x/sys/windows: use SyscallN in mkwinsyscall

The mkwinsyscall command has a hard limit of 15 on the
number of syscall arguments. Windows has several system
calls with more than 15 arguments, for example CreateFontPackage
has 18 arguments.

Another limiting factor is that mkwinsyscall uses the various
SyscallX functions, that generates extra complexity that can
be avoided by relying on SyscallN.

Updates golang/go#57914

windows/mkwinsyscall/mkwinsyscall.go

@@ -57,13 +57,11 @@ import (
 	"go/parser"
 	"go/token"
 	"io"
-	"io/ioutil"
 	"log"
 	"os"
 	"path/filepath"
 	"runtime"
 	"sort"
-	"strconv"
 	"strings"
 	"text/template"
 )
@@ -553,42 +551,18 @@ func (f *Fn) ParamCount() int {
 	return n
 }
 
-// SyscallParamCount determines which version of Syscall/Syscall6/Syscall9/...
-// to use. It returns parameter count for correspondent SyscallX function.
-func (f *Fn) SyscallParamCount() int {
-	n := f.ParamCount()
-	switch {
-	case n <= 3:
-		return 3
-	case n <= 6:
-		return 6
-	case n <= 9:
-		return 9
-	case n <= 12:
-		return 12
-	case n <= 15:
-		return 15
-	default:
-		panic("too many arguments to system call")
-	}
-}
-
 // Syscall determines which SyscallX function to use for function f.
 func (f *Fn) Syscall() string {
-	c := f.SyscallParamCount()
-	if c == 3 {
-		return syscalldot() + "Syscall"
-	}
-	return syscalldot() + "Syscall" + strconv.Itoa(c)
+	return syscalldot() + "SyscallN"
 }
 
-// SyscallParamList returns source code for SyscallX parameters for function f.
+// SyscallParamList returns source code for SyscallN parameters for function f.
 func (f *Fn) SyscallParamList() string {
 	a := make([]string, 0)
 	for _, p := range f.Params {
 		a = append(a, p.SyscallArgList()...)
 	}
-	for len(a) < f.SyscallParamCount() {
+	for len(a) == 0 {
 		a = append(a, "0")
 	}
 	return strings.Join(a, ", ")
@@ -923,7 +897,7 @@ func main() {
 	if *filename == "" {
 		_, err = os.Stdout.Write(data)
 	} else {
-		err = ioutil.WriteFile(*filename, data, 0644)
+		err = os.WriteFile(*filename, data, 0644)
 	}
 	if err != nil {
 		log.Fatal(err)
@@ -1011,7 +985,7 @@ func {{.HelperName}}({{.HelperParamList}}) {{template "results" .}}{
 
 {{define "results"}}{{if .Rets.List}}{{.Rets.List}} {{end}}{{end}}
 
-{{define "syscall"}}{{.Rets.SetReturnValuesCode}}{{.Syscall}}(proc{{.DLLFuncName}}.Addr(), {{.ParamCount}}, {{.SyscallParamList}}){{end}}
+{{define "syscall"}}{{.Rets.SetReturnValuesCode}}{{.Syscall}}(proc{{.DLLFuncName}}.Addr(), {{.SyscallParamList}}){{end}}
 
 {{define "tmpvarsreadback"}}{{range .Params}}{{if .TmpVarReadbackCode}}
 {{.TmpVarReadbackCode}}{{end}}{{end}}{{end}}