Add faster specialization for deserializing vector<float>

Author: pkolaczk

scylla-cql/src/frame/response/result.rs

@@ -7,6 +7,7 @@ use crate::frame::{frame_errors::ParseError, types};
 use crate::types::deserialize::result::{RowIterator, TypedRowIterator};
 use crate::types::deserialize::value::{
     mk_deser_err, BuiltinDeserializationErrorKind, DeserializeValue, MapIterator, UdtIterator,
+    VectorIterator,
 };
 use crate::types::deserialize::{DeserializationError, FrameSlice};
 use bytes::{Buf, Bytes};
@@ -829,9 +830,17 @@ pub fn deser_cql_value(
                 .collect::<StdResult<_, _>>()?;
             CqlValue::Tuple(t)
         }
-        Vector(_type_name, _) => {
-            let l = Vec::<CqlValue>::deserialize(typ, v)?;
-            CqlValue::Vector(l)
+        // Specialization for faster deserialization of vectors of floats, which are currently
+        // the only type of vector
+        Vector(elem_type, _) if matches!(elem_type.as_ref(), Float) => {
+            let v = VectorIterator::<CqlValue>::deserialize_vector_of_float_to_vec_of_cql_value(
+                typ, v,
+            )?;
+            CqlValue::Vector(v)
+        }
+        Vector(_, _) => {
+            let v = Vec::<CqlValue>::deserialize(typ, v)?;
+            CqlValue::Vector(v)
         }
     })
 }

scylla-cql/src/types/deserialize/value.rs

@@ -1,12 +1,7 @@
 //! Provides types for dealing with CQL value deserialization.
 
-use std::{
-    collections::{BTreeMap, BTreeSet, HashMap, HashSet},
-    hash::{BuildHasher, Hash},
-    net::IpAddr,
-};
-
 use bytes::Bytes;
+use std::{collections::{BTreeMap, BTreeSet, HashMap, HashSet}, hash::{BuildHasher, Hash}, mem, net::IpAddr};
 use uuid::Uuid;
 
 use std::fmt::{Display, Pointer};
@@ -759,7 +754,7 @@ pub struct VectorIterator<'frame, T> {
 }
 
 impl<'frame, T> VectorIterator<'frame, T> {
-    fn new(
+    pub fn new(
         coll_typ: &'frame ColumnType,
         elem_typ: &'frame ColumnType,
         count: usize,
@@ -774,6 +769,57 @@ impl<'frame, T> VectorIterator<'frame, T> {
             phantom_data: std::marker::PhantomData,
         }
     }
+
+    /// Faster specialization for deserializing a `vector<float>` into `Vec<CqlValue>`.  
+    /// The generic code `Vec<CqlValue>::deserialize(...)` is much slower because it has to
+    /// match on the element type for every item in the vector.
+    /// Here we just hardcode `f32` and we can shortcut a lot of code.
+    ///
+    /// This could be nicer if Rust had generic type specialization in stable,
+    /// but for now we need a separate method.    
+    pub fn deserialize_vector_of_float_to_vec_of_cql_value(
+        typ: &'frame ColumnType,
+        v: Option<FrameSlice<'frame>>,
+    ) -> Result<Vec<CqlValue>, DeserializationError> {
+
+        // Typecheck would make sure those never happen:
+        let ColumnType::Vector(elem_type, elem_count) = typ else {
+            panic!("Wrong column type: {:?}. Expected vector<>", typ);
+        };
+        if !matches!(elem_type.as_ref(), ColumnType::Float) {
+            panic!("Wrong element type: {:?}. Expected float", typ);
+        }
+
+        let elem_count = *elem_count as usize;
+        let mut frame = v.map(|s| s.as_slice()).unwrap_or_default();
+        let mut result = Vec::with_capacity(elem_count);
+
+        unsafe {
+            type Float = f32;
+            
+            // Check length only once
+            if frame.len() < size_of::<Float>() * elem_count {
+                return Err(mk_deser_err::<Vec<CqlValue>>(
+                    typ,
+                    BuiltinDeserializationErrorKind::RawCqlBytesReadError(
+                        LowLevelDeserializationError::TooFewBytesReceived {
+                            expected: size_of::<Float>() * elem_count,
+                            received: frame.len(),
+                        },
+                    ),
+                ));
+            }
+            // We know we have enough elements in the buffer, so now we can skip the checks
+            for _ in 0..elem_count {
+                // we did check for frame length earlier, so we can safely not check again
+                let (elem, remaining) = frame.split_first_chunk().unwrap_unchecked();
+                let elem = Float::from_be_bytes(*elem);
+                result.push(CqlValue::Float(elem));
+                frame = remaining;
+            }
+        }
+        Ok(result)
+    }
 }
 
 impl<'frame, T> DeserializeValue<'frame> for VectorIterator<'frame, T>
@@ -828,6 +874,7 @@ where
 {
     type Item = Result<T, DeserializationError>;
 
+    #[inline]
     fn next(&mut self) -> Option<Self::Item> {
         let raw = self.raw_iter.next()?.map_err(|err| {
             mk_deser_err::<Self>(
@@ -883,6 +930,7 @@ where
                     .and_then(|it| it.collect::<Result<_, DeserializationError>>())
                     .map_err(deser_error_replace_rust_name::<Self>)
             }
+
             ColumnType::Vector(_, _) => VectorIterator::<'frame, T>::deserialize(typ, v)
                 .and_then(|it| it.collect::<Result<_, DeserializationError>>())
                 .map_err(deser_error_replace_rust_name::<Self>),
@@ -1435,6 +1483,7 @@ impl<'frame> VectorBytesSequenceIterator<'frame> {
 impl<'frame> Iterator for VectorBytesSequenceIterator<'frame> {
     type Item = Result<Option<FrameSlice<'frame>>, LowLevelDeserializationError>;
 
+    #[inline]
     fn next(&mut self) -> Option<Self::Item> {
         self.remaining = self.remaining.checked_sub(1)?;
         Some(self.slice.read_subslice(self.elem_len))