cmd/compile/internal/gc: avoid allocation in bnum

Although bnum was being called with a Bits value, a limitation
of the escape analyser (golang/go#12588) meant that taking the
address of the Bits.b array in the range statement caused the
formal parameter to escape to the heap.

Passing the a pointer to a Bits, as with all the other Bits helper
methods avoids the allocation.

Before:
BenchmarkBnum1-4        20000000                69.6 ns/op            32 B/op          1 allocs/op

After:
BenchmarkBnum1-4        100000000               10.1 ns/op             0 B/op          0 allocs/op

Change-Id: I673bd57ddc032ee67d09474156d795fb1ba72018
Reviewed-on: https://go-review.googlesource.com/14501
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/reg.go

diff --git a/src/cmd/compile/internal/gc/reg.go b/src/cmd/compile/internal/gc/reg.go
index b3e9621..ff6ec32 100644
--- a/src/cmd/compile/internal/gc/reg.go
+++ b/src/cmd/compile/internal/gc/reg.go
@@ -170,7 +170,7 @@
 
 	for bany(&bit) {
 		// convert each bit to a variable
-		i = bnum(bit)
+		i = bnum(&bit)
 
 		node = vars[i].node
 		n = int(vars[i].name)
@@ -1321,7 +1321,7 @@
 			bit.b[z] = LOAD(r, z) &^ (r.act.b[z] | addrs.b[z])
 		}
 		for bany(&bit) {
-			i = bnum(bit)
+			i = bnum(&bit)
 			change = 0
 			paint1(f, i)
 			biclr(&bit, uint(i))
@@ -1465,7 +1465,7 @@
 }
 
 // bnum reports the lowest index of a 1 bit in a.
-func bnum(a Bits) int {
+func bnum(a *Bits) int {
 	for i, x := range &a.b { // & to avoid making a copy of a.b
 		if x != 0 {
 			return 64*i + Bitno(x)
@@ -1541,7 +1541,7 @@
 	var buf bytes.Buffer
 	sep := ""
 	for bany(&bits) {
-		i := bnum(bits)
+		i := bnum(&bits)
 		buf.WriteString(sep)
 		sep = " "
 		v := &vars[i]