diff --git a/.gitignore b/.gitignore
index 9ceaac3ed5..765772864e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,3 +5,4 @@
 tags
 target
 Cargo.lock
+.*.rustfmt
diff --git a/lib/cretonne/src/isa/constraints.rs b/lib/cretonne/src/isa/constraints.rs
index bdd2bea958..43a0ddb271 100644
--- a/lib/cretonne/src/isa/constraints.rs
+++ b/lib/cretonne/src/isa/constraints.rs
@@ -21,6 +21,7 @@ pub struct OperandConstraint {
 }
 
 /// The different kinds of operand constraints.
+#[derive(Clone, Copy, PartialEq, Eq)]
 pub enum ConstraintKind {
     /// This operand or result must be a register from the given register class.
     Reg,
diff --git a/lib/cretonne/src/isa/mod.rs b/lib/cretonne/src/isa/mod.rs
index 0bd9e5e9fa..ee33bb4c47 100644
--- a/lib/cretonne/src/isa/mod.rs
+++ b/lib/cretonne/src/isa/mod.rs
@@ -41,8 +41,8 @@
 //! concurrent function compilations.
 
 pub use isa::encoding::Encoding;
-pub use isa::registers::{RegInfo, RegUnit, RegClass};
-pub use isa::constraints::RecipeConstraints;
+pub use isa::registers::{RegInfo, RegUnit, RegClass, RegClassIndex};
+pub use isa::constraints::{RecipeConstraints, OperandConstraint, ConstraintKind};
 
 use settings;
 use ir::{InstructionData, DataFlowGraph};
diff --git a/lib/cretonne/src/isa/registers.rs b/lib/cretonne/src/isa/registers.rs
index f8c2ee5731..5e89110284 100644
--- a/lib/cretonne/src/isa/registers.rs
+++ b/lib/cretonne/src/isa/registers.rs
@@ -133,6 +133,12 @@ impl RegClassData {
             Some(RegClassIndex(mask.trailing_zeros() as u8))
         }
     }
+
+    /// Returns true if `other` is a subclass of this register class.
+    /// A register class is considerd to be a subclass of itself.
+    pub fn has_subclass<RCI: Into<RegClassIndex>>(&self, other: RCI) -> bool {
+        self.subclasses & (1 << other.into().0) != 0
+    }
 }
 
 /// A small reference to a register class.
diff --git a/lib/cretonne/src/regalloc/affinity.rs b/lib/cretonne/src/regalloc/affinity.rs
new file mode 100644
index 0000000000..529828759e
--- /dev/null
+++ b/lib/cretonne/src/regalloc/affinity.rs
@@ -0,0 +1,68 @@
+//! Value affinity for register allocation.
+//!
+//! An SSA value's affinity is a hint used to guide the register allocator. It specifies the class
+//! of allocation that is likely to cause the least amount of fixup moves in order to satisfy
+//! instruction operand constraints.
+//!
+//! For values that want to be in registers, the affinity hint includes a register class or
+//! subclass. This is just a hint, and the register allocator is allowed to pick a register from a
+//! larger register class instead.
+
+use isa::{RegInfo, RegClassIndex, OperandConstraint, ConstraintKind};
+
+/// Preferred register allocation for an SSA value.
+#[derive(Clone, Copy)]
+pub enum Affinity {
+    /// Don't care. This value can go anywhere.
+    Any,
+
+    /// This value should be placed in a spill slot on the stack.
+    Stack,
+
+    /// This value prefers a register from the given register class.
+    Reg(RegClassIndex),
+}
+
+impl Default for Affinity {
+    fn default() -> Self {
+        Affinity::Any
+    }
+}
+
+impl Affinity {
+    /// Create an affinity that satisfies a single constraint.
+    ///
+    /// This will never create the indifferent `Affinity::Any`.
+    /// Use the `Default` implementation for that.
+    pub fn new(constraint: &OperandConstraint) -> Affinity {
+        if constraint.kind == ConstraintKind::Stack {
+            Affinity::Stack
+        } else {
+            Affinity::Reg(constraint.regclass.into())
+        }
+    }
+
+    /// Merge an operand constraint into this affinity.
+    ///
+    /// Note that this does not guarantee that the register allocator will pick a register that
+    /// satisfies the constraint.
+    pub fn merge(&mut self, constraint: &OperandConstraint, reg_info: &RegInfo) {
+        match *self {
+            Affinity::Any => *self = Affinity::new(constraint),
+            Affinity::Reg(rc) => {
+                // If the preferred register class is a subclass of the constraint, there's no need
+                // to change anything.
+                if constraint.kind != ConstraintKind::Stack &&
+                   !constraint.regclass.has_subclass(rc) {
+                    // If the register classes don't overlap, `intersect` returns `None`, and we
+                    // just keep our previous affinity.
+                    if let Some(subclass) = constraint.regclass.intersect(reg_info.rc(rc)) {
+                        // This constraint shrinks our preferred register class.
+                        *self = Affinity::Reg(subclass);
+                    }
+                }
+            }
+            Affinity::Stack => {}
+        }
+    }
+}
diff --git a/lib/cretonne/src/regalloc/liverange.rs b/lib/cretonne/src/regalloc/liverange.rs
index e192efdc4d..67667a3a47 100644
--- a/lib/cretonne/src/regalloc/liverange.rs
+++ b/lib/cretonne/src/regalloc/liverange.rs
@@ -73,9 +73,9 @@
 //! 3. A numerical order by EBB number. Performant because it doesn't need to indirect through the
 //!    `ProgramOrder` for comparisons.
 //!
-//! These orderings will cause small differences in coalescing opportinities, but all of them would
+//! These orderings will cause small differences in coalescing opportunities, but all of them would
 //! do a decent job of compressing a long live range. The numerical order might be preferable
-//! beacuse:
+//! because:
 //!
 //! - It has better performance because EBB numbers can be compared directly without any table
 //!   lookups.
@@ -92,7 +92,7 @@
 //!
 //! Coalescing is an important compression technique because some live ranges can span thousands of
 //! EBBs. We can represent that by switching to a sorted `Vec<ProgramPoint>` representation where
-//! an `[Ebb, Inst]` pair represents a coalesced range, while an `Inst` entry without a preceeding
+//! an `[Ebb, Inst]` pair represents a coalesced range, while an `Inst` entry without a preceding
 //! `Ebb` entry represents a single live-in interval.
 //!
 //! This representation is more compact for a live range with many uncoalesced live-in intervals.
@@ -109,6 +109,7 @@
 
