Don't coalesce incoming stack arguments.

A function parameter in an incoming_arg stack slot should not be
coalesced into any virtual registers. We don't want to force the whole
virtual register to spill to the incoming_arg slot.

cranelift/filetests/regalloc/coalesce.cton

@@ -91,3 +91,21 @@ ebb1(v10: i32, v11: i32):
     brnz v13, ebb1(v11, v12)
     return v12
 }
+
+; Function arguments passed on the stack aren't allowed to be part of a virtual
+; register, at least for now. This is because the other values in the virtual
+; register would need to be spilled to the incoming_arg stack slot which we treat
+; as belonging to the caller.
+function %stackarg(i32, i32, i32, i32, i32, i32, i32, i32, i32) -> i32 {
+; check: ss0 = incoming_arg 4
+; not: incoming_arg
+ebb0(v0: i32, v1: i32, v2: i32, v3: i32, v4: i32, v5: i32, v6: i32, v7: i32, v8: i32):
+    ; check: fill v8
+    ; not: v8
+    brnz v0, ebb1(v8)
+    jump ebb1(v7)
+
+ebb1(v10: i32):
+    v11 = iadd_imm v10, 1
+    return v11
+}

lib/cretonne/src/regalloc/coalescing.rs

@@ -8,7 +8,7 @@
 use dbg::DisplayList;
 use dominator_tree::DominatorTree;
 use flowgraph::{ControlFlowGraph, BasicBlock};
-use ir::{DataFlowGraph, Layout, Cursor, InstBuilder};
+use ir::{DataFlowGraph, Layout, Cursor, InstBuilder, ValueDef};
 use ir::{Function, Ebb, Inst, Value, ExpandedProgramPoint};
 use regalloc::affinity::Affinity;
 use regalloc::liveness::Liveness;
@@ -400,6 +400,22 @@ impl<'a> Context<'a> {
                  pred_val,
                  pred_ebb,
                  self.func.dfg.display_inst(pred_inst));
+
+            // Never coalesce incoming function arguments on the stack. These arguments are
+            // pre-spilled, and the rest of the virtual register would be forced to spill to the
+            // `incoming_arg` stack slot too.
+            if let ValueDef::Arg(def_ebb, def_num) = self.func.dfg.value_def(pred_val) {
+                if Some(def_ebb) == self.func.layout.entry_block() &&
+                   self.func.signature.argument_types[def_num]
+                       .location
+                       .is_stack() {
+                    dbg!("Isolating incoming stack parameter {}", pred_val);
+                    let new_val = self.split_pred(pred_inst, pred_ebb, argnum, pred_val);
+                    assert!(self.add_class(new_val).is_ok());
+                    continue;
+                }
+            }
+
             if let Err((a, b)) = self.add_class(pred_val) {
                 dbg!("Found conflict between {} and {}", a, b);
                 // We have a conflict between the already merged value `a` and one of the new