cmd/compile/internal/ssa: use math/bits in countRegs and pickReg

Makes code simpler and faster (at least on x86).

	name               old time/op  new time/op  delta
	CountRegs-8        7.40ns ± 1%  0.59ns ± 0%  -92.02%  (p=0.000 n=9+9)
	PickReg/(1<<0)-8   2.07ns ± 0%  0.37ns ± 0%  -82.13%  (p=0.000 n=9+10)
	PickReg/(1<<16)-8  11.8ns ± 0%   0.4ns ± 0%  -96.86%  (p=0.002 n=8+10)

Change-Id: Ic780b615b75c25b6e7632a0de93b16a8e9ed0f8f
Reviewed-on: https://go-review.googlesource.com/120318
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

diff --git a/src/cmd/compile/internal/ssa/regalloc.go b/src/cmd/compile/internal/ssa/regalloc.go
index bbf19329814583cd9182fbe52f8caba0cf19fbe7..278da6fe99fd35358adc7407436d92f2f19c93c3 100644 (file)
--- a/src/cmd/compile/internal/ssa/regalloc.go
+++ b/src/cmd/compile/internal/ssa/regalloc.go
@@ -119,6 +119,7 @@ import (
        "cmd/internal/src"
        "cmd/internal/sys"
        "fmt"
+       "math/bits"
        "unsafe"
 )
 
@@ -183,26 +184,16 @@ func (s *regAllocState) RegMaskString(m regMask) string {
 
 // countRegs returns the number of set bits in the register mask.
 func countRegs(r regMask) int {
-       n := 0
-       for r != 0 {
-               n += int(r & 1)
-               r >>= 1
-       }
-       return n
+       return bits.OnesCount64(uint64(r))
 }
 
 // pickReg picks an arbitrary register from the register mask.
 func pickReg(r regMask) register {
-       // pick the lowest one
        if r == 0 {
                panic("can't pick a register from an empty set")
        }
-       for i := register(0); ; i++ {
-               if r&1 != 0 {
-                       return i
-               }
-               r >>= 1
-       }
+       // pick the lowest one
+       return register(bits.TrailingZeros64(uint64(r)))
 }
 
 type use struct {