Allow setting other Cranelift flags when running in Wasmtime.

crates/wasmtime/src/runtime.rs

@@ -355,6 +355,24 @@ impl Config {
         self
     }
 
+    /// Allows settings another Cranelift flag defined by a flag name and value. This allows
+    /// fine-tuning of Cranelift settings.
+    ///
+    /// Since Cranelift flags may be unstable, this method should not be considered to be stable
+    /// either; other `Config` functions should be preferred for stability.
+    ///
+    /// Note that this is marked as unsafe, because setting the wrong flag might break invariants,
+    /// resulting in execution hazards.
+    ///
+    /// # Errors
+    ///
+    /// This method can fail if the flag's name does not exist, or the value is not appropriate for
+    /// the flag type.
+    pub unsafe fn cranelift_other_flag(&mut self, name: &str, value: &str) -> Result<&mut Self> {
+        self.flags.set(name, value)?;
+        Ok(self)
+    }
+
     /// Loads cache configuration specified at `path`.
     ///
     /// This method will read the file specified by `path` on the filesystem and

src/lib.rs

@@ -153,6 +153,10 @@ struct CommonOptions {
     )]
     opt_level: wasmtime::OptLevel,
 
+    /// Other Cranelift flags to be passed down to Cranelift.
+    #[structopt(long, parse(try_from_str = parse_cranelift_flag))]
+    cranelift_flags: Vec<CraneliftFlag>,
+
     /// Maximum size in bytes of wasm memory before it becomes dynamically
     /// relocatable instead of up-front-reserved.
     #[structopt(long)]
@@ -181,6 +185,11 @@ impl CommonOptions {
             .cranelift_opt_level(self.opt_level())
             .strategy(pick_compilation_strategy(self.cranelift, self.lightbeam)?)?
             .profiler(pick_profiling_strategy(self.jitdump, self.vtune)?)?;
+        for CraneliftFlag { name, value } in &self.cranelift_flags {
+            unsafe {
+                config.cranelift_other_flag(name, value)?;
+            }
+        }
         if !self.disable_cache {
             match &self.config {
                 Some(path) => {
@@ -223,3 +232,23 @@ fn parse_opt_level(opt_level: &str) -> Result<wasmtime::OptLevel> {
         ),
     }
 }
+
+struct CraneliftFlag {
+    name: String,
+    value: String,
+}
+
+fn parse_cranelift_flag(name_and_value: &str) -> Result<CraneliftFlag> {
+    let mut split = name_and_value.splitn(2, '=');
+    let name = if let Some(name) = split.next() {
+        name.to_string()
+    } else {
+        bail!("missing name in cranelift flag");
+    };
+    let value = if let Some(value) = split.next() {
+        value.to_string()
+    } else {
+        bail!("missing value in cranelift flag");
+    };
+    Ok(CraneliftFlag { name, value })
+}