxLanes(dn);
+  HWY_LANES_CONSTEXPR size_t N = hn::Lanes(dn);
+  HWY_ALIGN int8_t in_a[kMaxN * 4];
+  HWY_ALIGN int8_t in_b[kMaxN * 4];
+  HWY_ALIGN int32_t expected[kMaxN];
+  hn::Store(a, di8, in_a);
+  hn::Store(b, di8, in_b);
+  hn::Store(c, dn, expected);
+
+  for (size_t block = 0; block < N; block += 4) {
+    const size_t block_i8 = block * 4;
+    for (int i = 0; i < 2; ++i) {
+      for (int j = 0; j < 2; ++j) {
+        int32_t sum = 0;
+        for (int k = 0; k < 8; ++k) {
+          sum += static_cast<int32_t>(in_a[block_i8 + i * 8 + k]) *
+                 static_cast<int32_t>(in_b[block_i8 + j * 8 + k]);
+        }
+        expected[block + i * 2 + j] += sum;
+      }
+    }
+  }
+  return hn::Load(dn, expected);
+#endif
+}
+
+template <class DN, class VBF, class VF, HWY_IF_F32_D(DN)>
+HWY_API VF PerBlock2x2MatMulMaybeEmulate(DN dn, VBF a, VBF b, VF c) {
+#if HWY_NATIVE_PER_BLOCK_2X2_MATMUL_BF16
+  return hn::PerBlock2x2MatMul(dn, a, b, c);
+#else
+  const hn::Repartition<hwy::bfloat16_t, DN> dbf;
+  const auto a_f = hn::BitCast(dn, a);
+  const auto a1 = hn::BitCast(dbf, hn::Per4LaneBlockShuffle<2, 2, 0, 0>(a_f));
+  const auto a2 = hn::BitCast(dbf, hn::Per4LaneBlockShuffle<3, 3, 1, 1>(a_f));
+
+  const auto b_f = hn::BitCast(dn, b);
+  const auto b1 = hn::BitCast(dbf, hn::Per4LaneBlockShuffle<2, 0, 2, 0>(b_f));
+  const auto b2 = hn::BitCast(dbf, hn::Per4LaneBlockShuffle<3, 1, 3, 1>(b_f));
+
+  VF sum1 = hn::Zero(dn);
+  VF sum0 = hn::ReorderWidenMulAccumulate(dn, a1, b1, c, sum1);
+  sum0 = hn::ReorderWidenMulAccumulate(dn, a2, b2, sum0, sum1);
+  return hn::RearrangeToOddPlusEven(sum0, sum1);
+#endif
+}
+
 // NOLINTNEXTLINE(google-readability-namespace-comments)
 }  // namespace HWY_NAMESPACE
 }  // namespace gcpp