cranelift: Add LibCalls to the interpreter (#4782)

* cranelift: Add libcall handlers to interpreter

* cranelift: Fuzz IshlI64 libcall

* cranelift: Revert back to fuzzing udivi64

* cranelift: Use sdiv as a fuzz libcall

* cranelift: Register Sdiv in fuzzgen

* cranelift: Add multiple libcalls to fuzzer

* cranelift: Register a single libcall handler

* cranelift: Simplify args checking in interpreter

* cranelift: Remove unused LibCalls

* cranelift: Cleanup interpreter libcall types

* cranelift: Fix Interpreter Docs

cranelift/fuzzgen/src/function_generator.rs

@@ -465,6 +465,16 @@ const OPCODE_SIGNATURES: &'static [(
     (Opcode::Call, &[], &[], insert_call),
 ];
 
+/// These libcalls need a interpreter implementation in `cranelift-fuzzgen.rs`
+const ALLOWED_LIBCALLS: &'static [LibCall] = &[
+    LibCall::CeilF32,
+    LibCall::CeilF64,
+    LibCall::FloorF32,
+    LibCall::FloorF64,
+    LibCall::TruncF32,
+    LibCall::TruncF64,
+];
+
 pub struct FunctionGenerator<'r, 'data>
 where
     'data: 'r,
@@ -506,6 +516,12 @@ where
         Ok(CallConv::SystemV)
     }
 
+    fn system_callconv(&mut self) -> CallConv {
+        // TODO: This currently only runs on linux, so this is the only choice
+        // We should improve this once we generate flags and targets
+        CallConv::SystemV
+    }
+
     fn generate_type(&mut self) -> Result<Type> {
         // TODO: It would be nice if we could get these directly from cranelift
         let scalars = [
@@ -833,12 +849,11 @@ where
                 let signature = self.generate_signature()?;
                 (name, signature)
             } else {
-                // Use udivi64 as an example of a libcall function.
-                let mut signature = Signature::new(CallConv::Fast);
-                signature.params.push(AbiParam::new(I64));
-                signature.params.push(AbiParam::new(I64));
-                signature.returns.push(AbiParam::new(I64));
-                (ExternalName::LibCall(LibCall::UdivI64), signature)
+                let libcall = *self.u.choose(ALLOWED_LIBCALLS)?;
+                // TODO: Use [CallConv::for_libcall] once we generate flags.
+                let callconv = self.system_callconv();
+                let signature = libcall.signature(callconv);
+                (ExternalName::LibCall(libcall), signature)
             };
 
             let sig_ref = builder.import_signature(sig.clone());