Update wasmparser requirement from 0.205 to 0.206 (#103)

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.github/CODEOWNERS

@@ -1,4 +1,4 @@
 # For details about syntax, see:
 # https://help.github.com/en/articles/about-code-owners
 
-/ @athei @pepyakin
+* @athei @pepyakin

.github/workflows/check.yml

@@ -11,7 +11,7 @@ env:
 
 jobs:
   rustfmt:
-    runs-on: "ubuntu-latest"
+    runs-on: "ubuntu_20_64_core"
     steps:
       - name: Install Rust toolchain
         uses: actions-rs/toolchain@v1
@@ -20,9 +20,9 @@ jobs:
           toolchain: nightly
           components: rustfmt
 
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
 
-      - name: Cargo fmt
+      - name: Fmt
         uses: actions-rs/cargo@v1
         with:
           toolchain: nightly
@@ -30,7 +30,7 @@ jobs:
           args: --all -- --check
 
   clippy:
-    runs-on: "ubuntu-latest"
+    runs-on: "ubuntu_20_64_core"
     steps:
       - name: Install Rust toolchain
         uses: actions-rs/toolchain@v1
@@ -38,60 +38,75 @@ jobs:
           profile: minimal
           toolchain: stable
           components: clippy
+          default: true
 
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
 
-      - name: Cargo clippy
-        uses: actions-rs/cargo@v1
+      - name: Clippy
+        uses: actions-rs/clippy-check@v1
         with:
-          toolchain: stable
-          command: clippy
+          token: ${{ secrets.GITHUB_TOKEN }}
           args: --all-targets --all-features -- -D warnings
 
-  test:
-    strategy:
-      matrix:
-        os: ["ubuntu-latest", "macos-latest", "windows-latest"]
-        toolchain: ["stable", "nightly"]
-    runs-on: ${{ matrix.os }}
+  build:
+    runs-on: "ubuntu_20_64_core"
     steps:
       - name: Install Rust toolchain
         uses: actions-rs/toolchain@v1
         with:
           profile: minimal
           target: wasm32-unknown-unknown
-          toolchain: ${{ matrix.toolchain }}
+          toolchain: stable
+          default: true
 
-      - name: Set git to use LF
-        run: |
-          git config --global core.autocrlf false
-          git config --global core.eol lf
-
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
 
       - name: Cargo build
         uses: actions-rs/cargo@v1
         with:
-          toolchain: ${{ matrix.toolchain }}
           command: build
 
+      - name: Cargo build (std)
+        uses: actions-rs/cargo@v1
+        with:
+          command: build
+          args: --all-features
+
       - name: Cargo build (no_std)
         uses: actions-rs/cargo@v1
         with:
-          toolchain: ${{ matrix.toolchain }}
           command: build
           args: --no-default-features
 
       - name: Cargo build (wasm)
         uses: actions-rs/cargo@v1
         with:
-          toolchain: ${{ matrix.toolchain }}
           command: build
           args: --no-default-features --target wasm32-unknown-unknown
 
+  test:
+    strategy:
+      matrix:
+        os: ["ubuntu_20_64_core", "macos-latest", "windows-latest"]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - name: Install Rust toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          default: true
+
+      - name: Set git to use LF
+        run: |
+          git config --global core.autocrlf false
+          git config --global core.eol lf
+
+      - uses: actions/checkout@v4
+
       - name: Cargo test
         uses: actions-rs/cargo@v1
         with:
-          toolchain: ${{ matrix.toolchain }}
           command: test
           args: --all-features
+

.gitignore

@@ -4,3 +4,4 @@ target
 .DS_Store
 .idea
 .vscode
+*~

BENCHMARKS.md

@@ -0,0 +1,91 @@
+# Benchmarks
+
+## Table of Contents
+
+- [Benchmark Results](#benchmark-results)
+    - [coremark, instrumented](#coremark,-instrumented)
+    - [recursive_ok, instrumented](#recursive_ok,-instrumented)
+    - [fibonacci_recursive, instrumented](#fibonacci_recursive,-instrumented)
+    - [factorial_recursive, instrumented](#factorial_recursive,-instrumented)
+    - [count_until, instrumented](#count_until,-instrumented)
+    - [memory_vec_add, instrumented](#memory_vec_add,-instrumented)
+    - [wasm_kernel::tiny_keccak, instrumented](#wasm_kernel::tiny_keccak,-instrumented)
+    - [global_bump, instrumented](#global_bump,-instrumented)
+
+## Instrumented Modules sizes
+
+| fixture                      |  original size   | gas metered/host fn | gas metered/mut global | size diff |
+|------------------------------|------------------|---------------------|------------------------|-----------|
+| recursive_ok.wat             |             0 kb |         0 kb (137%) |            0 kb (177%) |      +29% |
+| count_until.wat              |             0 kb |         0 kb (125%) |            0 kb (153%) |      +21% |
+| global_bump.wat              |             0 kb |         0 kb (123%) |            0 kb (145%) |      +18% |
+| memory-vec-add.wat           |             0 kb |         0 kb (116%) |            0 kb (134%) |      +15% |
+| factorial.wat                |             0 kb |         0 kb (125%) |            0 kb (145%) |      +15% |
+| fibonacci.wat                |             0 kb |         0 kb (121%) |            0 kb (134%) |      +10% |
+| contract_terminate.wasm      |             1 kb |         1 kb (110%) |            1 kb (112%) |       +2% |
+| coremark_minimal.wasm        |             7 kb |         8 kb (114%) |            8 kb (115%) |       +0% |
+| trait_erc20.wasm             |            10 kb |        11 kb (108%) |           11 kb (108%) |       +0% |
+| rand_extension.wasm          |             4 kb |         5 kb (109%) |            5 kb (109%) |       +0% |
+| multisig.wasm                |            27 kb |        30 kb (110%) |           30 kb (110%) |       +0% |
+| wasm_kernel.wasm             |           779 kb |       787 kb (100%) |          795 kb (101%) |       +0% |
+| many_blocks.wasm             |          1023 kb |      2389 kb (233%) |         2389 kb (233%) |       +0% |
+| contract_transfer.wasm       |             7 kb |         8 kb (113%) |            8 kb (113%) |       +0% |
+| erc1155.wasm                 |            26 kb |        29 kb (111%) |           29 kb (111%) |       +0% |
+| erc20.wasm                   |             9 kb |        10 kb (108%) |           10 kb (109%) |       +0% |
+| dns.wasm                     |            10 kb |        11 kb (108%) |           11 kb (108%) |       +0% |
+| proxy.wasm                   |             3 kb |         4 kb (108%) |            4 kb (109%) |       +0% |
+| erc721.wasm                  |            13 kb |        14 kb (108%) |           14 kb (108%) |       +0% |
+
+## Benchmark Results
+
+### coremark, instrumented
+
+|        | `with host_function::Injector`          | `with mutable_global::Injector`           |
+|:-------|:----------------------------------------|:----------------------------------------- |
+|        | `20.81 s` (✅ **1.00x**)                 | `20.20 s` (✅ **1.03x faster**)            |
+
+### recursive_ok, instrumented
+
+|        | `with host_function::Injector`          | `with mutable_global::Injector`           |
+|:-------|:----------------------------------------|:----------------------------------------- |
+|        | `367.11 us` (✅ **1.00x**)               | `585.39 us` (❌ *1.59x slower*)            |
+
+### fibonacci_recursive, instrumented
+
+|        | `with host_function::Injector`          | `with mutable_global::Injector`           |
+|:-------|:----------------------------------------|:----------------------------------------- |
+|        | `9.15 us` (✅ **1.00x**)                 | `13.56 us` (❌ *1.48x slower*)             |
+
+### factorial_recursive, instrumented
+
+|        | `with host_function::Injector`          | `with mutable_global::Injector`           |
+|:-------|:----------------------------------------|:----------------------------------------- |
+|        | `1.50 us` (✅ **1.00x**)                 | `1.98 us` (❌ *1.32x slower*)              |
+
+### count_until, instrumented
+
+|        | `with host_function::Injector`          | `with mutable_global::Injector`           |
+|:-------|:----------------------------------------|:----------------------------------------- |
+|        | `5.03 ms` (✅ **1.00x**)                 | `8.13 ms` (❌ *1.62x slower*)              |
+
+### memory_vec_add, instrumented
+
+|        | `with host_function::Injector`          | `with mutable_global::Injector`           |
+|:-------|:----------------------------------------|:----------------------------------------- |
+|        | `6.21 ms` (✅ **1.00x**)                 | `8.45 ms` (❌ *1.36x slower*)              |
+
+### wasm_kernel::tiny_keccak, instrumented
+
+|        | `with host_function::Injector`          | `with mutable_global::Injector`           |
+|:-------|:----------------------------------------|:----------------------------------------- |
+|        | `925.22 us` (✅ **1.00x**)               | `1.08 ms` (❌ *1.17x slower*)              |
+
+### global_bump, instrumented
+
+|        | `with host_function::Injector`          | `with mutable_global::Injector`           |
+|:-------|:----------------------------------------|:----------------------------------------- |
+|        | `3.79 ms` (✅ **1.00x**)                 | `7.03 ms` (❌ *1.86x slower*)              |
+
+---
+Made with [criterion-table](https://github.com/nu11ptr/criterion-table)
+

CHANGELOG.md

@@ -16,7 +16,26 @@ The interface provided to smart contracts will adhere to semver with one excepti
 major version bumps will be backwards compatible with regard to already deployed contracts.
 In other words: Upgrading this pallet will not break pre-existing contracts.
 
