Convert some Vecs in RDO to ArrayVecs.

These are fixed size and stack allocated rather than malloc()'ed,
hopefully improving speed and reducing memory fragmentation.

Vecs that are passed across functions have not been converted.

Cargo.toml

@@ -39,6 +39,7 @@ scan_fmt = { version = "0.1.3", optional = true }
 ivf = { path = "ivf/", optional = true }
 rayon = "1.0"
 bincode = "=1.0.1"
+arrayvec = "0.4.10"
 
 [target.'cfg(target_arch = "x86_64")'.build-dependencies]
 nasm-rs = { git = "https://github.com/tdaede/nasm-rs", branch="msvc-OUT", optional = true }

src/rdo.rs

@@ -39,6 +39,7 @@ use std;
 use std::cmp;
 use std::vec::Vec;
 use crate::partition::PartitionType::*;
+use arrayvec::*;
 
 #[derive(Copy,Clone,PartialEq)]
 pub enum RDOType {
@@ -461,9 +462,12 @@ pub fn rdo_mode_decision<T: Pixel>(
 
   let cw_checkpoint = cw.checkpoint();
 
-  let mut ref_frames_set = Vec::new();
-  let mut ref_slot_set = Vec::new();
-  let mut mvs_from_me = Vec::new();
+  // we can never have more than 7 reference frame sets
+  let mut ref_frames_set = ArrayVec::<[_; 7]>::new();
+  // again, max of 7 ref slots
+  let mut ref_slot_set = ArrayVec::<[_; 7]>::new();
+  // our implementation never returns more than 3 at the moment
+  let mut mvs_from_me = ArrayVec::<[_; 3]>::new();
   let mut fwdref = None;
   let mut bwdref = None;
 
@@ -496,9 +500,9 @@ pub fn rdo_mode_decision<T: Pixel>(
     assert!(!ref_frames_set.is_empty());
   }
 
-  let mut mode_set: Vec<(PredictionMode, usize)> = Vec::new();
-  let mut mv_stacks = Vec::new();
-  let mut mode_contexts = Vec::new();
+  let mut mode_set = ArrayVec::<[(PredictionMode, usize); 20]>::new();
+  let mut mv_stacks = ArrayVec::<[_; 20]>::new();
+  let mut mode_contexts = ArrayVec::<[_; 7]>::new();
 
   let motion_estimation = if fi.config.speed_settings.diamond_me {
     crate::me::DiamondSearch::motion_estimation
@@ -777,7 +781,7 @@ pub fn rdo_mode_decision<T: Pixel>(
     let mut probs = intra_mode_set.iter().map(|&a| (a, probs_all[a as usize])).collect::<Vec<_>>();
     probs.sort_by_key(|a| !a.1);
 
-    let mut modes = Vec::new();
+    let mut modes = ArrayVec::<[_;7]>::new();
     probs
       .iter()
       .take(num_modes_rdo / 2)