Implement IaddPairwise for the interpreter

Implemented `IaddPairwise` for the Cranelift interpreter, to add pairs
of adjacent values in two SIMD vectors, concatenating them at the end
(preserving both lane size and number of lanes).

Copyright (c) 2021, Arm Limited

cranelift/interpreter/src/step.rs

@@ -709,7 +709,7 @@ where
         Opcode::Fence => unimplemented!("Fence"),
         Opcode::WideningPairwiseDotProductS => unimplemented!("WideningPairwiseDotProductS"),
         Opcode::SqmulRoundSat => unimplemented!("SqmulRoundSat"),
-        Opcode::IaddPairwise => unimplemented!("IaddPairwise"),
+        Opcode::IaddPairwise => assign(binary_pairwise(arg(0)?, arg(1)?, ctrl_ty, Value::add)?),
 
         // TODO: these instructions should be removed once the new backend makes these obsolete
         // (see https://github.com/bytecodealliance/wasmtime/issues/1936); additionally, the
@@ -962,3 +962,31 @@ where
     }
     vectorizelanes(&result, vector_type)
 }
+
+/// Performs the supplied pairwise arithmetic `op` on two SIMD vectors, where
+/// pairs are formed from adjacent vector elements and the vectors are
+/// concatenated at the end.
+fn binary_pairwise<V, F>(x: V, y: V, vector_type: types::Type, op: F) -> ValueResult<V>
+where
+    V: Value,
+    F: Fn(V, V) -> ValueResult<V>,
+{
+    let arg0 = extractlanes(&x, vector_type.lane_type())?;
+    let arg1 = extractlanes(&y, vector_type.lane_type())?;
+    let mut result = SimdVec::new();
+    for pair in arg0.chunks(2) {
+        let lhs: V = Value::int(pair[0], vector_type.lane_type())?;
+        let rhs: V = Value::int(pair[1], vector_type.lane_type())?;
+        let sum = op(lhs, rhs)?;
+        let sum = sum.into_int()?;
+        result.push(sum);
+    }
+    for pair in arg1.chunks(2) {
+        let lhs: V = Value::int(pair[0], vector_type.lane_type())?;
+        let rhs: V = Value::int(pair[1], vector_type.lane_type())?;
+        let sum = op(lhs, rhs)?;
+        let sum = sum.into_int()?;
+        result.push(sum);
+    }
+    vectorizelanes(&result, vector_type)
+}