-## [v0.1.2] 2022-06-06
+## [Unreleased]
+
+### New
+
+- Add new gas metering method: mutable global + local gas function
+[#34](https://github.com/paritytech/wasm-instrument/pull/34)
+- Account for locals initialization costs
+[#38](https://github.com/paritytech/wasm-instrument/pull/38)
+
+## [v0.3.0]
+
+### Changed
+
+- Use 64bit arithmetic for per-block gas counter
+[#30](https://github.com/paritytech/wasm-instrument/pull/30)
+
+## [v0.2.0] 2022-06-06
+
+### Changed
+
 - Adjust debug information (if already parsed) when injecting gas metering
 [#16](https://github.com/paritytech/wasm-instrument/pull/16)
 

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "wasm-instrument"
-version = "0.1.2"
+version = "0.4.0"
 edition = "2021"
 rust-version = "1.56.1"
 authors = ["Parity Technologies <admin@parity.io>"]
@@ -12,8 +12,14 @@ repository = "https://github.com/paritytech/wasm-instrument"
 include = ["src/**/*", "LICENSE-*", "README.md"]
 
 [[bench]]
-name = "benches"
+name = "instrumentation"
 harness = false
+path = "benches/instrumentation.rs"
+
+[[bench]]
+name = "execution"
+harness = false
+path = "benches/execution.rs"
 
 [profile.bench]
 lto = "fat"
@@ -24,14 +30,19 @@ parity-wasm = { version = "0.45", default-features = false }
 
 [dev-dependencies]
 binaryen = "0.12"
-criterion = "0.3"
+criterion = "0.5"
 diff = "0.1"
+pretty_assertions = "1"
 rand = "0.8"
 wat = "1"
-wasmparser = "0.84"
-wasmprinter = "0.2"
+wasmparser = "0.206"
+wasmprinter = "0.200"
+wasmi = "0.31"
 
 [features]
 default = ["std"]
 std = ["parity-wasm/std"]
 sign_ext = ["parity-wasm/sign_ext"]
+
+[lib]
+bench = false

