os: parse command line without shell32.dll

Go uses CommandLineToArgV from shell32.dll to parse command
line parameters. But shell32.dll is slow to load. Implement
Windows command line parsing in Go. This should make starting
Go programs faster.

I can see these speed ups for runtime.BenchmarkRunningGoProgram

on my Windows 7 amd64:
name                old time/op  new time/op  delta
RunningGoProgram-2  11.2ms ± 1%  10.4ms ± 2%  -6.63%  (p=0.000 n=9+10)

on my Windows XP 386:
name                old time/op  new time/op  delta
RunningGoProgram-2  19.0ms ± 3%  12.1ms ± 1%  -36.20%  (p=0.000 n=10+10)

on @egonelbre Windows 10 amd64:
name                old time/op  new time/op  delta
RunningGoProgram-8  17.0ms ± 1%  15.3ms ± 2%  -9.71%  (p=0.000 n=10+10)

This CL is based on CL 22932 by John Starks.

Fixes #15588.

Change-Id: Ib14be0206544d0d4492ca1f0d91fac968be52241
Reviewed-on: https://go-review.googlesource.com/37915
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 71 insertions, 9 deletions

diff --git a/src/os/exec_windows.go b/src/os/exec_windows.go
index d89db2022c..d5d553a2f6 100644
--- a/src/os/exec_windows.go
+++ b/src/os/exec_windows.go
@@ -97,17 +97,79 @@ func findProcess(pid int) (p *Process, err error) {
 }
 
 func init() {
-	var argc int32
-	cmd := syscall.GetCommandLine()
-	argv, e := syscall.CommandLineToArgv(cmd, &argc)
-	if e != nil {
-		return
+	p := syscall.GetCommandLine()
+	cmd := syscall.UTF16ToString((*[0xffff]uint16)(unsafe.Pointer(p))[:])
+	if len(cmd) == 0 {
+		arg0, _ := Executable()
+		Args = []string{arg0}
+	} else {
+		Args = commandLineToArgv(cmd)
 	}
-	defer syscall.LocalFree(syscall.Handle(uintptr(unsafe.Pointer(argv))))
-	Args = make([]string, argc)
-	for i, v := range (*argv)[:argc] {
-		Args[i] = syscall.UTF16ToString((*v)[:])
+}
+
+// appendBSBytes appends n '\\' bytes to b and returns the resulting slice.
+func appendBSBytes(b []byte, n int) []byte {
+	for ; n > 0; n-- {
+		b = append(b, '\\')
+	}
+	return b
+}
+
+// readNextArg splits command line string cmd into next
+// argument and command line remainder.
+func readNextArg(cmd string) (arg []byte, rest string) {
+	var b []byte
+	var inquote bool
+	var nslash int
+	for ; len(cmd) > 0; cmd = cmd[1:] {
+		c := cmd[0]
+		switch c {
+		case ' ', '\t':
+			if !inquote {
+				return appendBSBytes(b, nslash), cmd[1:]
+			}
+		case '"':
+			b = appendBSBytes(b, nslash/2)
+			if nslash%2 == 0 {
+				// use "Prior to 2008" rule from
+				// http://daviddeley.com/autohotkey/parameters/parameters.htm
+				// section 5.2 to deal with double double quotes
+				if inquote && len(cmd) > 1 && cmd[1] == '"' {
+					b = append(b, c)
+					cmd = cmd[1:]
+				}
+				inquote = !inquote
+			} else {
+				b = append(b, c)
+			}
+			nslash = 0
+			continue
+		case '\\':
+			nslash++
+			continue
+		}
+		b = appendBSBytes(b, nslash)
+		nslash = 0
+		b = append(b, c)
+	}
+	return appendBSBytes(b, nslash), ""
+}
+
+// commandLineToArgv splits a command line into individual argument
+// strings, following the Windows conventions documented
+// at http://daviddeley.com/autohotkey/parameters/parameters.htm#WINARGV
+func commandLineToArgv(cmd string) []string {
+	var args []string
+	for len(cmd) > 0 {
+		if cmd[0] == ' ' || cmd[0] == '\t' {
+			cmd = cmd[1:]
+			continue
+		}
+		var arg []byte
+		arg, cmd = readNextArg(cmd)
+		args = append(args, string(arg))
 	}
+	return args
 }
 
 func ftToDuration(ft *syscall.Filetime) time.Duration {