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 <bcmills@google.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>

diff --git a/src/syscall/exec_windows.go b/src/syscall/exec_windows.go
index b311a5c74684bf398b4057961ec70d80f2a7f241..1220de4cdf1e36cd816d043336c20bd4a5d064c8 100644 (file)
--- a/src/syscall/exec_windows.go
+++ b/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