 use std::cmp::Ordering;
 use ir::{Inst, Ebb, Value, ProgramPoint, ProgramOrder};
+use regalloc::affinity::Affinity;
 use sparse_map::SparseMapValue;
 
 /// Global live range of a single SSA value.
@@ -143,6 +144,9 @@ pub struct LiveRange {
     /// This member can't be modified in case the live range is stored in a `SparseMap`.
     value: Value,
 
+    /// The preferred register allocation for this value.
+    pub affinity: Affinity,
+
     /// The instruction or EBB header where this value is defined.
     def_begin: ProgramPoint,
 
@@ -167,7 +171,7 @@ pub struct LiveRange {
 /// An additional contiguous interval of a global live range.
 ///
 /// This represents a live-in interval for a single EBB, or a coalesced set of live-in intervals
-/// for contiguous EBBs where all but the last live-in inteval covers the whole EBB.
+/// for contiguous EBBs where all but the last live-in interval covers the whole EBB.
 ///
 struct Interval {
     /// Interval starting point.
@@ -205,6 +209,7 @@ impl LiveRange {
         let def = def.into();
         LiveRange {
             value: value,
+            affinity: Default::default(),
             def_begin: def,
             def_end: def,
             liveins: Vec::new(),
@@ -238,7 +243,7 @@ impl LiveRange {
     /// is live-in to `ebb`, extending to `to`. Return true.
     ///
     /// The return value can be used to detect if we just learned that the value is live-in to
-    /// `ebb`. This can trigger recursive extensions in `ebb`'s CFG precedessor blocks.
+    /// `ebb`. This can trigger recursive extensions in `ebb`'s CFG predecessor blocks.
     pub fn extend_in_ebb<PO: ProgramOrder>(&mut self, ebb: Ebb, to: Inst, order: &PO) -> bool {
         // First check if we're extending the def interval.
         //
diff --git a/lib/cretonne/src/regalloc/mod.rs b/lib/cretonne/src/regalloc/mod.rs
index 9e0f7b820d..bd1c34cd23 100644
--- a/lib/cretonne/src/regalloc/mod.rs
+++ b/lib/cretonne/src/regalloc/mod.rs
@@ -5,3 +5,5 @@
 pub mod liverange;
 pub mod liveness;
 pub mod allocatable_set;
+
+mod affinity;