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>

diff --git a/src/cmd/compile/internal/gc/reg.go b/src/cmd/compile/internal/gc/reg.go
index b3e96216bdbcb8ce0464adcfe16754a2926ffe76..ff6ec321a1c58811cd071f7c37710feca5e0aad4 100644 (file)
--- a/src/cmd/compile/internal/gc/reg.go
+++ b/src/cmd/compile/internal/gc/reg.go
@@ -170,7 +170,7 @@ func setaddrs(bit Bits) {
 
        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 @@ loop2:
                        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 @@ func bany(a *Bits) bool {
 }
 
 // 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 @@ func (bits Bits) String() string {
        var buf bytes.Buffer
        sep := ""
        for bany(&bits) {
-               i := bnum(bits)
+               i := bnum(&bits)
                buf.WriteString(sep)
                sep = " "
                v := &vars[i]