cmd/compile: added stats printing to stackalloc

This is controlled by the "regalloc" stats flag, since regalloc
calls stackalloc.  The plan is for this to allow comparison
of cheaper stack allocation algorithms with what we have now.

Change-Id: Ibf64a780344c69babfcbb328fd6d053ea2e02cfc
Reviewed-on: https://go-review.googlesource.com/21393
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/stackalloc.go

@@ -22,6 +22,13 @@ type stackAllocState struct {
 	names     []LocalSlot
 	slots     []int
 	used      []bool
+
+	nArgSlot, // Number of Values sourced to arg slot
+	nNotNeed, // Number of Values not needing a stack slot
+	nNamedSlot, // Number of Values using a named stack slot
+	nReuse, // Number of values reusing a stack slot
+	nAuto, // Number of autos allocated for stack slots.
+	nSelfInterfere int32 // Number of self-interferences
 }
 
 func newStackAllocState(f *Func) *stackAllocState {
@@ -54,6 +61,7 @@ func putStackAllocState(s *stackAllocState) {
 	s.f.Config.stackAllocState = s
 	s.f = nil
 	s.live = nil
+	s.nArgSlot, s.nNotNeed, s.nNamedSlot, s.nReuse, s.nAuto, s.nSelfInterfere = 0, 0, 0, 0, 0, 0
 }
 
 type stackValState struct {
@@ -75,6 +83,13 @@ func stackalloc(f *Func, spillLive [][]ID) [][]ID {
 	defer putStackAllocState(s)
 
 	s.stackalloc()
+	if f.pass.stats > 0 {
+		f.logStat("stack_alloc_stats",
+			s.nArgSlot, "arg_slots", s.nNotNeed, "slot_not_needed",
+			s.nNamedSlot, "named_slots", s.nAuto, "auto_slots",
+			s.nReuse, "reused_slots", s.nSelfInterfere, "self_interfering")
+	}
+
 	return s.live
 }
 
@@ -170,9 +185,11 @@ func (s *stackAllocState) stackalloc() {
 	for _, b := range f.Blocks {
 		for _, v := range b.Values {
 			if !s.values[v.ID].needSlot {
+				s.nNotNeed++
 				continue
 			}
 			if v.Op == OpArg {
+				s.nArgSlot++
 				continue // already picked
 			}
 
@@ -190,12 +207,14 @@ func (s *stackAllocState) stackalloc() {
 					if h != nil && h.(LocalSlot).N == name.N && h.(LocalSlot).Off == name.Off {
 						// A variable can interfere with itself.
 						// It is rare, but but it can happen.
+						s.nSelfInterfere++
 						goto noname
 					}
 				}
 				if f.pass.debug > stackDebug {
 					fmt.Printf("stackalloc %s to %s\n", v, name.Name())
 				}
+				s.nNamedSlot++
 				f.setHome(v, name)
 				continue
 			}
@@ -217,11 +236,13 @@ func (s *stackAllocState) stackalloc() {
 			var i int
 			for i = 0; i < len(locs); i++ {
 				if !used[i] {
+					s.nReuse++
 					break
 				}
 			}
 			// If there is no unused stack slot, allocate a new one.
 			if i == len(locs) {
+				s.nAuto++
 				locs = append(locs, LocalSlot{N: f.Config.fe.Auto(v.Type), Type: v.Type, Off: 0})
 				locations[v.Type] = locs
 			}