benches/execution.rs

@@ -0,0 +1,437 @@
+use criterion::{
+	criterion_group, criterion_main, measurement::Measurement, Bencher, BenchmarkGroup, Criterion,
+};
+use std::{
+	fs::read,
+	path::PathBuf,
+	time::{Duration, SystemTime, UNIX_EPOCH},
+};
+use wasm_instrument::{
+	gas_metering::{self, host_function, mutable_global, ConstantCostRules},
+	parity_wasm::{deserialize_buffer, elements::Module, serialize},
+};
+use wasmi::{
+	core::{Pages, TrapCode, F32},
+	Caller, Config, Engine, Instance, Linker, Memory, StackLimits, Store, TypedFunc, Value,
+};
+
+/// Describes a gas metering strategy we want to benchmark.
+///
+/// Most strategies just need a subset of these functions. Hence we added default
+/// implementations for all of them.
+trait MeteringStrategy {
+	/// The wasmi config we should be using for this strategy.
+	fn config() -> Config {
+		Config::default()
+	}
+
+	/// The strategy may or may not want to instrument the module.
+	fn instrument_module(module: Module) -> Module {
+		module
+	}
+
+	/// The strategy might need to define additional host functions.
+	fn define_host_funcs(_linker: &mut Linker<u64>) {}
+
+	/// The strategy might need to do some initialization of the wasm instance.
+	fn init_instance(_module: &mut BenchInstance) {}
+}
+
+/// Don't do any metering at all. This is helpful as a baseline.
+struct NoMetering;
+
+/// Use wasmi's builtin fuel metering.
+struct WasmiMetering;
+
+/// Instrument the module using [`host_function::Injector`].
+struct HostFunctionMetering;
+
+/// Instrument the module using [`mutable_global::Injector`].
+struct MutableGlobalMetering;
+
+impl MeteringStrategy for NoMetering {}
+
+impl MeteringStrategy for WasmiMetering {
+	fn config() -> Config {
+		let mut config = Config::default();
+		config.consume_fuel(true);
+		config
+	}
+
+	fn init_instance(module: &mut BenchInstance) {
+		module.store.add_fuel(u64::MAX).unwrap();
+	}
+}
+
+impl MeteringStrategy for HostFunctionMetering {
+	fn instrument_module(module: Module) -> Module {
+		let backend = host_function::Injector::new("env", "gas");
+		gas_metering::inject(module, backend, &ConstantCostRules::default()).unwrap()
+	}
+
+	fn define_host_funcs(linker: &mut Linker<u64>) {
+		// the instrumentation relies on the existing of this function
+		linker
+			.func_wrap("env", "gas", |mut caller: Caller<'_, u64>, amount_consumed: u64| {
+				let gas_remaining = caller.data_mut();
+				*gas_remaining =
+					gas_remaining.checked_sub(amount_consumed).ok_or(TrapCode::OutOfFuel)?;
+				Ok(())
+			})
+			.unwrap();
+	}
+}
+
+impl MeteringStrategy for MutableGlobalMetering {
+	fn instrument_module(module: Module) -> Module {
+		let backend = mutable_global::Injector::new("gas_left");
+		gas_metering::inject(module, backend, &ConstantCostRules::default()).unwrap()
+	}
+
+	fn init_instance(module: &mut BenchInstance) {
+		// the instrumentation relies on the host to initialize the global with the gas limit
+		// we just init to the maximum so it will never run out
+		module
+			.instance
+			.get_global(&mut module.store, "gas_left")
+			.unwrap()
+			.set(&mut module.store, Value::I64(-1i64)) // the same as u64::MAX
+			.unwrap();
+	}
+}
+
+/// A wasm instance ready to be benchmarked.
+struct BenchInstance {
+	store: Store<u64>,
+	instance: Instance,
+}
+
+impl BenchInstance {
+	/// Create a new instance for the supplied metering strategy.
+	///
+	/// `wasm`: The raw wasm module for the benchmark.
+	/// `define_host_func`: In here the caller can define additional host function.
+	fn new<S, H>(wasm: &[u8], define_host_funcs: &H) -> Self
+	where
+		S: MeteringStrategy,
+		H: Fn(&mut Linker<u64>),
+	{
+		let module = deserialize_buffer(wasm).unwrap();
+		let instrumented_module = S::instrument_module(module);
+		let input = serialize(instrumented_module).unwrap();
+		let mut config = S::config();
+		config.set_stack_limits(StackLimits::new(1024, 1024 * 1024, 64 * 1024).unwrap());
+		let engine = Engine::new(&config);
+		let module = wasmi::Module::new(&engine, &mut &input[..]).unwrap();
+		let mut linker = Linker::new(&engine);
+		S::define_host_funcs(&mut linker);
+		define_host_funcs(&mut linker);
+		// init host state with maximum gas_left (only used by host_function instrumentation)
+		let mut store = Store::new(&engine, u64::MAX);
+		let instance = linker.instantiate(&mut store, &module).unwrap().start(&mut store).unwrap();
+		let mut bench_module = Self { store, instance };
+		S::init_instance(&mut bench_module);
+		bench_module
+	}
+}
+
+/// Runs a benchmark for every strategy.
+///
+/// We require the closures to implement `Fn` as they are executed for every strategy and we
+/// don't want them to change in between.
+///
+/// `group`: The benchmark group within the benchmarks will be executed.
+/// `wasm`: The raw wasm module for the benchmark.
+/// `define_host_func`: In here the caller can define additional host function.
+/// `f`: In here the user should perform the benchmark. Will be executed for every strategy.
+fn for_strategies<M, H, F>(group: &mut BenchmarkGroup<M>, wasm: &[u8], define_host_funcs: H, f: F)
+where
+	M: Measurement,
+	H: Fn(&mut Linker<u64>),
+	F: Fn(&mut Bencher<M>, &mut BenchInstance),
+{
+	let mut module = BenchInstance::new::<NoMetering, _>(wasm, &define_host_funcs);
+	group.bench_function("no_metering", |bench| f(bench, &mut module));
+
+	let mut module = BenchInstance::new::<WasmiMetering, _>(wasm, &define_host_funcs);
+	group.bench_function("wasmi_builtin", |bench| f(bench, &mut module));
+
+	let mut module = BenchInstance::new::<HostFunctionMetering, _>(wasm, &define_host_funcs);
+	group.bench_function("host_function", |bench| f(bench, &mut module));
+
+	let mut module = BenchInstance::new::<MutableGlobalMetering, _>(wasm, &define_host_funcs);
+	group.bench_function("mutable_global", |bench| f(bench, &mut module));
+}
+
+/// Converts the `.wat` encoded `bytes` into `.wasm` encoded bytes.
+fn wat2wasm(bytes: &[u8]) -> Vec<u8> {
+	wat::parse_bytes(bytes).unwrap().into_owned()
+}
+
+fn fixture_dir() -> PathBuf {
+	let mut path = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
+	path.push("benches");
+	path.push("fixtures");
+	path.push("wasm");
+	path
+}
+
+fn gas_metered_coremark(c: &mut Criterion) {
+	let mut group = c.benchmark_group("coremark");
+	// Benchmark host_function::Injector
+	let wasm_filename = "coremark_minimal.wasm";
+	let bytes = read(fixture_dir().join(wasm_filename)).unwrap();
+	let define_host_funcs = |linker: &mut Linker<u64>| {
+		linker
+			.func_wrap("env", "clock_ms", || {
+				SystemTime::now().duration_since(UNIX_EPOCH).unwrap().as_millis() as u64
+			})
+			.unwrap();
+	};
+	for_strategies(&mut group, &bytes, define_host_funcs, |bench, module| {
+		bench.iter(|| {
+			let run = module.instance.get_typed_func::<(), F32>(&mut module.store, "run").unwrap();
+			// Call the wasm!
+			run.call(&mut module.store, ()).unwrap();
+		})
+	});
+}
+
+fn gas_metered_recursive_ok(c: &mut Criterion) {
+	let mut group = c.benchmark_group("recursive_ok");
+	const RECURSIVE_DEPTH: i32 = 8000;
+	let wasm_bytes = wat2wasm(include_bytes!("fixtures/wat/recursive_ok.wat"));
+	for_strategies(
+		&mut group,
+		&wasm_bytes,
+		|_| {},
+		|bench, module| {
+			let bench_call =
+				module.instance.get_typed_func::<i32, i32>(&module.store, "call").unwrap();
+			bench.iter(|| {
+				let result = bench_call.call(&mut module.store, RECURSIVE_DEPTH).unwrap();
+				assert_eq!(result, 0);
+			})
+		},
+	);
+}
+
+fn gas_metered_fibonacci_recursive(c: &mut Criterion) {
+	let mut group = c.benchmark_group("fibonacci_recursive");
+	const FIBONACCI_REC_N: i64 = 10;
+	let wasm_bytes = wat2wasm(include_bytes!("fixtures/wat/fibonacci.wat"));
+	for_strategies(
+		&mut group,
+		&wasm_bytes,
+		|_| {},
+		|bench, module| {
+			let bench_call = module
+				.instance
+				.get_typed_func::<i64, i64>(&module.store, "fib_recursive")
+				.unwrap();
+			bench.iter(|| {
+				bench_call.call(&mut module.store, FIBONACCI_REC_N).unwrap();
+			});
+		},
+	);
+}
+
+fn gas_metered_fac_recursive(c: &mut Criterion) {
+	let mut group = c.benchmark_group("factorial_recursive");
+	let wasm_bytes = wat2wasm(include_bytes!("fixtures/wat/factorial.wat"));
+	for_strategies(
+		&mut group,
+		&wasm_bytes,
+		|_| {},
+		|bench, module| {
+			let fac = module
+				.instance
+				.get_typed_func::<i64, i64>(&module.store, "recursive_factorial")
+				.unwrap();
+			bench.iter(|| {
+				let result = fac.call(&mut module.store, 25).unwrap();
+				assert_eq!(result, 7034535277573963776);
+			})
+		},
+	);
+}
+
+fn gas_metered_count_until(c: &mut Criterion) {
+	const COUNT_UNTIL: i32 = 100_000;
+	let mut group = c.benchmark_group("count_until");
+	let wasm_bytes = wat2wasm(include_bytes!("fixtures/wat/count_until.wat"));
+	for_strategies(
+		&mut group,
+		&wasm_bytes,
+		|_| {},
+		|bench, module| {
+			let count_until = module
+				.instance
+				.get_typed_func::<i32, i32>(&module.store, "count_until")
+				.unwrap();
+			bench.iter(|| {
+				let result = count_until.call(&mut module.store, COUNT_UNTIL).unwrap();
+				assert_eq!(result, COUNT_UNTIL);
+			})
+		},
+	);
+}
+
+fn gas_metered_vec_add(c: &mut Criterion) {
+	fn test_for<A, B>(
+		b: &mut Bencher,
+		vec_add: TypedFunc<(i32, i32, i32, i32), ()>,
+		mut store: &mut Store<u64>,
+		mem: Memory,
+		len: usize,
+		vec_a: A,
+		vec_b: B,
+	) where
+		A: IntoIterator<Item = i32>,
+		B: IntoIterator<Item = i32>,
+	{
+		use core::mem::size_of;
+
+		let ptr_result = 10;
+		let len_result = len * size_of::<i64>();
+		let ptr_a = ptr_result + len_result;
+		let len_a = len * size_of::<i32>();
+		let ptr_b = ptr_a + len_a;
+
+		// Reset `result` buffer to zeros:
+		mem.data_mut(&mut store)[ptr_result..ptr_result + (len * size_of::<i32>())].fill(0);
+		// Initialize `a` buffer:
+		for (n, a) in vec_a.into_iter().take(len).enumerate() {
+			mem.write(&mut store, ptr_a + (n * size_of::<i32>()), &a.to_le_bytes()).unwrap();
+		}
+		// Initialize `b` buffer:
+		for (n, b) in vec_b.into_iter().take(len).enumerate() {
+			mem.write(&mut store, ptr_b + (n * size_of::<i32>()), &b.to_le_bytes()).unwrap();
+		}
+
+		// Prepare parameters and all Wasm `vec_add`:
+		let params = (ptr_result as i32, ptr_a as i32, ptr_b as i32, len as i32);
+		b.iter(|| {
+			vec_add.call(&mut store, params).unwrap();
+		});
+
+		// Validate the result buffer:
+		for n in 0..len {
+			let mut buffer4 = [0x00; 4];
+			let mut buffer8 = [0x00; 8];
+			let a = {
+				mem.read(&store, ptr_a + (n * size_of::<i32>()), &mut buffer4).unwrap();
+				i32::from_le_bytes(buffer4)
+			};
+			let b = {
+				mem.read(&store, ptr_b + (n * size_of::<i32>()), &mut buffer4).unwrap();
+				i32::from_le_bytes(buffer4)
+			};
+			let actual_result = {
+				mem.read(&store, ptr_result + (n * size_of::<i64>()), &mut buffer8).unwrap();
+				i64::from_le_bytes(buffer8)
+			};
+			let expected_result = (a as i64) + (b as i64);
+			assert_eq!(
+				expected_result, actual_result,
+				"given a = {a} and b = {b}, results diverge at index {n}"
+			);
+		}
+	}
+
+	let mut group = c.benchmark_group("memory_vec_add");
+	let wasm_bytes = wat2wasm(include_bytes!("fixtures/wat/memory-vec-add.wat"));
+	const LEN: usize = 100_000;
+
+	for_strategies(
+		&mut group,
+		&wasm_bytes,
+		|_| {},
+		|bench, module| {
+			let vec_add = module
+				.instance
+				.get_typed_func::<(i32, i32, i32, i32), ()>(&module.store, "vec_add")
+				.unwrap();
+			let mem = module.instance.get_memory(&module.store, "mem").unwrap();
+			mem.grow(&mut module.store, Pages::new(25).unwrap()).unwrap();
+			test_for(
+				bench,
+				vec_add,
+				&mut module.store,
+				mem,
+				LEN,
+				(0..LEN).map(|i| (i * i) as i32),
+				(0..LEN).map(|i| (i * 10) as i32),
+			)
+		},
+	);
+}
+
+fn gas_metered_tiny_keccak(c: &mut Criterion) {
+	let mut group = c.benchmark_group("wasm_kernel::tiny_keccak");
+	let wasm_filename = "wasm_kernel.wasm";
+	let wasm_bytes = read(fixture_dir().join(wasm_filename)).unwrap();
+	for_strategies(
+		&mut group,
+		&wasm_bytes,
+		|_| {},
+		|bench, module| {
+			let prepare = module
+				.instance
+				.get_typed_func::<(), i32>(&module.store, "prepare_tiny_keccak")
+				.unwrap();
+			let keccak = module
+				.instance
+				.get_typed_func::<i32, ()>(&module.store, "bench_tiny_keccak")
+				.unwrap();
+			let test_data_ptr = prepare.call(&mut module.store, ()).unwrap();
+			bench.iter(|| {
+				keccak.call(&mut module.store, test_data_ptr).unwrap();
+			})
+		},
+	);
+}
+
+fn gas_metered_global_bump(c: &mut Criterion) {
+	const BUMP_AMOUNT: i32 = 100_000;
+	let mut group = c.benchmark_group("global_bump");
+	let wasm_bytes = wat2wasm(include_bytes!("fixtures/wat/global_bump.wat"));
+	for_strategies(
+		&mut group,
+		&wasm_bytes,
+		|_| {},
+		|bench, module| {
+			let bump = module.instance.get_typed_func::<i32, i32>(&module.store, "bump").unwrap();
+			bench.iter(|| {
+				let result = bump.call(&mut module.store, BUMP_AMOUNT).unwrap();
+				assert_eq!(result, BUMP_AMOUNT);
+			})
+		},
+	);
+}
+
+criterion_group!(
+	name = coremark;
+	config = Criterion::default()
+		.sample_size(10)
+		.measurement_time(Duration::from_millis(275000))
+		.warm_up_time(Duration::from_millis(1000));
+	targets =
+		 gas_metered_coremark,
+);
+criterion_group!(
+	name = wasmi_fixtures;
+	config = Criterion::default()
+		.sample_size(10)
+		.measurement_time(Duration::from_millis(250000))
+	.warm_up_time(Duration::from_millis(1000));
+	targets =
+		gas_metered_recursive_ok,
+		gas_metered_fibonacci_recursive,
+		gas_metered_fac_recursive,
+		gas_metered_count_until,
+		gas_metered_vec_add,
+		gas_metered_tiny_keccak,
+		gas_metered_global_bump,
+);
+criterion_main!(coremark, wasmi_fixtures);

benches/fixtures/wat/count_until.wat

@@ -0,0 +1,25 @@
+;; Exports a function `count_until` that takes an input `n`.
+;; The exported function counts an integer `n` times and then returns `n`.
+(module
+  (func $count_until (export "count_until") (param $limit i32) (result i32)
+    (local $counter i32)
+    (block
+        (loop
+            (br_if
+                1
+                (i32.eq
+                    (local.tee $counter
+                        (i32.add
+                            (local.get $counter)
+                            (i32.const 1)
+                        )
+                    )
+                    (local.get $limit)
+                )
+            )
+            (br 0)
+        )
+    )
+    (return (local.get $counter))
+  )
+)

