Reorg so that Get() is implemented fully in assembly on amd64

This is a minor perf win and reduces future maintenance. The runtime
structures are now encapsulated in the runtime_goX.Y.go files. The same
assembly implementation of Get() can be used until the runtime structure
changes significantly (i.e. `g.goid` is moved, which is unlikely).

name   old time/op  new time/op  delta
Get-8  2.88ns ± 3%  1.70ns ± 2%  -41.05%  (p=0.000 n=10+9)

Author: petermattis

goid_go1.5_amd64.go

@@ -0,0 +1,21 @@
+// Copyright 2016 Peter Mattis.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+// implied. See the License for the specific language governing
+// permissions and limitations under the License. See the AUTHORS file
+// for names of contributors.
+
+// +build amd64 amd64p32
+// +build go1.5
+
+package goid
+
+func Get() int64

goid_go1.5plus.s goid_go1.5_amd64.s

@@ -18,10 +18,12 @@
 // +build amd64 amd64p32
 // +build go1.5
 
+#include "go_asm.h"
 #include "textflag.h"
 
-// func getg() *g
-TEXT ·getg(SB),NOSPLIT,$0-8
-	MOVQ (TLS), BX
-	MOVQ BX, ret+0(FP)
+// func Get() int64
+TEXT ·Get(SB),NOSPLIT,$0-8
+	MOVQ (TLS), R14
+	MOVQ g_goid(R14), R13
+	MOVQ R13, ret+0(FP)
 	RET

goid_go1.5_arm.go

@@ -0,0 +1,26 @@
+// Copyright 2016 Peter Mattis.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+// implied. See the License for the specific language governing
+// permissions and limitations under the License. See the AUTHORS file
+// for names of contributors.
+
+// +build arm
+// +build go1.5
+
+package goid
+
+// Backdoor access to runtime·getg().
+func getg() *g // in goid_go1.5plus.s
+
+func Get() int64 {
+	return getg().goid
+}

goid_go1.5plus_arm.s goid_go1.5_arm.s

@@ -42,3 +42,9 @@ func TestGet(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkGet(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		Get()
+	}
+}

goid_test.go

@@ -13,7 +13,6 @@
 // permissions and limitations under the License. See the AUTHORS file
 // for names of contributors.
 
-// +build amd64 amd64p32 arm
 // +build go1.5,!go1.6
 
 package goid
@@ -55,10 +54,3 @@ type g struct {
 	stackLock    uint32
 	goid         int64 // Here it is!
 }
-
-// Backdoor access to runtime·getg().
-func getg() *g // in goid_go1.5plus.s
-
-func Get() int64 {
-	return getg().goid
-}

goid_go1.5.go runtime_go1.5.go

@@ -1,4 +1,3 @@
-// +build amd64 amd64p32 arm
 // +build go1.6,!go1.9
 
 package goid
@@ -41,10 +40,3 @@ type g struct {
 	stackLock    uint32
 	goid         int64 // Here it is!
 }
-
-// Backdoor access to runtime·getg().
-func getg() *g // in goid_go1.5plus{,_arm}.s
-
-func Get() int64 {
-	return getg().goid
-}

goid_go1.6plus.go runtime_go1.6.go

@@ -1,4 +1,3 @@
-// +build amd64 amd64p32 arm
 // +build go1.9
 
 package goid
@@ -35,10 +34,3 @@ type g struct {
 	stackLock    uint32
 	goid         int64 // Here it is!
 }
-
-// Backdoor access to runtime·getg().
-func getg() *g // in goid_go1.5plus{,_arm}.s
-
-func Get() int64 {
-	return getg().goid
-}