Allow for special purpose function arguments and return values.

Enumerate a set of special purposes for function arguments that general
purpose code needs to know about. Some of these argument purposes will
only appear in the signature of the current function, representing
things the prologue and epilogues need to know about like the link
register and callee-saved registers.

Get rid of the 'inreg' argument flag. Arguments can be pre-assigned to a
specific register instead.

lib/cretonne/src/ir/extfunc.rs

@@ -9,6 +9,7 @@ use ir::{Type, FunctionName, SigRef, ArgumentLoc};
 use isa::RegInfo;
 use std::cmp;
 use std::fmt;
+use std::str::FromStr;
 
 /// Function signature.
 ///
@@ -111,10 +112,10 @@ impl fmt::Display for Signature {
 pub struct ArgumentType {
     /// Type of the argument value.
     pub value_type: Type,
+    /// Special purpose of argument, or `Normal`.
+    pub purpose: ArgumentPurpose,
     /// Method for extending argument to a full register.
     pub extension: ArgumentExtension,
-    /// Place this argument in a register if possible.
-    pub inreg: bool,
 
     /// ABI-specific location of this argument, or `Unassigned` for arguments that have not yet
     /// been legalized.
@@ -127,7 +128,7 @@ impl ArgumentType {
         ArgumentType {
             value_type: vt,
             extension: ArgumentExtension::None,
-            inreg: false,
+            purpose: ArgumentPurpose::Normal,
             location: Default::default(),
         }
     }
@@ -149,8 +150,8 @@ impl<'a> fmt::Display for DisplayArgumentType<'a> {
             ArgumentExtension::Uext => write!(f, " uext")?,
             ArgumentExtension::Sext => write!(f, " sext")?,
         }
-        if self.0.inreg {
-            write!(f, " inreg")?;
+        if self.0.purpose != ArgumentPurpose::Normal {
+            write!(f, " {}", self.0.purpose)?;
         }
 
         if self.0.location.is_assigned() {
@@ -181,6 +182,75 @@ pub enum ArgumentExtension {
     Sext,
 }
 
+/// The special purpose of a function argument.
+///
+/// Function arguments and return values are used to pass user program values between functions,
+/// but they are also used to represent special registers with significance to the ABI such as
+/// frame pointers and callee-saved registers.
+///
+/// The argument purpose is used to indicate any special meaning of an argument or return value.
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+pub enum ArgumentPurpose {
+    /// A normal user program value passed to or from a function.
+    Normal,
+
+    /// Struct return pointer.
+    ///
+    /// When a function needs to return more data than will fit in registers, the caller passes a
+    /// pointer to a memory location where the return value can be written. In some ABIs, this
+    /// struct return pointer is passed in a specific register.
+    ///
+    /// This argument kind can also appear as a return value for ABIs that require a function with
+    /// a `StructReturn` pointer argument to also return that pointer in a register.
+    StructReturn,
+
+    /// The link register.
+    ///
+    /// Most RISC architectures implement calls by saving the return address in a designated
+    /// register rather than pushing it on the stack. This is represented with a `Link` argument.
+    ///
+    /// Similarly, some return instructions expect the return address in a register represented as
+    /// a `Link` return value.
+    Link,
+
+    /// The frame pointer.
+    ///
+    /// This indicates the frame pointer register which has a special meaning in some ABIs.
+    ///
+    /// The frame pointer appears as an argument and as a return value since it is a callee-saved
+    /// register.
+    FramePointer,
+
+    /// A callee-saved register.
+    ///
+    /// Some calling conventions have registers that must be saved by the callee. These registers
+    /// are represented as `CalleeSaved` arguments and return values.
+    CalleeSaved,
+}
+
+/// Text format names of the `ArgumentPurpose` variants.
+static PURPOSE_NAMES: [&'static str; 5] = ["normal", "sret", "link", "fp", "csr"];
+
+impl fmt::Display for ArgumentPurpose {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str(PURPOSE_NAMES[*self as usize])
+    }
+}
+
+impl FromStr for ArgumentPurpose {
+    type Err = ();
+    fn from_str(s: &str) -> Result<ArgumentPurpose, ()> {
+        match s {
+            "normal" => Ok(ArgumentPurpose::Normal),
+            "sret" => Ok(ArgumentPurpose::StructReturn),
+            "link" => Ok(ArgumentPurpose::Link),
+            "fp" => Ok(ArgumentPurpose::FramePointer),
+            "csr" => Ok(ArgumentPurpose::CalleeSaved),
+            _ => Err(()),
+        }
+    }
+}
+
 /// An external function.
 ///
 /// Information about a function that can be called directly with a direct `call` instruction.
@@ -209,8 +279,21 @@ mod tests {
         assert_eq!(t.to_string(), "i32");
         t.extension = ArgumentExtension::Uext;
         assert_eq!(t.to_string(), "i32 uext");
-        t.inreg = true;
-        assert_eq!(t.to_string(), "i32 uext inreg");
+        t.purpose = ArgumentPurpose::StructReturn;
+        assert_eq!(t.to_string(), "i32 uext sret");
+    }
+
+    #[test]
+    fn argument_purpose() {
+        let all_purpose = [ArgumentPurpose::Normal,
+                           ArgumentPurpose::StructReturn,
+                           ArgumentPurpose::Link,
+                           ArgumentPurpose::FramePointer,
+                           ArgumentPurpose::CalleeSaved];
+        for (&e, &n) in all_purpose.iter().zip(PURPOSE_NAMES.iter()) {
+            assert_eq!(e.to_string(), n);
+            assert_eq!(Ok(e), n.parse());
+        }
     }
 
     #[test]

lib/cretonne/src/ir/mod.rs

@@ -18,7 +18,7 @@ mod progpoint;
 mod valueloc;
 
 pub use ir::funcname::FunctionName;
-pub use ir::extfunc::{Signature, ArgumentType, ArgumentExtension, ExtFuncData};
+pub use ir::extfunc::{Signature, ArgumentType, ArgumentExtension, ArgumentPurpose, ExtFuncData};
 pub use ir::types::Type;
 pub use ir::entities::{Ebb, Inst, Value, StackSlot, JumpTable, FuncRef, SigRef};
 pub use ir::instructions::{Opcode, InstructionData, VariableArgs, ValueList, ValueListPool};