benches/fixtures/wat/factorial.wat

@@ -0,0 +1,35 @@
+(module
+    ;; Iterative factorial function, does not use recursion.
+    (func (export "iterative_factorial") (param i64) (result i64)
+        (local i64)
+        (local.set 1 (i64.const 1))
+        (block
+            (br_if 0 (i64.lt_s (local.get 0) (i64.const 2)))
+            (loop
+                (local.set 1 (i64.mul (local.get 1) (local.get 0)))
+                (local.set 0 (i64.add (local.get 0) (i64.const -1)))
+                (br_if 0 (i64.gt_s (local.get 0) (i64.const 1)))
+            )
+        )
+        (local.get 1)
+    )
+
+    ;; Recursive trivial factorial function.
+    (func $rec_fac (export "recursive_factorial") (param i64) (result i64)
+        (if (result i64)
+            (i64.eq (local.get 0) (i64.const 0))
+            (then (i64.const 1))
+            (else
+                (i64.mul
+                    (local.get 0)
+                    (call $rec_fac
+                        (i64.sub
+                            (local.get 0)
+                            (i64.const 1)
+                        )
+                    )
+                )
+            )
+        )
+    )
+)

benches/fixtures/wat/fibonacci.wat

@@ -0,0 +1,47 @@
+(module
+    (func $fib_recursive (export "fib_recursive") (param $N i64) (result i64)
+        (if
+            (i64.le_s (local.get $N) (i64.const 1))
+            (then (return (local.get $N)))
+        )
+        (return
+            (i64.add
+                (call $fib_recursive
+                  (i64.sub (local.get $N) (i64.const 1))
+                )
+                (call $fib_recursive
+                  (i64.sub (local.get $N) (i64.const 2))
+                )
+            )
+        )
+    )
+
+    (func $fib_iterative (export "fib_iterative") (param $N i64) (result i64)
+        (local $n1 i64)
+        (local $n2 i64)
+        (local $tmp i64)
+        (local $i i64)
+        ;; return $N for N <= 1
+        (if
+            (i64.le_s (local.get $N) (i64.const 1))
+            (then (return (local.get $N)))
+        )
+        (local.set $n1 (i64.const 1))
+        (local.set $n2 (i64.const 1))
+        (local.set $i (i64.const 2))
+        ;;since we normally return n2, handle n=1 case specially
+        (loop $again
+            (if
+                (i64.lt_s (local.get $i) (local.get $N))
+                (then
+                    (local.set $tmp (i64.add (local.get $n1) (local.get $n2)))
+                    (local.set $n1 (local.get $n2))
+                    (local.set $n2 (local.get $tmp))
+                    (local.set $i (i64.add (local.get $i) (i64.const 1)))
+                    (br $again)
+                )
+            )
+        )
+        (local.get $n2)
+    )
+)

benches/fixtures/wat/global_bump.wat

@@ -0,0 +1,27 @@
+;; Exports a function `bump` that takes an input `n`.
+;; The exported function bumps a global variable `n` times and then returns it.
+(module
+    (global $g (mut i32) (i32.const 0))
+    (func $bump (export "bump") (param $n i32) (result i32)
+        (global.set $g (i32.const 0))
+        (block $break
+            (loop $continue
+                (br_if ;; if $g == $n then break
+                    $break
+                    (i32.eq
+                        (global.get $g)
+                        (local.get $n)
+                    )
+                )
+                (global.set $g ;; $g += 1
+                    (i32.add
+                        (global.get $g)
+                        (i32.const 1)
+                    )
+                )
+                (br $continue)
+            )
+        )
+        (return (global.get $g))
+    )
+)

benches/fixtures/wat/memory-vec-add.wat

@@ -0,0 +1,58 @@
+;; Exports a function `vec_add` that computes the addition of 2 vectors
+;; of length `len` starting at `ptr_a` and `ptr_b` and stores the result
+;; into a buffer of the same length starting at `ptr_result`.
+(module
+    (memory (export "mem") 1)
+    (func (export "vec_add")
+        (param $ptr_result i32)
+        (param $ptr_a i32)
+        (param $ptr_b i32)
+        (param $len i32)
+        (local $n i32)
+        (block $exit
+            (loop $loop
+                (br_if ;; exit loop if $n == $len
+                    $exit
+                    (i32.eq
+                        (local.get $n)
+                        (local.get $len)
+                    )
+                )
+                (i64.store offset=0 ;; ptr_result[n] = ptr_a[n] + ptr_b[n]
+                    (i32.add
+                        (local.get $ptr_result)
+                        (i32.mul
+                            (local.get $n)
+                            (i32.const 8)
+                        )
+                    )
+                    (i64.add
+                        (i64.load32_s offset=0 ;; load ptr_a[n]
+                            (i32.add
+                                (local.get $ptr_a)
+                                (i32.mul
+                                    (local.get $n)
+                                    (i32.const 4)
+                                )
+                            )
+                        )
+                        (i64.load32_s offset=0 ;; load ptr_b[n]
+                            (i32.add
+                                (local.get $ptr_b)
+                                (i32.mul
+                                    (local.get $n)
+                                    (i32.const 4)
+                                )
+                            )
+                        )
+                    )
+                )
+                (local.set $n ;; increment n
+                    (i32.add (local.get $n) (i32.const 1))
+                 )
+                (br $loop) ;; continue loop
+            )
+        )
+        (return)
+    )
+)

benches/fixtures/wat/recursive_ok.wat

@@ -0,0 +1,22 @@
+;; Exports a function `call` that takes an input `n`.
+;; The exported function calls itself `n` times.
+(module
+    (func $call (export "call") (param $n i32) (result i32)
+        (if (result i32)
+            (local.get $n)
+            (then
+                (return
+                    (call $call
+                        (i32.sub
+                            (local.get $n)
+                            (i32.const 1)
+                        )
+                    )
+                )
+            )
+            (else
+                (return (local.get $n))
+            )
+        )
+    )
+)

benches/instrumentation.rs

@@ -7,7 +7,8 @@ use std::{
 	path::PathBuf,
 };
 use wasm_instrument::{
-	gas_metering, inject_stack_limiter,
+	gas_metering::{self, host_function, ConstantCostRules},
+	inject_stack_limiter,
 	parity_wasm::{deserialize_buffer, elements::Module},
 };
 
@@ -15,17 +16,18 @@ fn fixture_dir() -> PathBuf {
 	let mut path = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
 	path.push("benches");
 	path.push("fixtures");
+	path.push("wasm");
 	path
 }
 
-fn any_fixture<F, M>(group: &mut BenchmarkGroup<M>, f: F)
+fn for_fixtures<F, M>(group: &mut BenchmarkGroup<M>, f: F)
 where
 	F: Fn(Module),
 	M: Measurement,
 {
 	for entry in read_dir(fixture_dir()).unwrap() {
 		let entry = entry.unwrap();
-		let bytes = read(&entry.path()).unwrap();
+		let bytes = read(entry.path()).unwrap();
 		group.throughput(Throughput::Bytes(bytes.len().try_into().unwrap()));
 		group.bench_with_input(entry.file_name().to_str().unwrap(), &bytes, |bench, input| {
 			bench.iter(|| f(deserialize_buffer(input).unwrap()))
@@ -35,14 +37,19 @@ where
 
 fn gas_metering(c: &mut Criterion) {
 	let mut group = c.benchmark_group("Gas Metering");
-	any_fixture(&mut group, |module| {
-		gas_metering::inject(module, &gas_metering::ConstantCostRules::default(), "env").unwrap();
+	for_fixtures(&mut group, |module| {
+		gas_metering::inject(
+			module,
+			host_function::Injector::new("env", "gas"),
+			&ConstantCostRules::default(),
+		)
+		.unwrap();
 	});
 }
 
 fn stack_height_limiter(c: &mut Criterion) {
 	let mut group = c.benchmark_group("Stack Height Limiter");
-	any_fixture(&mut group, |module| {
+	for_fixtures(&mut group, |module| {
 		inject_stack_limiter(module, 128).unwrap();
 	});
 }

