From ce51995828d6040c371dd78e187a7a98aeec6266 Mon Sep 17 00:00:00 2001
From: Sergei Shulepov <s.pepyakin@gmail.com>
Date: Thu, 2 Jul 2020 18:24:03 +0200
Subject: [PATCH] Use smallvec for avoid allocations in the trampoline (#1965)

---
 Cargo.lock                  |  1 +
 crates/wasmtime/Cargo.toml  |  1 +
 crates/wasmtime/src/func.rs | 14 +++++++++++---
 3 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index 260306058a..9692147912 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2329,6 +2329,7 @@ dependencies = [
  "log",
  "region",
  "rustc-demangle",
+ "smallvec",
  "target-lexicon",
  "tempfile",
  "wasmparser 0.58.0",
diff --git a/crates/wasmtime/Cargo.toml b/crates/wasmtime/Cargo.toml
index 9973906c70..88c6a822c5 100644
--- a/crates/wasmtime/Cargo.toml
+++ b/crates/wasmtime/Cargo.toml
@@ -25,6 +25,7 @@ rustc-demangle = "0.1.16"
 lazy_static = "1.4"
 log = "0.4.8"
 wat = { version = "1.0.18", optional = true }
+smallvec = "1.4.0"
 
 [target.'cfg(target_os = "windows")'.dependencies]
 winapi = "0.3.7"
diff --git a/crates/wasmtime/src/func.rs b/crates/wasmtime/src/func.rs
index 9c2081846f..e9e7833569 100644
--- a/crates/wasmtime/src/func.rs
+++ b/crates/wasmtime/src/func.rs
@@ -2,6 +2,7 @@ use crate::runtime::StoreInner;
 use crate::trampoline::StoreInstanceHandle;
 use crate::{Extern, FuncType, Memory, Store, Trap, Val, ValType};
 use anyhow::{bail, ensure, Context as _, Result};
+use smallvec::{smallvec, SmallVec};
 use std::cmp::max;
 use std::fmt;
 use std::mem;
@@ -247,14 +248,21 @@ impl Func {
             // We have a dynamic guarantee that `values_vec` has the right
             // number of arguments and the right types of arguments. As a result
             // we should be able to safely run through them all and read them.
-            let mut args = Vec::with_capacity(ty_clone.params().len());
+            const STACK_ARGS: usize = 4;
+            const STACK_RETURNS: usize = 2;
+            let mut args: SmallVec<[Val; STACK_ARGS]> =
+                SmallVec::with_capacity(ty_clone.params().len());
             let store = Store::upgrade(&store_weak).unwrap();
             for (i, ty) in ty_clone.params().iter().enumerate() {
                 unsafe {
-                    args.push(Val::read_value_from(&store, values_vec.add(i), ty));
+                    let val = Val::read_value_from(&store, values_vec.add(i), ty);
+                    args.push(val);
                 }
             }
-            let mut returns = vec![Val::null(); ty_clone.results().len()];
+
+            let mut returns: SmallVec<[Val; STACK_RETURNS]> =
+                smallvec![Val::null(); ty_clone.results().len()];
+
             func(
                 Caller {
                     store: &store_weak,