Implement Imm64 in an 'immediates.rs' module.

Format larger immediates as hexadecimal with a multiple of 4 digits and '_'
group separators.

src/libcretonne/immediates.rs

@@ -0,0 +1,53 @@
+
+//! Immediate operands for Cretonne instructions
+//!
+//! This module defines the types of immediate operands that can appear on Cretonne instructions.
+//! Each type here should have a corresponding definition in the `cretonne.immediates` Python
+//! module in the meta language.
+
+use std::fmt::{self, Display, Formatter};
+
+/// 64-bit immediate integer operand.
+///
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+pub struct Imm64(i64);
+
+impl Display for Imm64 {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        let x = self.0;
+        if -10_000 < x && x < 10_000 {
+            // Use decimal for small numbers.
+            write!(f, "{}", x)
+        } else {
+            // Hexadecimal with a multiple of 4 digits and group separators:
+            //
+            //   0xfff0
+            //   0x0001_ffff
+            //   0xffff_ffff_fff8_4400
+            //
+            let mut pos = (64 - x.leading_zeros() - 1) & 0xf0;
+            try!(write!(f, "0x{:04x}", (x >> pos) & 0xffff));
+            while pos > 0 {
+                pos -= 16;
+                try!(write!(f, "_{:04x}", (x >> pos) & 0xffff));
+            }
+            Ok(())
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn format_imm64() {
+        assert_eq!(format!("{}", Imm64(0)), "0");
+        assert_eq!(format!("{}", Imm64(9999)), "9999");
+        assert_eq!(format!("{}", Imm64(10000)), "0x2710");
+        assert_eq!(format!("{}", Imm64(-9999)), "-9999");
+        assert_eq!(format!("{}", Imm64(-10000)), "0xffff_ffff_ffff_d8f0");
+        assert_eq!(format!("{}", Imm64(0xffff)), "0xffff");
+        assert_eq!(format!("{}", Imm64(0x10000)), "0x0001_0000");
+    }
+}

src/libcretonne/lib.rs

@@ -6,3 +6,4 @@
 // ====------------------------------------------------------------------------------------==== //
 
 pub mod types;
+pub mod immediates;