Revert "windows/mkwinsyscall: use syscall.SyscallN instead of syscall.Syscall{6,9,12,15}"

This reverts CL 614082.

Reason for revert: syscall.SyscallN allocates more than its syscall.SyscallX counterparts, producing perf-related test failures across the board.

Updates #70197

Change-Id: I51107d909fcdbef4e65ee3f84932b2a0e7804f1b
Reviewed-on: https://go-review.googlesource.com/c/sys/+/625375
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

Author: qmuntal

windows/mkwinsyscall/mkwinsyscall.go

@@ -62,6 +62,7 @@ import (
 	"path/filepath"
 	"runtime"
 	"sort"
+	"strconv"
 	"strings"
 	"text/template"
 )
@@ -542,9 +543,47 @@ func (f *Fn) ParamPrintList() string {
 	return join(f.Params, func(p *Param) string { return fmt.Sprintf(`"%s=", %s, `, p.Name, p.Name) }, `", ", `)
 }
 
-// SyscallN returns a string representing the SyscallN function.
-func (f *Fn) SyscallN() string {
-	return syscalldot() + "SyscallN"
+// ParamCount return number of syscall parameters for function f.
+func (f *Fn) ParamCount() int {
+	n := 0
+	for _, p := range f.Params {
+		n += len(p.SyscallArgList())
+	}
+	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
+	case n <= 42: // current SyscallN limit
+		return n
+	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"
+	}
+	if c > 15 {
+		return syscalldot() + "SyscallN"
+	}
+	return syscalldot() + "Syscall" + strconv.Itoa(c)
 }
 
 // SyscallParamList returns source code for SyscallX parameters for function f.
@@ -553,12 +592,9 @@ func (f *Fn) SyscallParamList() string {
 	for _, p := range f.Params {
 		a = append(a, p.SyscallArgList()...)
 	}
-
-	// Check if the number exceeds the current SyscallN limit
-	if len(a) > 42 {
-		panic("too many arguments to system call")
+	for len(a) < f.SyscallParamCount() {
+		a = append(a, "0")
 	}
-
 	return strings.Join(a, ", ")
 }
 
@@ -979,7 +1015,7 @@ func {{.HelperName}}({{.HelperParamList}}) {{template "results" .}}{
 
 {{define "results"}}{{if .Rets.List}}{{.Rets.List}} {{end}}{{end}}
 
-{{define "syscall"}}{{.Rets.SetReturnValuesCode}}{{.SyscallN}}(proc{{.DLLFuncName}}.Addr(), {{.SyscallParamList}}){{end}}
+{{define "syscall"}}{{.Rets.SetReturnValuesCode}}{{.Syscall}}(proc{{.DLLFuncName}}.Addr(),{{if le .ParamCount 15}} {{.ParamCount}},{{end}} {{.SyscallParamList}}){{end}}
 
 {{define "tmpvarsreadback"}}{{range .Params}}{{if .TmpVarReadbackCode}}
 {{.TmpVarReadbackCode}}{{end}}{{end}}{{end}}

windows/mkwinsyscall/mkwinsyscall_test.go

@@ -50,25 +50,25 @@ func TestDLLFilenameEscaping(t *testing.T) {
 	}
 }
 
-func TestSyscallNGeneration(t *testing.T) {
+func TestSyscallXGeneration(t *testing.T) {
 	tests := []struct {
 		name        string
 		wantsysfunc string
 		sig         string
 	}{
 		{
 			name:        "syscall with 2 params",
-			wantsysfunc: "syscall.SyscallN",
+			wantsysfunc: "syscall.Syscall",
 			sig:         "Example(a1 *uint16, a2 *uint16) = ",
 		},
 		{
 			name:        "syscall with 6 params",
-			wantsysfunc: "syscall.SyscallN",
+			wantsysfunc: "syscall.Syscall6",
 			sig:         "Example(a1 *uint, a2 *uint, a3 *uint, a4 *uint, a5 *uint, a6 *uint) = ",
 		},
 		{
 			name:        "syscall with 15 params",
-			wantsysfunc: "syscall.SyscallN",
+			wantsysfunc: "syscall.Syscall15",
 			sig: strings.ReplaceAll(`Example(a1 *uint, a2 *uint, a3 *uint, a4 *uint, a5 *uint, a6 *uint,
 						a7 *uint, a8 *uint, a9 *uint, a10 *uint, a11 *uint, a12 *uint,
 						a13 *uint, a14 *uint, a15 *uint) = `, "\n", ""),