Recommend -O opt-level=0 when debugging wasm

docs/examples-debugging-native-debugger.md

@@ -13,15 +13,16 @@ the same time:
 
 2. Run Wasmtime with the debug info enabled; this is `-D debug-info` from the
    CLI and `Config::debug_info(true)` in an embedding (e.g. see [debugging in a
-   Rust embedding](./examples-rust-debugging.md))
+   Rust embedding](./examples-rust-debugging.md)). It's also recommended to use
+   `-O opt-level=0` for better inspection of local variables if desired.
 
 3. Use a supported debugger:
 
     ```sh
     lldb -- wasmtime run -D debug-info foo.wasm
     ```
     ```sh
-    gdb --args wasmtime run -D debug-info foo.wasm
+    gdb --args wasmtime run -D debug-info -O opt-level=0 foo.wasm
     ```
 
 If you run into trouble, the following discussions might help:

examples/fib-debug/main.c

@@ -16,6 +16,7 @@ int main(int argc, const char *argv[]) {
   // original fib-wasm.c source code and variables.
   wasm_config_t *config = wasm_config_new();
   wasmtime_config_debug_info_set(config, true);
+  wasmtime_config_cranelift_opt_level_set(config, WASMTIME_OPT_LEVEL_NONE);
 
   // Initialize.
   printf("Initializing...\n");

examples/fib-debug/main.rs

@@ -14,7 +14,11 @@ fn main() -> Result<()> {
     // Load our previously compiled wasm file (built previously with Cargo) and
     // also ensure that we generate debuginfo so this executable can be
     // debugged in GDB.
-    let engine = Engine::new(Config::new().debug_info(true))?;
+    let engine = Engine::new(
+        Config::new()
+            .debug_info(true)
+            .cranelift_opt_level(OptLevel::None),
+    )?;
     let mut store = Store::new(&engine, ());
     let module = Module::from_file(&engine, "target/wasm32-unknown-unknown/debug/fib.wasm")?;
     let instance = Instance::new(&mut store, &module, &[])?;