Add helper routines for special-purpose arguments.

- ArgumentType::special() creates a new special-purpose argument without
  assigning it to a register location.
- Signature::special_arg_index() funds a unique special-purpose
  argument.
- Function::special_arg() finds a special-purpose argument by value.

Also add a new "sigid" argument purpose which will be used for runtime
signature checks in WebAssembly indirect calls.

lib/cretonne/src/ir/extfunc.rs

@@ -76,6 +76,13 @@ impl Signature {
     pub fn display<'a, R: Into<Option<&'a RegInfo>>>(&'a self, regs: R) -> DisplaySignature<'a> {
         DisplaySignature(self, regs.into())
     }
+
+    /// Find the index of a presumed unique special-purpose argument.
+    pub fn special_arg_index(&self, purpose: ArgumentPurpose) -> Option<usize> {
+        self.argument_types.iter().rposition(
+            |arg| arg.purpose == purpose,
+        )
+    }
 }
 
 /// Wrapper type capable of displaying a `Signature` with correct register names.
@@ -146,6 +153,16 @@ impl ArgumentType {
         }
     }
 
+    /// Create a special-purpose argument type that is not (yet) bound to a specific register.
+    pub fn special(vt: Type, purpose: ArgumentPurpose) -> ArgumentType {
+        ArgumentType {
+            value_type: vt,
+            extension: ArgumentExtension::None,
+            purpose,
+            location: Default::default(),
+        }
+    }
+
     /// Create an argument type for a special-purpose register.
     pub fn special_reg(vt: Type, purpose: ArgumentPurpose, regunit: RegUnit) -> ArgumentType {
         ArgumentType {
@@ -255,10 +272,16 @@ pub enum ArgumentPurpose {
     /// This is a pointer to a context struct containing details about the current sandbox. It is
     /// used as a base pointer for `vmctx` global variables.
     VMContext,
+
+    /// A signature identifier.
+    ///
+    /// This is a special-purpose argument used to identify the calling convention expected by the
+    /// caller in an indirect call. The callee can verify that the expected signature ID matches.
+    SignatureId,
 }
 
 /// Text format names of the `ArgumentPurpose` variants.
-static PURPOSE_NAMES: [&str; 6] = ["normal", "sret", "link", "fp", "csr", "vmctx"];
+static PURPOSE_NAMES: [&str; 7] = ["normal", "sret", "link", "fp", "csr", "vmctx", "sigid"];
 
 impl fmt::Display for ArgumentPurpose {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
@@ -276,6 +299,7 @@ impl FromStr for ArgumentPurpose {
             "fp" => Ok(ArgumentPurpose::FramePointer),
             "csr" => Ok(ArgumentPurpose::CalleeSaved),
             "vmctx" => Ok(ArgumentPurpose::VMContext),
+            "sigid" => Ok(ArgumentPurpose::SignatureId),
             _ => Err(()),
         }
     }