src/gas_metering/backend.rs

@@ -0,0 +1,139 @@
+//! Provides backends for the gas metering instrumentation
+use parity_wasm::elements;
+
+/// Implementation details of the specific method of the gas metering.
+#[derive(Clone)]
+pub enum GasMeter {
+	/// Gas metering with an external function.
+	External {
+		/// Name of the module to import the gas function from.
+		module: &'static str,
+		/// Name of the external gas function to be imported.
+		function: &'static str,
+	},
+	/// Gas metering with a local function and a mutable global.
+	Internal {
+		/// Name of the mutable global to be exported.
+		global: &'static str,
+		/// Body of the local gas counting function to be injected.
+		func_instructions: elements::Instructions,
+		/// Cost of the gas function execution.
+		cost: u64,
+	},
+}
+
+use super::Rules;
+/// Under the hood part of the gas metering mechanics.
+pub trait Backend {
+	/// Provides the gas metering implementation details.  
+	fn gas_meter<R: Rules>(self, module: &elements::Module, rules: &R) -> GasMeter;
+}
+
+/// Gas metering with an external host function.
+pub mod host_function {
+	use super::{Backend, GasMeter, Rules};
+	use parity_wasm::elements::Module;
+	/// Injects invocations of the gas charging host function into each metering block.
+	pub struct Injector {
+		/// The name of the module to import the gas function from.
+		module: &'static str,
+		/// The name of the gas function to import.
+		name: &'static str,
+	}
+
+	impl Injector {
+		pub fn new(module: &'static str, name: &'static str) -> Self {
+			Self { module, name }
+		}
+	}
+
+	impl Backend for Injector {
+		fn gas_meter<R: Rules>(self, _module: &Module, _rules: &R) -> GasMeter {
+			GasMeter::External { module: self.module, function: self.name }
+		}
+	}
+}
+
+/// Gas metering with a mutable global.
+///
+/// # Note
+///
+/// Not for all execution engines this method gives performance wins compared to using an [external
+/// host function](host_function). See benchmarks and size overhead tests for examples of how to
+/// make measurements needed to decide which gas metering method is better for your particular case.
+///
+/// # Warning
+///
+/// It is not recommended to apply [stack limiter](crate::inject_stack_limiter) instrumentation to a
+/// module instrumented with this type of gas metering. This could lead to a massive module size
+/// bloat. This is a known issue to be fixed in upcoming versions.
+pub mod mutable_global {
+	use super::{Backend, GasMeter, Rules};
+	use alloc::vec;
+	use parity_wasm::elements::{self, Instruction, Module};
+	/// Injects a mutable global variable and a local function to the module to track
+	/// current gas left.
+	///
+	/// The function is called in every metering block. In case of falling out of gas, the global is
+	/// set to the sentinel value `U64::MAX` and `unreachable` instruction is called. The execution
+	/// engine should take care of getting the current global value and setting it back in order to
+	/// sync the gas left value during an execution.
+	pub struct Injector {
+		/// The export name of the gas tracking global.
+		pub global_name: &'static str,
+	}
+
+	impl Injector {
+		pub fn new(global_name: &'static str) -> Self {
+			Self { global_name }
+		}
+	}
+
+	impl Backend for Injector {
+		fn gas_meter<R: Rules>(self, module: &Module, rules: &R) -> GasMeter {
+			let gas_global_idx = module.globals_space() as u32;
+
+			let func_instructions = vec![
+				Instruction::GetGlobal(gas_global_idx),
+				Instruction::GetLocal(0),
+				Instruction::I64GeU,
+				Instruction::If(elements::BlockType::NoResult),
+				Instruction::GetGlobal(gas_global_idx),
+				Instruction::GetLocal(0),
+				Instruction::I64Sub,
+				Instruction::SetGlobal(gas_global_idx),
+				Instruction::Else,
+				// sentinel val u64::MAX
+				Instruction::I64Const(-1i64),           // non-charged instruction
+				Instruction::SetGlobal(gas_global_idx), // non-charged instruction
+				Instruction::Unreachable,               // non-charged instruction
+				Instruction::End,
+				Instruction::End,
+			];
+
+			// calculate gas used for the gas charging func execution itself
+			let mut gas_fn_cost = func_instructions.iter().fold(0, |cost: u64, instruction| {
+				cost.saturating_add(rules.instruction_cost(instruction).unwrap_or(u32::MAX).into())
+			});
+			// don't charge for the instructions used to fail when out of gas
+			let fail_cost = [
+				Instruction::I64Const(-1i64),           // non-charged instruction
+				Instruction::SetGlobal(gas_global_idx), // non-charged instruction
+				Instruction::Unreachable,               // non-charged instruction
+			]
+			.iter()
+			.fold(0, |cost: u64, instruction| {
+				cost.saturating_add(rules.instruction_cost(instruction).unwrap_or(u32::MAX).into())
+			});
+
+			// the fail costs are a subset of the overall costs and hence this never underflows
+			gas_fn_cost -= fail_cost;
+
+			GasMeter::Internal {
+				global: self.global_name,
+				func_instructions: elements::Instructions::new(func_instructions),
+				cost: gas_fn_cost,
+			}
+		}
+	}
+}

src/gas_metering/validation.rs

@@ -23,10 +23,10 @@ struct ControlFlowNode {
 	first_instr_pos: Option<usize>,
 
 	/// The actual gas cost of executing all instructions in the basic block.
-	actual_cost: u32,
+	actual_cost: u64,
 
 	/// The amount of gas charged by the injected metering instructions within this basic block.
-	charged_cost: u32,
+	charged_cost: u64,
 
 	/// Whether there are any other nodes in the graph that loop back to this one. Every cycle in
 	/// the control flow graph contains at least one node with this flag set.
@@ -68,10 +68,10 @@ impl ControlFlowGraph {
 	}
 
 	fn increment_actual_cost(&mut self, node_id: NodeId, cost: u32) {
-		self.get_node_mut(node_id).actual_cost += cost;
+		self.get_node_mut(node_id).actual_cost += u64::from(cost);
 	}
 
-	fn increment_charged_cost(&mut self, node_id: NodeId, cost: u32) {
+	fn increment_charged_cost(&mut self, node_id: NodeId, cost: u64) {
 		self.get_node_mut(node_id).charged_cost += cost;
 	}
 
@@ -134,6 +134,14 @@ fn build_control_flow_graph(
 
 	let mut stack = vec![ControlFrame::new(entry_node_id, terminal_node_id, false)];
 	let mut metered_blocks_iter = blocks.iter().peekable();
+
+	let locals_count = body
+		.locals()
+		.iter()
+		.try_fold(0u32, |count, val_type| count.checked_add(val_type.count()))
+		.ok_or(())?;
+	let locals_init_cost = rules.call_per_local_cost().checked_mul(locals_count).ok_or(())?;
+
 	for (cursor, instruction) in body.code().elements().iter().enumerate() {
 		let active_node_id = stack
 			.last()
@@ -149,6 +157,10 @@ fn build_control_flow_graph(
 			graph.increment_charged_cost(active_node_id, next_metered_block.cost);
 		}
 
+		// Add locals initialization cost to the function block.
+		if cursor == 0 {
+			graph.increment_actual_cost(active_node_id, locals_init_cost);
+		}
 		let instruction_cost = rules.instruction_cost(instruction).ok_or(())?;
 		match instruction {
 			Instruction::Block(_) => {
@@ -267,9 +279,9 @@ fn validate_graph_gas_costs(graph: &ControlFlowGraph) -> bool {
 	fn visit(
 		graph: &ControlFlowGraph,
 		node_id: NodeId,
-		mut total_actual: u32,
-		mut total_charged: u32,
-		loop_costs: &mut Map<NodeId, (u32, u32)>,
+		mut total_actual: u64,
+		mut total_charged: u64,
+		loop_costs: &mut Map<NodeId, (u64, u64)>,
 	) -> bool {
 		let node = graph.get_node(node_id);
 
@@ -342,8 +354,10 @@ mod tests {
 
 			for func_body in module.code_section().iter().flat_map(|section| section.bodies()) {
 				let rules = ConstantCostRules::default();
+				let locals_count = func_body.locals().iter().map(|val_type| val_type.count()).sum();
 
-				let metered_blocks = determine_metered_blocks(func_body.code(), &rules).unwrap();
+				let metered_blocks =
+					determine_metered_blocks(func_body.code(), &rules, locals_count).unwrap();
 				let success =
 					validate_metering_injections(func_body, &rules, &metered_blocks).unwrap();
 				assert!(success);

src/stack_limiter/max_height.rs

@@ -176,7 +176,7 @@ pub fn compute(func_idx: u32, module: &elements::Module) -> Result<u32, &'static
 		match opcode {
 			Nop => {},
 			Block(ty) | Loop(ty) | If(ty) => {
-				let end_arity = if *ty == BlockType::NoResult { 0 } else { 1 };
+				let end_arity = u32::from(*ty != BlockType::NoResult);
 				let branch_arity = if let Loop(_) = *opcode { 0 } else { end_arity };
 				if let If(_) = *opcode {
 					stack.pop_values(1)?;
@@ -465,7 +465,7 @@ mod tests {
   (memory 0)
   (func (result i32)
 	unreachable
-	grow_memory
+	memory.grow
   )
 )
 "#,

src/stack_limiter/mod.rs

@@ -222,8 +222,8 @@ fn instrument_functions(
 /// Before:
 ///
 /// ```text
-/// get_local 0
-/// get_local 1
+/// local.get 0
+/// local.get 1
 /// call 228
 /// drop
 /// ```
@@ -231,8 +231,8 @@ fn instrument_functions(
 /// After:
 ///
 /// ```text
-/// get_local 0
-/// get_local 1
+/// local.get 0
+/// local.get 1
 ///
 /// < ... preamble ... >
 ///
@@ -366,9 +366,9 @@ mod tests {
 			r#"
 (module
 	(func (export "i32.add") (param i32 i32) (result i32)
-		get_local 0
-	get_local 1
-	i32.add
+		local.get 0
+		local.get 1
+		i32.add
 	)
 )
 "#,

src/stack_limiter/thunk.rs

@@ -1,12 +1,8 @@
-#[cfg(not(features = "std"))]
-use alloc::collections::BTreeMap as Map;
-use alloc::vec::Vec;
+use alloc::{collections::BTreeMap as Map, vec::Vec};
 use parity_wasm::{
 	builder,
 	elements::{self, FunctionType, Internal},
 };
-#[cfg(features = "std")]
-use std::collections::HashMap as Map;
 
 use super::{resolve_func_type, Context};
 

tests/diff.rs

@@ -1,10 +1,9 @@
-use parity_wasm::elements::Module;
 use std::{
 	fs,
 	io::{self, Read, Write},
 	path::{Path, PathBuf},
 };
-use wasm_instrument::{self as instrument, parity_wasm::elements};
+use wasm_instrument::{self as instrument, gas_metering, parity_wasm::elements};
 use wasmparser::validate;
 
 fn slurp<P: AsRef<Path>>(path: P) -> io::Result<Vec<u8>> {
@@ -20,18 +19,23 @@ fn dump<P: AsRef<Path>>(path: P, buf: &[u8]) -> io::Result<()> {
 	Ok(())
 }
 
