Spill values live across calls.

Calls clobber many registers, so spill everything that is live across a
call for now.

In the future, we may add support for callee-saved registers.

filetests/regalloc/spill.cton

@@ -62,3 +62,16 @@ ebb0(v1: i32):
     return v21
     ; check: return $v21, $rlink2
 }
+
+; All values live across a call must be spilled
+function %across_call(i32) {
+    fn0 = function %foo(i32)
+ebb0(v1: i32):
+    ; check: $v1 = spill
+    call fn0(v1)
+    ; check: call $fn0
+    call fn0(v1)
+    ; check: fill $v1
+    ; check: call $fn0
+    return
+}

lib/cretonne/src/regalloc/affinity.rs

@@ -64,6 +64,14 @@ impl Affinity {
         }
     }
 
+    /// Is this the `Reg` affinity?
+    pub fn is_reg(self) -> bool {
+        match self {
+            Affinity::Reg(_) => true,
+            _ => false,
+        }
+    }
+
     /// Is this the `Stack` affinity?
     pub fn is_stack(self) -> bool {
         match self {

lib/cretonne/src/regalloc/spilling.rs

@@ -152,14 +152,25 @@ impl<'a> Context<'a> {
         //
         // Spills should be removed from tracker. Otherwise they could be double-counted by
         // free_regs below.
+        let call_sig = dfg.call_signature(inst);
 
         // Update the live value tracker with this instruction.
         let (throughs, kills, defs) = tracker.process_inst(inst, dfg, self.liveness);
 
-
         // Remove kills from the pressure tracker.
         self.free_regs(kills);
 
+        // If inst is a call, spill all register values that are live across the call.
+        // This means that we don't currently take advantage of callee-saved registers.
+        // TODO: Be more sophisticated.
+        if call_sig.is_some() {
+            for lv in throughs {
+                if lv.affinity.is_reg() && !self.spills.contains(&lv.value) {
+                    self.spill_reg(lv.value);
+                }
+            }
+        }
+
         // Make sure we have enough registers for the register defs.
         // Dead defs are included here. They need a register too.
         // No need to process call return values, they are in fixed registers.