[dev.ssa] cmd/internal/ssa: Add register allocation

Add a simple register allocator.  It does only intra-basicblock
allocation.  It uses a greedy one-pass allocation treating the
register file as a cache.

Change-Id: Ib6b52f48270e08dfda98f2dd842b05afc3ab01ce
Reviewed-on: https://go-review.googlesource.com/9761
Reviewed-by: Alan Donovan <adonovan@google.com>

src/cmd/internal/ssa/stackalloc.go

@@ -0,0 +1,51 @@
+package ssa
+
+// stackalloc allocates storage in the stack frame for
+// all Values that did not get a register.
+func stackalloc(f *Func) {
+	home := f.RegAlloc
+
+	var n int64 = 8 // 8 = space for return address.  TODO: arch-dependent
+
+	// Assign stack locations to phis first, because we
+	// must also assign the same locations to the phi copies
+	// introduced during regalloc.
+	for _, b := range f.Blocks {
+		for _, v := range b.Values {
+			if v.Op != OpPhi {
+				continue
+			}
+			n += v.Type.Size()
+			// a := v.Type.Align()
+			// n = (n + a - 1) / a * a  TODO
+			loc := &LocalSlot{n}
+			home = setloc(home, v, loc)
+			for _, w := range v.Args {
+				home = setloc(home, w, loc)
+			}
+		}
+	}
+
+	// Now do all other unassigned values.
+	for _, b := range f.Blocks {
+		for _, v := range b.Values {
+			if v.ID < ID(len(home)) && home[v.ID] != nil {
+				continue
+			}
+			if v.Type.IsMemory() { // TODO: only "regallocable" types
+				continue
+			}
+			// a := v.Type.Align()
+			// n = (n + a - 1) / a * a  TODO
+			n += v.Type.Size()
+			loc := &LocalSlot{n}
+			home = setloc(home, v, loc)
+		}
+	}
+	f.RegAlloc = home
+
+	// TODO: share stack slots among noninterfering (& gc type compatible) values
+	// TODO: align final n
+	// TODO: compute total frame size: n + max paramout space
+	// TODO: save total size somewhere
+}