-fn run_diff_test<F: FnOnce(&[u8]) -> Vec<u8>>(test_dir: &str, name: &str, test: F) {
+fn run_diff_test<F: FnOnce(&[u8]) -> Vec<u8>>(
+	test_dir: &str,
+	in_name: &str,
+	out_name: &str,
+	test: F,
+) {
 	let mut fixture_path = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
 	fixture_path.push("tests");
 	fixture_path.push("fixtures");
 	fixture_path.push(test_dir);
-	fixture_path.push(name);
+	fixture_path.push(in_name);
 
 	let mut expected_path = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
 	expected_path.push("tests");
 	expected_path.push("expectations");
 	expected_path.push(test_dir);
-	expected_path.push(name);
+	expected_path.push(out_name);
 
 	let fixture_wasm = wat::parse_file(&fixture_path).expect("Failed to read fixture");
 	validate(&fixture_wasm).expect("Fixture is invalid");
@@ -48,7 +52,7 @@ fn run_diff_test<F: FnOnce(&[u8]) -> Vec<u8>>(test_dir: &str, name: &str, test:
 	if actual_wat != expected_wat {
 		println!("difference!");
 		println!("--- {}", expected_path.display());
-		println!("+++ {} test {}", test_dir, name);
+		println!("+++ {} test {}", test_dir, out_name);
 		for diff in diff::lines(expected_wat, &actual_wat) {
 			match diff {
 				diff::Result::Left(l) => println!("-{}", l),
@@ -72,13 +76,18 @@ mod stack_height {
 		( $name:ident ) => {
 			#[test]
 			fn $name() {
-				run_diff_test("stack-height", concat!(stringify!($name), ".wat"), |input| {
-					let module =
-						elements::deserialize_buffer(input).expect("Failed to deserialize");
-					let instrumented = instrument::inject_stack_limiter(module, 1024)
-						.expect("Failed to instrument with stack counter");
-					elements::serialize(instrumented).expect("Failed to serialize")
-				});
+				run_diff_test(
+					"stack-height",
+					concat!(stringify!($name), ".wat"),
+					concat!(stringify!($name), ".wat"),
+					|input| {
+						let module =
+							elements::deserialize_buffer(input).expect("Failed to deserialize");
+						let instrumented = instrument::inject_stack_limiter(module, 1024)
+							.expect("Failed to instrument with stack counter");
+						elements::serialize(instrumented).expect("Failed to serialize")
+					},
+				);
 			}
 		};
 	}
@@ -96,27 +105,53 @@ mod gas {
 	use super::*;
 
 	macro_rules! def_gas_test {
-		( $name:ident ) => {
+		( ($input:ident, $name1:ident, $name2:ident) ) => {
 			#[test]
-			fn $name() {
-				run_diff_test("gas", concat!(stringify!($name), ".wat"), |input| {
-					let rules = instrument::gas_metering::ConstantCostRules::default();
+			fn $name1() {
+				run_diff_test(
+					"gas",
+					concat!(stringify!($input), ".wat"),
+					concat!(stringify!($name1), ".wat"),
+					|input| {
+						let rules = gas_metering::ConstantCostRules::default();
 
-					let module: Module =
-						elements::deserialize_buffer(input).expect("Failed to deserialize");
-					let module = module.parse_names().expect("Failed to parse names");
+						let module: elements::Module =
+							elements::deserialize_buffer(input).expect("Failed to deserialize");
+						let module = module.parse_names().expect("Failed to parse names");
+						let backend = gas_metering::host_function::Injector::new("env", "gas");
 
-					let instrumented = instrument::gas_metering::inject(module, &rules, "env")
-						.expect("Failed to instrument with gas metering");
-					elements::serialize(instrumented).expect("Failed to serialize")
-				});
+						let instrumented = gas_metering::inject(module, backend, &rules)
+							.expect("Failed to instrument with gas metering");
+						elements::serialize(instrumented).expect("Failed to serialize")
+					},
+				);
+			}
+
+			#[test]
+			fn $name2() {
+				run_diff_test(
+					"gas",
+					concat!(stringify!($input), ".wat"),
+					concat!(stringify!($name2), ".wat"),
+					|input| {
+						let rules = gas_metering::ConstantCostRules::default();
+
+						let module: elements::Module =
+							elements::deserialize_buffer(input).expect("Failed to deserialize");
+						let module = module.parse_names().expect("Failed to parse names");
+						let backend = gas_metering::mutable_global::Injector::new("gas_left");
+						let instrumented = gas_metering::inject(module, backend, &rules)
+							.expect("Failed to instrument with gas metering");
+						elements::serialize(instrumented).expect("Failed to serialize")
+					},
+				);
 			}
 		};
 	}
 
-	def_gas_test!(ifs);
-	def_gas_test!(simple);
-	def_gas_test!(start);
-	def_gas_test!(call);
-	def_gas_test!(branch);
+	def_gas_test!((ifs, ifs_host_fn, ifs_mut_global));
+	def_gas_test!((simple, simple_host_fn, simple_mut_global));
+	def_gas_test!((start, start_host_fn, start_mut_global));
+	def_gas_test!((call, call_host_fn, call_mut_global));
+	def_gas_test!((branch, branch_host_fn, branch_mut_global));
 }

tests/expectations/gas/branch_host_fn.wat

@@ -1,12 +1,12 @@
 (module
   (type (;0;) (func (result i32)))
-  (type (;1;) (func (param i32)))
+  (type (;1;) (func (param i64)))
   (import "env" "gas" (func (;0;) (type 1)))
   (func $fibonacci_with_break (;1;) (type 0) (result i32)
     (local i32 i32)
-    i32.const 13
+    i64.const 15
     call 0
-    block  ;; label = @1
+    block ;; label = @1
       i32.const 0
       local.set 0
       i32.const 1
@@ -18,7 +18,7 @@
       local.set 1
       i32.const 1
       br_if 0 (;@1;)
-      i32.const 5
+      i64.const 5
       call 0
       local.get 0
       local.get 1

tests/expectations/gas/branch_mut_global.wat

@@ -0,0 +1,47 @@
+(module
+  (type (;0;) (func (result i32)))
+  (type (;1;) (func (param i64)))
+  (func $fibonacci_with_break (;0;) (type 0) (result i32)
+    (local $x i32) (local $y i32)
+    i64.const 26
+    call 1
+    block ;; label = @1
+      i32.const 0
+      local.set $x
+      i32.const 1
+      local.set $y
+      local.get $x
+      local.get $y
+      local.tee $x
+      i32.add
+      local.set $y
+      i32.const 1
+      br_if 0 (;@1;)
+      i64.const 16
+      call 1
+      local.get $x
+      local.get $y
+      local.tee $x
+      i32.add
+      local.set $y
+    end
+    local.get $y
+  )
+  (func (;1;) (type 1) (param i64)
+    global.get 0
+    local.get 0
+    i64.ge_u
+    if ;; label = @1
+      global.get 0
+      local.get 0
+      i64.sub
+      global.set 0
+    else
+      i64.const -1
+      global.set 0
+      unreachable
+    end
+  )
+  (global (;0;) (mut i64) i64.const 0)
+  (export "gas_left" (global 0))
+)

tests/expectations/gas/call_host_fn.wat

@@ -1,10 +1,10 @@
 (module
   (type (;0;) (func (param i32 i32) (result i32)))
-  (type (;1;) (func (param i32)))
+  (type (;1;) (func (param i64)))
   (import "env" "gas" (func (;0;) (type 1)))
   (func $add_locals (;1;) (type 0) (param $x i32) (param $y i32) (result i32)
     (local i32)
-    i32.const 5
+    i64.const 6
     call 0
     local.get $x
     local.get $y
@@ -13,7 +13,7 @@
     local.get 2
   )
   (func $add (;2;) (type 0) (param i32 i32) (result i32)
-    i32.const 3
+    i64.const 3
     call 0
     local.get 0
     local.get 1

tests/expectations/gas/call_mut_global.wat

@@ -0,0 +1,38 @@
+(module
+  (type (;0;) (func (param i32 i32) (result i32)))
+  (type (;1;) (func (param i64)))
+  (func $add_locals (;0;) (type 0) (param $x i32) (param $y i32) (result i32)
+    (local $t i32)
+    i64.const 17
+    call 2
+    local.get $x
+    local.get $y
+    call $add
+    local.set $t
+    local.get $t
+  )
+  (func $add (;1;) (type 0) (param $x i32) (param $y i32) (result i32)
+    i64.const 14
+    call 2
+    local.get $x
+    local.get $y
+    i32.add
+  )
+  (func (;2;) (type 1) (param i64)
+    global.get 0
+    local.get 0
+    i64.ge_u
+    if ;; label = @1
+      global.get 0
+      local.get 0
+      i64.sub
+      global.set 0
+    else
+      i64.const -1
+      global.set 0
+      unreachable
+    end
+  )
+  (global (;0;) (mut i64) i64.const 0)
+  (export "gas_left" (global 0))
+)

tests/expectations/gas/ifs_host_fn.wat

@@ -1,19 +1,19 @@
 (module
   (type (;0;) (func (param i32) (result i32)))
-  (type (;1;) (func (param i32)))
+  (type (;1;) (func (param i64)))
   (import "env" "gas" (func (;0;) (type 1)))
   (func (;1;) (type 0) (param i32) (result i32)
-    i32.const 2
+    i64.const 2
     call 0
     i32.const 1
-    if (result i32)  ;; label = @1
-      i32.const 3
+    if (result i32) ;; label = @1
+      i64.const 3
       call 0
       local.get 0
       i32.const 1
       i32.add
     else
-      i32.const 2
+      i64.const 2
       call 0
       local.get 0
       i32.popcnt

tests/expectations/gas/ifs_mut_global.wat

@@ -0,0 +1,38 @@
+(module
+  (type (;0;) (func (param i32) (result i32)))
+  (type (;1;) (func (param i64)))
+  (func (;0;) (type 0) (param $x i32) (result i32)
+    i64.const 13
+    call 1
+    i32.const 1
+    if (result i32) ;; label = @1
+      i64.const 14
+      call 1
+      local.get $x
+      i32.const 1
+      i32.add
+    else
+      i64.const 13
+      call 1
+      local.get $x
+      i32.popcnt
+    end
+  )
+  (func (;1;) (type 1) (param i64)
+    global.get 0
+    local.get 0
+    i64.ge_u
+    if ;; label = @1
+      global.get 0
+      local.get 0
+      i64.sub
+      global.set 0
+    else
+      i64.const -1
+      global.set 0
+      unreachable
+    end
+  )
+  (global (;0;) (mut i64) i64.const 0)
+  (export "gas_left" (global 0))
+)

