wasmtime-bench-api: Randomize the locations of heap objects

This helps us avoid measurement bias due to accidental locality of unrelated
heap objects. See *Stabilizer: Statistically Sound Performance Evaluation* by
Curtsinger and Berger for details (although Stabilizer deals with much more than
just the location of heap allocations):
https://people.cs.umass.edu/~emery/pubs/stabilizer-asplos13.pdf

Cargo.lock

@@ -1964,6 +1964,18 @@ version = "0.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7fdf1b9db47230893d76faad238fd6097fd6d6a9245cd7a4d90dbd639536bbd2"
 
+[[package]]
+name = "shuffling-allocator"
+version = "1.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "848c0a454373d16ebfaa740c99d4faebe25ea752a35e0c6e341168fe67f987f8"
+dependencies = [
+ "cfg-if 1.0.0",
+ "libc",
+ "rand",
+ "winapi",
+]
+
 [[package]]
 name = "smallvec"
 version = "1.6.1"
@@ -2430,6 +2442,7 @@ name = "wasmtime-bench-api"
 version = "0.19.0"
 dependencies = [
  "anyhow",
+ "shuffling-allocator",
  "wasi-common",
  "wasmtime",
  "wasmtime-wasi",

crates/bench-api/Cargo.toml

@@ -16,10 +16,13 @@ crate-type = ["rlib", "cdylib"]
 
 [dependencies]
 anyhow = "1.0"
+shuffling-allocator = { version = "1.1.1", optional = true }
 wasmtime = { path = "../wasmtime", default-features = false }
 wasmtime-wasi = { path = "../wasi" }
 wasi-common = { path = "../wasi-common" }
 
-
 [dev-dependencies]
 wat = "1.0"
+
+[features]
+default = ["shuffling-allocator"]

crates/bench-api/src/lib.rs

@@ -83,6 +83,14 @@ pub type ExitCode = c_int;
 pub const OK: ExitCode = 0;
 pub const ERR: ExitCode = -1;
 
+// Randomize the location of heap objects to avoid accidental locality being an
+// uncontrolled variable that obscures performance evaluation in our
+// experiments.
+#[cfg(feature = "shuffling-allocator")]
+#[global_allocator]
+static ALLOC: shuffling_allocator::ShufflingAllocator<std::alloc::System> =
+    shuffling_allocator::wrap!(&std::alloc::System);
+
 /// Exposes a C-compatible way of creating the engine from the bytes of a single
 /// Wasm module.
 ///