Enable the tail calling convention by default

crates/wasmtime/src/config.rs

@@ -1728,6 +1728,24 @@ impl Config {
         self
     }
 
+    pub(crate) fn conditionally_enable_defaults(&mut self) {
+        // If tail calls were not explicitly enabled/disabled (i.e. tail_callable is None), enable
+        // them if we are targeting a backend that supports them. Currently the Cranelift
+        // compilation strategy is the only one that supports tail calls, but not targeting s390x.
+        if self.tunables.tail_callable.is_none() {
+            #[cfg(feature = "cranelift")]
+            let default_tail_calls = self.compiler_config.strategy == Strategy::Cranelift
+                && self.compiler_config.target.as_ref().map_or_else(
+                    || target_lexicon::Triple::host().architecture,
+                    |triple| triple.architecture,
+                ) != Architecture::S390x;
+            #[cfg(not(feature = "cranelift"))]
+            let default_tail_calls = false;
+
+            self.wasm_tail_call(default_tail_calls);
+        }
+    }
+
     pub(crate) fn validate(&self) -> Result<Tunables> {
         if self.features.contains(WasmFeatures::REFERENCE_TYPES)
             && !self.features.contains(WasmFeatures::BULK_MEMORY)

crates/wasmtime/src/engine.rs

@@ -101,7 +101,12 @@ impl Engine {
             crate::runtime::vm::debug_builtins::ensure_exported();
         }
 
-        let config = config.clone();
+        let config = {
+            let mut config = config.clone();
+            config.conditionally_enable_defaults();
+            config
+        };
+
         let tunables = config.validate()?;
 
         #[cfg(any(feature = "cranelift", feature = "winch"))]

tests/all/defaults.rs

@@ -0,0 +1,62 @@
+use wasmtime::*;
+
+#[test]
+fn test_tail_call_default() -> Result<()> {
+    for (expected, cfg) in [
+        (
+            true,
+            Config::new()
+                .strategy(Strategy::Cranelift)
+                .target("x86_64")?,
+        ),
+        (
+            true,
+            Config::new()
+                .strategy(Strategy::Cranelift)
+                .target("aarch64")?,
+        ),
+        (
+            true,
+            Config::new()
+                .strategy(Strategy::Cranelift)
+                .target("riscv64")?,
+        ),
+        (
+            false,
+            Config::new()
+                .strategy(Strategy::Cranelift)
+                .target("s390x")?,
+        ),
+        (
+            false,
+            Config::new().strategy(Strategy::Winch).target("x86_64")?,
+        ),
+        (
+            false,
+            Config::new().strategy(Strategy::Winch).target("aarch64")?,
+        ),
+        (
+            false,
+            Config::new()
+                .strategy(Strategy::Cranelift)
+                .wasm_tail_call(false)
+                .target("x86_64")?,
+        ),
+    ] {
+        let engine = Engine::new(cfg)?;
+
+        let wat = r#"
+            (module $from_name_section
+                (func (export "run") (return_call 0))
+            )
+        "#;
+
+        let result = engine.precompile_module(wat.as_bytes()).map(|_| ());
+
+        eprintln!("for config {cfg:?}, got: {result:?}");
+
+        assert_eq!(expected, result.is_ok());
+    }
+
+    Ok(())
+}

tests/all/main.rs

@@ -7,6 +7,7 @@ mod code_too_large;
 mod component_model;
 mod coredump;
 mod debug;
+mod defaults;
 mod epoch_interruption;
 mod externals;
 mod fuel;