tests/expectations/gas/simple_host_fn.wat

@@ -1,16 +1,16 @@
 (module
   (type (;0;) (func))
-  (type (;1;) (func (param i32)))
+  (type (;1;) (func (param i64)))
   (import "env" "gas" (func (;0;) (type 1)))
   (func (;1;) (type 0)
-    i32.const 2
+    i64.const 2
     call 0
     i32.const 1
-    if  ;; label = @1
-      i32.const 1
+    if ;; label = @1
+      i64.const 1
       call 0
-      loop  ;; label = @2
-        i32.const 2
+      loop ;; label = @2
+        i64.const 2
         call 0
         i32.const 123
         drop
@@ -18,9 +18,9 @@
     end
   )
   (func (;2;) (type 0)
-    i32.const 1
+    i64.const 1
     call 0
-    block  ;; label = @1
+    block ;; label = @1
     end
   )
   (export "simple" (func 1))

tests/expectations/gas/simple_mut_global.wat

@@ -0,0 +1,43 @@
+(module
+  (type (;0;) (func))
+  (type (;1;) (func (param i64)))
+  (func (;0;) (type 0)
+    i64.const 13
+    call 2
+    i32.const 1
+    if ;; label = @1
+      i64.const 12
+      call 2
+      loop ;; label = @2
+        i64.const 13
+        call 2
+        i32.const 123
+        drop
+      end
+    end
+  )
+  (func (;1;) (type 0)
+    i64.const 12
+    call 2
+    block ;; label = @1
+    end
+  )
+  (func (;2;) (type 1) (param i64)
+    global.get 0
+    local.get 0
+    i64.ge_u
+    if ;; label = @1
+      global.get 0
+      local.get 0
+      i64.sub
+      global.set 0
+    else
+      i64.const -1
+      global.set 0
+      unreachable
+    end
+  )
+  (global (;0;) (mut i64) i64.const 0)
+  (export "simple" (func 0))
+  (export "gas_left" (global 0))
+)

tests/expectations/gas/start_host_fn.wat

@@ -1,12 +1,12 @@
 (module
   (type (;0;) (func (param i32 i32)))
   (type (;1;) (func))
-  (type (;2;) (func (param i32)))
+  (type (;2;) (func (param i64)))
   (import "env" "ext_return" (func $ext_return (;0;) (type 0)))
   (import "env" "memory" (memory (;0;) 1 1))
   (import "env" "gas" (func (;1;) (type 2)))
   (func $start (;2;) (type 1)
-    i32.const 4
+    i64.const 4
     call 1
     i32.const 8
     i32.const 4

tests/expectations/gas/start_mut_global.wat

@@ -0,0 +1,36 @@
+(module
+  (type (;0;) (func (param i32 i32)))
+  (type (;1;) (func))
+  (type (;2;) (func (param i64)))
+  (import "env" "ext_return" (func $ext_return (;0;) (type 0)))
+  (import "env" "memory" (memory (;0;) 1 1))
+  (func $start (;1;) (type 1)
+    i64.const 15
+    call 3
+    i32.const 8
+    i32.const 4
+    call $ext_return
+    unreachable
+  )
+  (func (;2;) (type 1))
+  (func (;3;) (type 2) (param i64)
+    global.get 0
+    local.get 0
+    i64.ge_u
+    if ;; label = @1
+      global.get 0
+      local.get 0
+      i64.sub
+      global.set 0
+    else
+      i64.const -1
+      global.set 0
+      unreachable
+    end
+  )
+  (global (;0;) (mut i64) i64.const 0)
+  (export "call" (func 2))
+  (export "gas_left" (global 0))
+  (start $start)
+  (data (;0;) (i32.const 8) "\01\02\03\04")
+)

tests/expectations/stack-height/empty_functions.wat

@@ -8,7 +8,7 @@
     global.get 0
     i32.const 1024
     i32.gt_u
-    if  ;; label = @1
+    if ;; label = @1
       unreachable
     end
     call 0
@@ -25,7 +25,7 @@
     global.get 0
     i32.const 1024
     i32.gt_u
-    if  ;; label = @1
+    if ;; label = @1
       unreachable
     end
     call 0
@@ -42,7 +42,7 @@
     global.get 0
     i32.const 1024
     i32.gt_u
-    if  ;; label = @1
+    if ;; label = @1
       unreachable
     end
     call 1

tests/expectations/stack-height/global.wat

@@ -24,7 +24,7 @@
     global.get 1
     i32.const 1024
     i32.gt_u
-    if  ;; label = @1
+    if ;; label = @1
       unreachable
     end
     call $i32.add
@@ -44,7 +44,7 @@
     global.get 1
     i32.const 1024
     i32.gt_u
-    if  ;; label = @1
+    if ;; label = @1
       unreachable
     end
     call $i32.add

tests/expectations/stack-height/imports.wat

@@ -20,7 +20,7 @@
     global.get 0
     i32.const 1024
     i32.gt_u
-    if  ;; label = @1
+    if ;; label = @1
       unreachable
     end
     call 2

tests/expectations/stack-height/many_locals.wat

@@ -11,7 +11,7 @@
     global.get 0
     i32.const 1024
     i32.gt_u
-    if  ;; label = @1
+    if ;; label = @1
       unreachable
     end
     call $one-group-many-locals

tests/expectations/stack-height/simple.wat

@@ -12,7 +12,7 @@
     global.get 0
     i32.const 1024
     i32.gt_u
-    if  ;; label = @1
+    if ;; label = @1
       unreachable
     end
     call 0

tests/expectations/stack-height/start.wat

@@ -15,7 +15,7 @@
     global.get 0
     i32.const 1024
     i32.gt_u
-    if  ;; label = @1
+    if ;; label = @1
       unreachable
     end
     call $start
@@ -32,7 +32,7 @@
     global.get 0
     i32.const 1024
     i32.gt_u
-    if  ;; label = @1
+    if ;; label = @1
       unreachable
     end
     call 2

tests/expectations/stack-height/table.wat

@@ -13,7 +13,7 @@
     global.get 0
     i32.const 1024
     i32.gt_u
-    if  ;; label = @1
+    if ;; label = @1
       unreachable
     end
     call $i32.add
@@ -37,7 +37,7 @@
     global.get 0
     i32.const 1024
     i32.gt_u
-    if  ;; label = @1
+    if ;; label = @1
       unreachable
     end
     call 1
@@ -56,7 +56,7 @@
     global.get 0
     i32.const 1024
     i32.gt_u
-    if  ;; label = @1
+    if ;; label = @1
       unreachable
     end
     call $i32.add

tests/fixtures/gas/branch.wat

@@ -3,25 +3,25 @@
 		(local $x i32) (local $y i32)
 
 		(block $unrolled_loop
-			(set_local $x (i32.const 0))
-			(set_local $y (i32.const 1))
+			(local.set $x (i32.const 0))
+			(local.set $y (i32.const 1))
 
-			get_local $x
-			get_local $y
-			tee_local $x
+			local.get $x
+			local.get $y
+			local.tee $x
 			i32.add
-			set_local $y
+			local.set $y
 
 			i32.const 1
 			br_if $unrolled_loop
 
-			get_local $x
-			get_local $y
-			tee_local $x
+			local.get $x
+			local.get $y
+			local.tee $x
 			i32.add
-			set_local $y
+			local.set $y
 		)
 
-		get_local $y
+		local.get $y
 	)
 )

tests/fixtures/gas/call.wat

@@ -2,18 +2,18 @@
 	(func $add_locals (param $x i32) (param $y i32) (result i32)
 		(local $t i32)
 
-		get_local $x
-		get_local $y
+		local.get $x
+		local.get $y
 		call $add
-		set_local $t
+		local.set $t
 
-		get_local $t
+		local.get $t
 	)
 
 	(func $add (param $x i32) (param $y i32) (result i32)
 		(i32.add
-			(get_local $x)
-			(get_local $y)
+			(local.get $x)
+			(local.get $y)
 		)
 	)
 )

tests/fixtures/gas/ifs.wat

@@ -2,8 +2,8 @@
 	(func (param $x i32) (result i32)
 		(if (result i32)
 			(i32.const 1)
-			(then (i32.add (get_local $x) (i32.const 1)))
-			(else (i32.popcnt (get_local $x)))
+			(then (i32.add (local.get $x) (i32.const 1)))
+			(else (i32.popcnt (local.get $x)))
 		)
 	)
 )

tests/fixtures/stack-height/global.wat

@@ -5,22 +5,22 @@
   (global $counter (mut i32) (i32.const 1))
 
   (func $i32.add (export "i32.add") (param i32 i32) (result i32)
-    get_local 0
-	  get_local 1
-	  i32.add
+    local.get 0
+    local.get 1
+    i32.add
   )
   (func (param $arg i32)
-     (local $tmp i32)
+    (local $tmp i32)
 
-     global.get 0
-     i32.const 1
-     i32.add
-	   tee_local $tmp
-     global.set $counter
+    global.get 0
+    i32.const 1
+    i32.add
+    local.tee $tmp
+    global.set $counter
 
