syscall: simplify and optimize environment block creation on Windows

createEnvBlock currently allocates multiple times: at least one to
convert the slice of strings into a NULL separated slice of bytes, and
then again to encode it as UTF-16. The logic to do so is also quite
complex.

This CL simplifies the logic by allocating only once by encoding the
slice of strings into UTF-16 directly using utf16.AppendRune.

goos: windows
goarch: amd64
pkg: syscall
cpu: Intel(R) Core(TM) i7-10850H CPU @ 2.70GHz
                  │   old.txt    │               new.txt               │
                  │    sec/op    │   sec/op     vs base                │
CreateEnvBlock-12   37.92µ ± 24%   21.36µ ± 8%  -43.66% (p=0.000 n=10)

                  │    old.txt    │               new.txt                │
                  │     B/op      │     B/op      vs base                │
CreateEnvBlock-12   109.12Ki ± 0%   26.62Ki ± 0%  -75.60% (p=0.000 n=10)

                  │  old.txt   │              new.txt               │
                  │ allocs/op  │ allocs/op   vs base                │
CreateEnvBlock-12   4.000 ± 0%   1.000 ± 0%  -75.00% (p=0.000 n=10)

Change-Id: If35f62c3926b486d5253a9ae23a33b979b2f02c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/531355
Reviewed-by: Bryan Mills <[email protected]>
Run-TryBot: Quim Muntal <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>

src/syscall/exec_windows.go

@@ -118,11 +118,11 @@ func makeCmdLine(args []string) string {
 // terminated strings followed by a nil.
 // Last bytes are two UCS-2 NULs, or four NUL bytes.
 // If any string contains a NUL, it returns (nil, EINVAL).
-func createEnvBlock(envv []string) (*uint16, error) {
+func createEnvBlock(envv []string) ([]uint16, error) {
 	if len(envv) == 0 {
-		return &utf16.Encode([]rune("\x00\x00"))[0], nil
+		return utf16.Encode([]rune("\x00\x00")), nil
 	}
-	length := 0
+	var length int
 	for _, s := range envv {
 		if bytealg.IndexByteString(s, 0) != -1 {
 			return nil, EINVAL
@@ -131,17 +131,15 @@ func createEnvBlock(envv []string) (*uint16, error) {
 	}
 	length += 1
 
-	b := make([]byte, length)
-	i := 0
+	b := make([]uint16, 0, length)
 	for _, s := range envv {
-		l := len(s)
-		copy(b[i:i+l], []byte(s))
-		copy(b[i+l:i+l+1], []byte{0})
-		i = i + l + 1
+		for _, c := range s {
+			b = utf16.AppendRune(b, c)
+		}
+		b = utf16.AppendRune(b, 0)
 	}
-	copy(b[i:i+1], []byte{0})
-
-	return &utf16.Encode([]rune(string(b)))[0], nil
+	b = utf16.AppendRune(b, 0)
+	return b, nil
 }
 
 func CloseOnExec(fd Handle) {
@@ -387,9 +385,9 @@ func StartProcess(argv0 string, argv []string, attr *ProcAttr) (pid int, handle
 	pi := new(ProcessInformation)
 	flags := sys.CreationFlags | CREATE_UNICODE_ENVIRONMENT | _EXTENDED_STARTUPINFO_PRESENT
 	if sys.Token != 0 {
-		err = CreateProcessAsUser(sys.Token, argv0p, argvp, sys.ProcessAttributes, sys.ThreadAttributes, willInheritHandles, flags, envBlock, dirp, &si.StartupInfo, pi)
+		err = CreateProcessAsUser(sys.Token, argv0p, argvp, sys.ProcessAttributes, sys.ThreadAttributes, willInheritHandles, flags, &envBlock[0], dirp, &si.StartupInfo, pi)
 	} else {
-		err = CreateProcess(argv0p, argvp, sys.ProcessAttributes, sys.ThreadAttributes, willInheritHandles, flags, envBlock, dirp, &si.StartupInfo, pi)
+		err = CreateProcess(argv0p, argvp, sys.ProcessAttributes, sys.ThreadAttributes, willInheritHandles, flags, &envBlock[0], dirp, &si.StartupInfo, pi)
 	}
 	if err != nil {
 		return 0, 0, err