cmd/compile: make Maxarg local

Passes toolstash -cmp. No compiler performance impact.

Updates #15756

Change-Id: I1294058716d83dd1be495d399ed7ab2277754dc6
Reviewed-on: https://go-review.googlesource.com/38329
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -1200,7 +1200,6 @@ func addmethod(msym *Sym, t *Type, local, nointerface bool) {
 
 func funccompile(n *Node) {
 	Stksize = BADWIDTH
-	Maxarg = 0
 
 	if n.Type == nil {
 		if nerrors == 0 {

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

@@ -245,8 +245,6 @@ var dclcontext Class // PEXTERN/PAUTO
 
 var statuniqgen int // name generator for static temps
 
-var Maxarg int64
-
 var Stksize int64 // stack size for current frame
 
 var stkptrsize int64 // prefix of stack containing pointers

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

@@ -90,8 +90,6 @@ func fninit(n []*Node) {
 	addvar(gatevar, Types[TUINT8], PEXTERN)
 
 	// (2)
-	Maxarg = 0
-
 	fn := nod(ODCLFUNC, nil, nil)
 	initsym := lookup("init")
 	fn.Func.Nname = newname(initsym)

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

@@ -4204,6 +4204,8 @@ type SSAGenState struct {
 	SSEto387 map[int16]int16
 	// Some architectures require a 64-bit temporary for FP-related register shuffling. Examples include x86-387, PPC, and Sparc V8.
 	ScratchFpMem *Node
+
+	maxarg int64 // largest frame size for arguments to calls made by the function
 }
 
 // Pc returns the current Prog.
@@ -4355,7 +4357,7 @@ func genssa(f *ssa.Func, ptxt *obj.Prog, gcargs, gclocals *Sym) {
 	liveness(e.curfn, ptxt, gcargs, gclocals)
 
 	// Add frame prologue. Zero ambiguously live variables.
-	thearch.Defframe(ptxt, e.curfn, Stksize+Maxarg)
+	thearch.Defframe(ptxt, e.curfn, Stksize+s.maxarg)
 	if Debug['f'] != 0 {
 		frame(0)
 	}
@@ -4631,8 +4633,8 @@ func (s *SSAGenState) Call(v *ssa.Value) *obj.Prog {
 		}
 		p.To.Reg = v.Args[0].Reg()
 	}
-	if Maxarg < v.AuxInt {
-		Maxarg = v.AuxInt
+	if s.maxarg < v.AuxInt {
+		s.maxarg = v.AuxInt
 	}
 	return p
 }