-     get_local $tmp
-     get_local $arg
-     call $i32.add
-     drop
+    local.get $tmp
+    local.get $arg
+    call $i32.add
+    drop
   )
 )

tests/fixtures/stack-height/imports.wat

@@ -8,8 +8,8 @@
     call $foo
     call $boo
 
-    get_local 0
-    get_local 1
+    local.get 0
+    local.get 1
     i32.add
   )
 )

tests/fixtures/stack-height/many_locals.wat

@@ -6,5 +6,5 @@
 		(call
 			$one-group-many-locals
 		)
-  	)
+	)
 )

tests/fixtures/stack-height/table.wat

@@ -1,17 +1,17 @@
 (module
   (import "env" "foo" (func $foo))
   (func (param i32)
-    get_local 0
+    local.get 0
     i32.const 0
     call $i32.add
     drop
   )
   (func $i32.add (export "i32.add") (param i32 i32) (result i32)
-    get_local 0
-	  get_local 1
-	  i32.add
+    local.get 0
+    local.get 1
+    i32.add
   )
-  (table 10 anyfunc)
+  (table 10 funcref)
 
   ;; Refer all types of functions: imported, defined not exported and defined exported.
   (elem (i32.const 0) 0 1 2)

tests/overhead.rs

@@ -1,9 +1,10 @@
 use std::{
-	fs::{read, read_dir},
+	fs::{read, read_dir, ReadDir},
 	path::PathBuf,
 };
 use wasm_instrument::{
-	gas_metering, inject_stack_limiter,
+	gas_metering::{self, host_function, mutable_global, ConstantCostRules},
+	inject_stack_limiter,
 	parity_wasm::{deserialize_buffer, elements::Module, serialize},
 };
 
@@ -14,60 +15,157 @@ fn fixture_dir() -> PathBuf {
 	path
 }
 
-/// Print the overhead of applying gas metering, stack height limiting or both.
-///
-/// Use `cargo test print_overhead -- --nocapture`.
-#[test]
-fn print_size_overhead() {
-	let mut results: Vec<_> = read_dir(fixture_dir())
-		.unwrap()
+use gas_metering::Backend;
+fn gas_metered_mod_len<B: Backend>(orig_module: Module, backend: B) -> (Module, usize) {
+	let module = gas_metering::inject(orig_module, backend, &ConstantCostRules::default()).unwrap();
+	let bytes = serialize(module.clone()).unwrap();
+	let len = bytes.len();
+	(module, len)
+}
+
+fn stack_limited_mod_len(module: Module) -> (Module, usize) {
+	let module = inject_stack_limiter(module, 128).unwrap();
+	let bytes = serialize(module.clone()).unwrap();
+	let len = bytes.len();
+	(module, len)
+}
+
+struct InstrumentedWasmResults {
+	filename: String,
+	original_module_len: usize,
+	stack_limited_len: usize,
+	gas_metered_host_fn_len: usize,
+	gas_metered_mut_glob_len: usize,
+	gas_metered_host_fn_then_stack_limited_len: usize,
+	gas_metered_mut_glob_then_stack_limited_len: usize,
+}
+
+fn size_overheads_all(files: ReadDir) -> Vec<InstrumentedWasmResults> {
+	files
 		.map(|entry| {
 			let entry = entry.unwrap();
-			let (orig_len, orig_module) = {
-				let bytes = read(&entry.path()).unwrap();
+			let filename = entry.file_name().into_string().unwrap();
+
+			let (original_module_len, orig_module) = {
+				let bytes = match entry.path().extension().unwrap().to_str() {
+					Some("wasm") => read(entry.path()).unwrap(),
+					Some("wat") =>
+						wat::parse_bytes(&read(entry.path()).unwrap()).unwrap().into_owned(),
+					_ => panic!("expected fixture_dir containing .wasm or .wat files only"),
+				};
+
 				let len = bytes.len();
 				let module: Module = deserialize_buffer(&bytes).unwrap();
 				(len, module)
 			};
-			let (gas_metering_len, gas_module) = {
-				let module = gas_metering::inject(
-					orig_module.clone(),
-					&gas_metering::ConstantCostRules::default(),
-					"env",
-				)
-				.unwrap();
-				let bytes = serialize(module.clone()).unwrap();
-				let len = bytes.len();
-				(len, module)
-			};
-			let stack_height_len = {
-				let module = inject_stack_limiter(orig_module, 128).unwrap();
-				let bytes = serialize(module).unwrap();
-				bytes.len()
-			};
-			let both_len = {
-				let module = inject_stack_limiter(gas_module, 128).unwrap();
-				let bytes = serialize(module).unwrap();
-				bytes.len()
-			};
 
-			let overhead = both_len * 100 / orig_len;
+			let (gm_host_fn_module, gas_metered_host_fn_len) = gas_metered_mod_len(
+				orig_module.clone(),
+				host_function::Injector::new("env", "gas"),
+			);
 
-			(
-				overhead,
-				format!(
-					"{:30}: orig = {:4} kb, gas_metering = {} %, stack_limiter = {} %, both = {} %",
-					entry.file_name().to_str().unwrap(),
-					orig_len / 1024,
-					gas_metering_len * 100 / orig_len,
-					stack_height_len * 100 / orig_len,
-					overhead,
-				),
-			)
+			let (gm_mut_global_module, gas_metered_mut_glob_len) =
+				gas_metered_mod_len(orig_module.clone(), mutable_global::Injector::new("gas_left"));
+
+			let stack_limited_len = stack_limited_mod_len(orig_module).1;
+
+			let (_gm_hf_sl_mod, gas_metered_host_fn_then_stack_limited_len) =
+				stack_limited_mod_len(gm_host_fn_module);
+
+			let (_gm_mg_sl_module, gas_metered_mut_glob_then_stack_limited_len) =
+				stack_limited_mod_len(gm_mut_global_module);
+
+			InstrumentedWasmResults {
+				filename,
+				original_module_len,
+				stack_limited_len,
+				gas_metered_host_fn_len,
+				gas_metered_mut_glob_len,
+				gas_metered_host_fn_then_stack_limited_len,
+				gas_metered_mut_glob_then_stack_limited_len,
+			}
 		})
-		.collect();
-	results.sort_unstable_by(|a, b| b.0.cmp(&a.0));
-	for entry in results {
-		println!("{}", entry.1);
+		.collect()
+}
+
+fn calc_size_overheads() -> Vec<InstrumentedWasmResults> {
+	let mut wasm_path = fixture_dir();
+	wasm_path.push("wasm");
+
+	let mut wat_path = fixture_dir();
+	wat_path.push("wat");
+
+	let mut results = size_overheads_all(read_dir(wasm_path).unwrap());
+	let results_wat = size_overheads_all(read_dir(wat_path).unwrap());
+
+	results.extend(results_wat);
+
+	results
+}
+
+/// Print the overhead of applying gas metering, stack
+/// height limiting or both.
+///
+/// Use `cargo test print_size_overhead -- --nocapture`.
+#[test]
+fn print_size_overhead() {
+	let mut results = calc_size_overheads();
+	results.sort_unstable_by(|a, b| {
+		b.gas_metered_mut_glob_then_stack_limited_len
+			.cmp(&a.gas_metered_mut_glob_then_stack_limited_len)
+	});
+
+	for r in results {
+		let filename = r.filename;
+		let original_size = r.original_module_len / 1024;
+		let stack_limit = r.stack_limited_len * 100 / r.original_module_len;
+		let host_fn = r.gas_metered_host_fn_len * 100 / r.original_module_len;
+		let mut_glob = r.gas_metered_mut_glob_len * 100 / r.original_module_len;
+		let host_fn_sl = r.gas_metered_host_fn_then_stack_limited_len * 100 / r.original_module_len;
+		let mut_glob_sl =
+			r.gas_metered_mut_glob_then_stack_limited_len * 100 / r.original_module_len;
+
+		println!(
+			"{filename:30}: orig = {original_size:4} kb, stack_limiter = {stack_limit} %, \
+			  gas_metered_host_fn =    {host_fn} %, both = {host_fn_sl} %,\n \
+			 {:69} gas_metered_mut_global = {mut_glob} %, both = {mut_glob_sl} %",
+			""
+		);
+	}
+}
+
+/// Compare module size overhead of applying gas metering with two methods.
+///
+/// Use `cargo test print_gas_metered_sizes -- --nocapture`.
+#[test]
+fn print_gas_metered_sizes() {
+	let overheads = calc_size_overheads();
+	let mut results = overheads
+		.iter()
+		.map(|r| {
+			let diff = (r.gas_metered_mut_glob_len * 100 / r.gas_metered_host_fn_len) as i32 - 100;
+			(diff, r)
+		})
+		.collect::<Vec<(i32, &InstrumentedWasmResults)>>();
+	results.sort_unstable_by(|a, b| b.0.cmp(&a.0));
+
+	println!(
+		"| {:28} | {:^16} | gas metered/host fn | gas metered/mut global | size diff |",
+		"fixture", "original size",
+	);
+	println!("|{:-^30}|{:-^18}|{:-^21}|{:-^24}|{:-^11}|", "", "", "", "", "",);
+	for r in results {
+		let filename = &r.1.filename;
+		let original_size = &r.1.original_module_len / 1024;
+		let host_fn = &r.1.gas_metered_host_fn_len / 1024;
+		let mut_glob = &r.1.gas_metered_mut_glob_len / 1024;
+		let host_fn_percent = &r.1.gas_metered_host_fn_len * 100 / r.1.original_module_len;
+		let mut_glob_percent = &r.1.gas_metered_mut_glob_len * 100 / r.1.original_module_len;
+		let host_fn = format!("{host_fn} kb ({host_fn_percent:}%)");
+		let mut_glob = format!("{mut_glob} kb ({mut_glob_percent:}%)");
+		let diff = &r.0;
+		println!(
+			"| {filename:28} | {original_size:13} kb | {host_fn:>19} | {mut_glob:>22} | {diff:+8}% |"
+		);
 	}
 }