cmd/compile: add global autogeneratedPos

We use an "autogenerated" position in several places.
Rather than recreate it each time, make one early on and reuse it.
This removes the creation of new positions during the backend,
which was not concurrency-safe.

Updates #15756

Change-Id: Ic116b2e60f0e99de1a2ea87fe763831b50b645f8
Reviewed-on: https://go-review.googlesource.com/38915
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -186,7 +186,7 @@ func genhash(sym *Sym, t *Type) {
 		fmt.Printf("genhash %v %v\n", sym, t)
 	}
 
-	lineno = makePos(nil, 1, 0) // less confusing than end of input
+	lineno = autogeneratedPos // less confusing than end of input
 	dclcontext = PEXTERN
 	markdcl()
 
@@ -364,7 +364,7 @@ func geneq(sym *Sym, t *Type) {
 		fmt.Printf("geneq %v %v\n", sym, t)
 	}
 
-	lineno = makePos(nil, 1, 0) // less confusing than end of input
+	lineno = autogeneratedPos // less confusing than end of input
 	dclcontext = PEXTERN
 	markdcl()
 

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

@@ -286,6 +286,8 @@ var nodfp *Node
 
 var disable_checknil int
 
+var autogeneratedPos src.XPos
+
 // interface to back end
 
 type Arch struct {

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

@@ -4,8 +4,6 @@
 
 package gc
 
-import "cmd/internal/src"
-
 // a function named init is a special case.
 // it is called by the initialization before
 // main is run. to make it unique within a
@@ -77,7 +75,7 @@ func anyinit(n []*Node) bool {
 }
 
 func fninit(n []*Node) {
-	lineno = makePos(src.NewFileBase("<autogenerated>", "<autogenerated>"), 1, 0)
+	lineno = autogeneratedPos
 	nf := initfix(n)
 	if !anyinit(nf) {
 		return

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

@@ -351,6 +351,8 @@ func Main(archInit func(*Arch)) {
 	dclcontext = PEXTERN
 	nerrors = 0
 
+	autogeneratedPos = makePos(src.NewFileBase("<autogenerated>", "<autogenerated>"), 1, 0)
+
 	timings.Start("fe", "loadsys")
 	loadsys()
 

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

@@ -1717,7 +1717,7 @@ func genwrapper(rcvr *Type, method *Field, newnam *Sym, iface int) {
 		fmt.Printf("genwrapper rcvrtype=%v method=%v newnam=%v\n", rcvr, method, newnam)
 	}
 
-	lineno = makePos(src.NewFileBase("<autogenerated>", "<autogenerated>"), 1, 0)
+	lineno = autogeneratedPos
 
 	dclcontext = PEXTERN
 	markdcl()