cmd/compile: improve regalloc live values debug printing

Before:

live values at end of each block
  b1: v3 v2 v7 avoid=0
  b2: v3 v13 avoid=81
  b3: v19[AX] v3 avoid=81
  b6: avoid=0
  b7: avoid=0
  b5: avoid=0
  b4: v3 v18 avoid=81

After:

live values at end of each block
  b1: v3 v2 v7
  b2: v3 v13 avoid=AX DI
  b3: v19[AX] v3 avoid=AX DI
  b6:
  b7:
  b5:
  b4: v3 v18 avoid=AX DI

Change-Id: Ibec5c76a16151832b8d49a21c640699fdc9a9d28
Reviewed-on: https://go-review.googlesource.com/109000
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -165,6 +165,21 @@ func (m regMask) String() string {
 	return s
 }
 
+func (s *regAllocState) RegMaskString(m regMask) string {
+	str := ""
+	for r := register(0); m != 0; r++ {
+		if m>>r&1 == 0 {
+			continue
+		}
+		m &^= regMask(1) << r
+		if str != "" {
+			str += " "
+		}
+		str += s.registers[r].String()
+	}
+	return str
+}
+
 // countRegs returns the number of set bits in the register mask.
 func countRegs(r regMask) int {
 	n := 0
@@ -793,7 +808,7 @@ func (s *regAllocState) regalloc(f *Func) {
 			}
 		}
 		if s.f.pass.debug > regDebug {
-			fmt.Printf("uses for %s:%s\n", s.f.Name, b)
+			fmt.Printf("use distances for %s\n", b)
 			for i := range s.values {
 				vi := &s.values[i]
 				u := vi.uses
@@ -2400,7 +2415,9 @@ func (s *regAllocState) computeLive() {
 					fmt.Printf("]")
 				}
 			}
-			fmt.Printf(" avoid=%x", int64(s.desired[b.ID].avoid))
+			if avoid := s.desired[b.ID].avoid; avoid != 0 {
+				fmt.Printf(" avoid=%v", s.RegMaskString(avoid))
+			}
 			fmt.Println()
 		}
 	}